Recent developments in chemical oligosaccharide synthesis GEERT-JAN BOONS The School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2Ti7 UK Reviewing the literature published up to October 1995 1 2 2.1 2.2 2.3 2.4 2.5 2.6 3 4 5 6 7 8 9 10 11 Introduction Methods for stereoselective glycosylation Glycosidic bond synthesis Neighbouring group assisted procedures In situ anomerisation Glycosylation with inversion of configuration Solvent participation Intramolecular aglycon delivery Glycosylation of 3-deoxy-2-ulopyranoso- nates (3-deoxy 2-ketoaldonic acids) Formation of 2-deoxy-glycosidic linkages Regioselective glycosylations Convergent block synthesis Chemoselective glycosylations Polycondensations and one-pot multi-step glycosylations Solid-phase oligosaccharide synthesis Concluding remarks References 1 Introduction Glycoconjugates are the most functionally and structurally diverse molecules in nature and it is now well established that protein- and lipid-bound saccharides play essential roles in many molecular processes impacting on eukaryotic biology and disease.' Examples of such processes include fertilisation, embryogenesis, neuronal development, hormone activities? the proliferation of cells and their organisation into specific tissues.Remarkable changes in the cell-surface carbohydrates occur with tumour progression, which appear to be intimately associated with the dreaded state of metastasis.' Furthermore, carbohydrates are capable of inducing a protective antibody response and this immunological reaction is a major contributor to the survival of the organism during infe~tion.~ Oligosaccharides have also been found to control the development and defence mechanisms of plank4 The increased appreciation of the role of carbohydrates in the biological and pharmaceutical sciences has resulted in a revival of interest in carbohydrate chemistry.The chemical synthesis of oligosaccharides is much more complicated than the synthesis of other biopolymers such as peptides and nucleic acids. The difficulties in the preparation of complex oligosaccharides are a result of a greater number of possibilities for the combination of monomeric units to form oligosaccharides. In addition, the glycosidic linkages have to be introduced stereospecifically ( a : p selectivity). To date, there are no general applicable methods or strategies for oligosaccharide synthesis and consequently the preparation of these molecules is very time consuming.Nevertheless, contemporary carbohydrate chemistry makes it now possible to execute complex multi-step synthetic sequences that give oligosaccharides consisting of as many as 20 monosaccharide units5 The preparation of oligosaccharides of this size is only possible when each synthetic step in the assembly of the oligosaccharide is high yielding and, furthermore, the formation of each glycosidic linkage is highly stereoselective. Apart from this, the assembly of the monomeric units should be highly convergent. This review describes the recent advances in chemical oligosaccharide synthesis. In the first part, methods for stereoselective glycosylation are reviewed and the scope and limitations are mentioned (Section 2).Next, the glycosylation of 3-deoxy-2-keto-ulo(pyranosyl)onates and 2-deoxy sugars are discussed (Sections 3 and 4). In Sections 5-8, examples of convergent oligosaccharide synthesis are presented and new glycosylation strategies for the facile preparation of saccharide building blocks are discussed. Finally, recent advances in solid supported oligosaccharide synthesis are summarised (Section 9). 2 Methods for stereoselective glycosylation 2.1 Glycosidic bond synthesis Inter-glycosidic bond formation is generally achieved by condensing a fully protected glycosyl donor, which bears a leaving group at its anomeric centre, with a suitably protected glycosyl acceptor that contains often only one free hydroxy group (Scheme l).637 Traditionally,Q the most widely used glycosylation methods utilised anomeric halide derivatives of carbohydrates as glycosyl donors.However, these compounds often suffer from instability and require relatively drastic conditions for their preparation. The introduction of the Boons: Recent developments in chemical oligosaccharide synthesis 173wo S / d X + HO-acceptor donor 1 promoter yo 0-acceptor s 4 4 R . . ors that tnfluence the a$ ratio 1. Substituent R: participating vs. non-padicipating 2. Orientation of substituent R: equatorial vs. axial 3. Type of substituents (S) in donor and acceptor 4. Type of leaving group (X) 5. Type of promoter 6. Solvent 7. Temperature 8. Pressure Scheme 1 orthoester' and imidate' procedures was the first attempt to fjnd alternatives to the glycosyl halide methodolegies.Since these original disclosures, many other leaving groups at the anomeric centre have been reported (Figure However, of these glycosyl donors, the anomeric fluorides, trichloroacetimidates and thioglycosides have been applied most widely. These compounds can be prepared under mild conditions, are sufficiently stable to be purified and stored for a considerable period of time, and undergo glycosylations under mild conditions. By selecting the appropriate reaction conditions, high yields and good a : /3 ratios can be obtained. The anomeric linkages can be classified according to the relative and absolute configuration at C-1 and C-2 (Figure 2) and they are: the 1,2-cis- and 1,2-trans-2-~-glycero series (allo-, gluco-, gulo- and galactopyranosides) and the 1,2-cis- and 1,2-trans- 2-~-glycero series (altro-, manno-, ido- and talo- galactopyranosides).Apart form these types, some miscellaneous glycosidic linkages can also be identified including: 2-deoxyglycosides and 3-deoxy- 2-keto-ulo(pyranosylic) acids. The stereoselective introduction of the glycosidic linkage is one of the most challenging aspects in oligosaccharide synthesis. The nature of the protecting group at C-2 of the glycosyl donor is a major determinant of the anomeric selectivity. A protecting group at C-2 which can perform neighbouring group participation during glycosylation will give 1,2-trans glycosidic linkages. On the other hand, when a non-assisting functionality is present at C-2 then the reaction conditions (e.g.solvent, temperature, promoter) will determine the anomeric selectivity. Also the constitution of the glycosyl donor and acceptor (e.g. type of saccharide, leaving group at the anomeric centre, protection and substitution pattern) have a major effect on the a : p selectivity. 2.2 Neighbouring group assisted procedures The most reliable method for the introduction of 1,2-trans-glycosidic linkages is based on I cosy1 halides = F, CI, Br) trichloroacetimidate !J selenoglycosides a S R thioglycosides (R = alkyl, aryl, cyanide, pyridyl) glycosyl xanthate glycosyl sulfoxide 1 ,2-epoxide lycosyl phoshorous e R = al I, Oalkyl; x = 0,'X lone pair) vinyl glycosides (R = H, Me) pentenyl glycoside N anomeric diazirines Figure 1 orthoester (R' = OR2, SR2, CN) anomeric acetate -OH reducing sugar X OR 0 pentenoyl glycoside 1 ,2-cis 1,2-mns 2-D-glycero 2-D-glycero 1 , 2 - m s 2-deoxy- 2-~-glycero glycosides Figure 2 1 ,Bcis 2-~-glycero OR 2-keto-3-deoxy- ulosonic acids 174 Contemporary Organic Synthesisneighbouring group participation of a 2-0-acyl functionality.The principle of this approach is schematically illustrated in Scheme 2. Thus, activation of the anomeric centre of 1 results in the formation of an oxonium ion 2. Subsequent neighbouring group participation of the 2-0-acetyl protecting group leads to a more stable acetoxonium ion 3. Attack of an alcohol at the anomeric centre results in the formation of a 1,2-trans-glycoside 4.Thus, in the case of glucosyl- type donors, P-linked products will be obtained and mannosides will give a-glycosides. The neighbouring group assisted glycosylation procedures are compatible with many different anomeric leaving groups and representative examples are depicted in Scheme 3. In some glycosylations, the alcohol will attack at the C-2 position of the dioxolane ring of 3 resulting in the formation of an undesired orthoester 6 (Scheme 2). For example, reaction of the glycosyl donor 7 with acceptor 8 gave only traces of coupling product 9 and a significant amount of orthoester 10 was isolated (Scheme 4).1° However, glycoconjugate 12 was isolated in a respectable 77% yield when glycosyl donor 11 was used. In this donor, the 2-0- BnO Bn:n* AcO + OAll O Y N H cc13 OBn + BzO BzO SMe BnO BzO BnO AcO AcO SMe +Ho& BnO OBn PhthN BnO PivO SPh + Q OH P i v 0 PivO ClAcO + CIA& HOB* SPh AcO PhthN R ' A o 1 R ' 4 0 4 1 HO-R' 6 5 Scheme 2 BF39Et2 - (73%) DMTST (85%) D CuBr2, Bu4NBr, AgOTf - (84%) Tf20 (80%) t SnC12, AgC104 w (72%) (All = allyl) (OBZ ,OBn B z o m i & O B n BzO BzO BnO PAC ,OBn OBn AcO PhthN BnO P i v o A o + PivO PivO PivO.PhthN BnO, Me3SiOTf,0°C ~ BnOB* + B n o & o ) BnO BnO BnO A&BnO&op BnO BnO AcO Brio Scheme 3 Boons: Recent developments in chemical oligosaccharide synthesis 175,OAc (OAc AcO F OAc AcO 7 R=Ac 11 R=PG + 'OAcw SnCI2, AgOTf 9 R=AC 12 R=Pw 10 Scheme 4 a Bz? Me I PhthN 14 AgOTf 1 13 PhthN BzO BzO 15 (43=2:1) b Bz? OBz I M e q B z + & ? ? HO SEt PhthN Br 14 I 16 a b AgOTf Figure 3 I 17 (a$ = 1:8.4) Scheme 5 acetyl protecting group has been replaced by a pivaloyl functionality.The increase in yield was explained as follows: the orthoester formation is disfavoured by the presence of the bulky rert-butyl group adjacent to the electrophilic carbon atom enhancing attack at the anomeric centre. A similar effect can be achieved by benzoyl protecting groups. In some cases, the glycosylation may also proceed via the oxonium ion 2 to give mixtures of anomers 5 (Scheme 2). For example, van Boeckel and co- workers showed" that coupling of bromide 13 with acceptor 14 in the presence of silver trifluoromethane sulfonate at -50 "C gave the dimer 15 with modest anomeric selectivity of a : /3 = 2 : 1 (Scheme 5a). Thus, although the participating benzoyl group present at C-2 of the glycosyl donor 13 should direct p-glycosidic bond formation, mainly the a-linked product was 18 (mismatched pair) I 19 (matched pair) obtained.It was reasoned that the transition state leading to the P-glycoside 18 was strongly disfavoured (mismatched pair) by severe steric hindrance (Figure 3a). It was anticipated that the use of glycosyl donor 16 with opposite chirality should give a different stereochemical outcome (double stereodifferentiation). Indeed, coupling of 16 with 14 under identical conditions afforded predominantly the P-linked dimer 17 (a: p = 1 : 8.4) (Scheme 5b). Computer modelling studies showed that in this case the transition state leading to the /?-product 19 is more favoured (matched pair, Figure 3b).In general, when in glycosylations unexpected a : P ratios or very low yields are obtained then unfavourable steric hindrance in the transition state should be considered. These unfavourable interactions may be reduced using sterically less demanding protecting groups. 2.3 In-situ anomerisation A major breakthrough in a-glycosidic bond synthesis came with the introduction of the in situ anomerisation procedure. This approach requires 176 Contemporuiy Organic Synthesisglycosyl donors with a non-participating protecting group at C-2. Lemieux and co-workers described'* that coupling of bromide 20 with glycosyl acceptor 21 in the presence of tetrabutylammonium bromide gave trisaccharide 22 mainly as the a-anomer (Scheme 6). The stereochemical outcome of this reaction was explained by the Curtin-Hammett prin~ip1e.l~ Thus, in this type of reaction, the tetrabutylammonium bromide catalyses the equilibration between the a- and /?-halides 23 and 24.This equilibrium is shifted strongly towards the a-bromide 23 since this compound is stabilised by the anomeric effect. However, the energy barrier for nucleophilic attack by an alcohol is lower for the P-halide 24. Therefore, glycosylation will take place from this intermediate and mainly x-glycosides 25 will be formed. However, an important requirement of this reaction is that the rate of equilibration is much faster than that of glycosylation. Furthermore, it is essential that the glycosylation is performed in a solvent of low polarity. In polar solvents, the reaction will proceed via an oxycarbonium ion and the anomeric selectivity will be reduced.Tetraalkylammonium halides react only with very reactive glycosyl halides. More reactive activators are required for more demanding glycosylations and nowadays a whole range of activators with different reactivities are available.6' High a-anomeric selectivities have been obtained with other anomeric leaving groups. For example, Ph ?r b @OBn + B z o w o : % AcHN OH 21 OBn BnO 20 Bu4NBr 1 Ph BzO b*o% OBn I F O B . 22 OBn BnO 26 Bn'O Bn\O I 23 Br 24 Brio Scheme 6 trimethylsilyl triflate mediated couplings of perbenzylated trichloroacetimidates at low temperature give in many cases excellent a-selectivities. Some examples have been reported in which thioglycosides and glycosyl fluorides also give high a-selectivities (Scheme 7).It has to be noted that the reaction mechanisms of these glycosylations have been less well studied. However, it is reasonable to assume that they proceed via an in situ anomerisation process. equilibration between the two ion pairs is faster than the glycosylation and many different parameters affect this requirement (see Scheme 2). Often many different reaction conditions have to be examined in order to obtain satisfactory results. Also small changes in the constitution of the glycosyl donor or acceptor may have a dramatic effect on the stereochemical outcome of a glycosylat ion. As mentioned above, it is very important that the 2.4 Glycosylation with inversion of configuration The in situ anomerisation procedure requires a fast equilibrium between an a- and P-ion pair.On the other hand, some glycosylation procedures are based on preventing this pre-equilibration and, hence, glycosylation will proceed via inversion of configuration. For example, glycosylation of a-halides 27 in the presence of an insoluble silver salt results mainly in P-glycoside formation 29.14 In this case, anomerisation of the halide is restricted because of lack of nucleophiles in the reaction mixture and therefore the reaction will proceed with inversion of configuration. A schematic illustration of the reaction mechanism is depicted in Scheme 8. Silver silicate and silver silicate-aluminate have often been applied as the heterogeneous catalyst. These catalysts have proven to be valuable in the preparation of P-linked mannosides which cannot be prepared by neighbouring group participation or in situ anomerisation.The presence of an non-participating substituent on C-2 is an important requirement for a glycosylation using a heterogenous catalyst. However, van Boeckel and co-workers have shown15 that the nature of the substituents at C-3, C-4 and C-6 also have a major effect on the anomeric ratios of the coupling products. As can be seen in Scheme 9, reaction of 30 with 31 gave dimer 32 as an mixture of anomers. However, when the acetyl group at C-3 was replaced by a benzyl group (33) then the dimer 34 was isolated mainly as the p-anomer. Reaction of a glycosyl donor having a trichloroacetyl group at C-3 (35) with 31 yielded dimer 36 as an anomeric mixture. When glycosyl donors with a 4-0-acyl group, 37 and 39, were used high levels of /?-products, 38 and 40, were obtained.Surprisingly, a glycosyl donor having a 4-0-alkyl group (41) gave 42 with lower levels of anomeric selectivity. Thus, these results indicate that an acyl group at C-3 decreases and at C-4 increases the P-selectivity. These observations were explained as Boons: Recent developments in chemical oligosaccharide synthesis 177BnoB% BnO MesSiOTf, Et20. room temp. BnO (83%, = 8:l) OBn BnO OBn BnO * o y " " BnO cc13 Brio% BnO OBn + HO BnO MeaSiOTf, Et20, mom temp. BnO (6670, a:B = %:I) v DI I U I m.n Bnb OBn BnO& BnO SEt + BnO N O M e IDCP. (83%. u J ~ Et2WCE = 5:l) .no$+ BnO BnO HoMe OBn BnO 0 OBn BnO ,OBn BZO Bn:% + H:* OTBS CuBr2, TBABr, AgOTf BnO A:%o& A d BnO * SMe (90%, a) BnO OTBS N3 SnCh.AgCIO,, -15 OC * Ad% (75%, a) BnO + H O T BnO Brio 0 7 OTPDPS BnO OTPDPS Scheme 7 Scheme 8 follows: anomeric mixtures will be obtained when the glycosylation proceeds via an oxonium ion and such an intermediate will be formed when the positive charge at the anomeric centre is well stabilised. A lone pair of the oxygen atom of the 4-O-acyl group can interact through bonds with the p(0) ring orbital. Thus, this interaction is stronger than expected with inductive effects only, and the electron withdrawing 4-O-acyl substituent suppresses oxy carbonium ion formation. Through- bond interactions are only significant when a succession of trans-bridges is available. Therefore, a 3-O-acyl group can interact only with the anomeric centre via inductive effects.Thus, destabilisation of 178 Contemporary Organic SynthesisR2tc% R'O + mo Br HO N3 30 R' = Ac; R2 = Bn 33 R' = R2 = Bn 35 R' = COCCI,; R2 = Bn 31 I 37 R' = Bn; R2 = Ac 39 R' = Bn; R2 = COCCIn silver silicate I 41 R' = Bn; R2 = All 1 7 0 (All = allyl) 32 R' = Ac; R2 = Bn; a$ = 1:l 34 R' = R2 = Bn; a:p = 1:6 36 R' = COCClj R2 = Bn; a:p = 1:l 38 R' = Bn; R2 = Ac; a$ = 1:9 40 R' = Bn; R2 = COCClj a$ = 1:lO 42 R' = Bn; R2 = All; a:P = 1:5 Scheme 9 an oxonium ion is less profound and hence more a-product will be obtained. P-Mannosides have also been prepared by direct substitution of an l-O-tosyl-a-mannoglycosyl donor.I6 However, this method has not been applied widely in oligosaccharide synthesis. Glycosylations may also proceed via inversion of configuration when performed in an apolar solvent and activated by a mild promotor.Schmidt and co- workers s h o ~ e d ' ~ that BF3 - Et,O mediated glycosylation of a-glucosyl and a-galactosyl trichloroacetimidate donors in dichloromethane or mixtures of dichloromethane-hexane give mainly P-glycosides. For example, BF3 Et20 mediated coupling of 43 with 44 in dichloromethane at -20 "C gave dimer 45 mainly as the p-anomer (Scheme 10). efficient approach for the synthesis of 1,2-cis- pyranosides employing l72-trans-g1ycosyl thiocyanates as glycosyl donors and tritylated sugar Kochetkov and co-workers have reported" an 44 CHzC12 I I 43 "'3 BF39Et2 Scheme 10 S-CEN + BnO 46 48 O-Tr d 49 S/-d + Tr-N=C=S + Tr+ Brio OR 50 Scheme 11 derivatives as glycosyl acceptors.For example, coupling of 46 with tritylated saccharide 47 gave dimer 48 as the a-anomer (Scheme 11). This coupling reaction is initiated by the reaction of the nitrogen atom of the thiocyanate 49 with a trityl cation with simultaneous nucleophilic attack by the oxygen atom of the trityl protected sugar alcohol at the anomeric centre to give an a-glycoside 50. It appears that this reaction proceeds by clean SN2 inversion at the anomeric centre. The substitution pattern of a glycosyl donor may also prevent in situ anomerisation and under appropriate conditions glycosylation will take place via SN2 substitution. For example, Danishefsky and co-workers have shown" that reaction of a 1,2-cis epoxide 52, obtained by epoxidation of a glucal 51 with dimethyldioxirane, with a sugar alcohol 53 in the presence of ZnC12 gives stereoselectively a 1,2-trans glycosidic linked product 54 (Scheme 12).However, van Boom and co-workers reported" that ZnC12 mediated glycosylation of 1,Zepoxides derived from galactal gives mixtures of anomers. Thus, in these cases, the reaction proceeds via a SN1 mechanism. Recently, Kiessling and co-workers used 172-cyclic sulfites as glycosyl donors.21 They argued that these compounds are more easily accessible and less labile than the corresponding epoxides and, hence, more appropriate glycosyl donors. Thus, osmylation of glucal 51 proceeded with high diastereofacial selectivity (19: 1) to give a 1,2-diol in high yield (91%) which upon treatment with thionyl Boons: Recent developments in chemical oligosaccharide synthesis 179BnO, BnO, 51 52 \o/ B n r a BnO / 53 BnO B n O a o BnO 7 OH I B n f n a 54 Scheme 12 Scheme 13 imidazolide stereospecifically gave the 1,2-cis cyclic sulfite 55 (Scheme 13).Exposure of 55 to lanthanide(I1r) triflate and reaction with benzyl alcohol resulted mainly in the formation of 1,2-trans glucoside 56 (10 : 1). It has not been reported whether 1,Zcyclic sulfites also give acceptable results with sugar alcohols as glycosyl acceptors. 2.5 Solvent participation The stereochemistry of a glycosylation can also be controlled by a participating marked example is the use of acetonitrile which in many cases leads to the formation of an equatorial glycosidic bond.23 Several groups have independently proposed24 that this reaction proceeds via an a-nitrilium ion 59 which is generated under SN1 conditions (Scheme 14).Nucleophilic substitution of the nitrilium ion by an alcohol will lead to P-glycosidic bond formation (60). An important requirement for the reaction is the absence of a participating functionality at C-2. donors (e.g trichloroacetimidates, fluorides, phosphates and pentenyl-, vinyl- and thio- glycosides) feature the ability to form the highly reactive nitrilium intermediates. As can be seen The most It has been shown that different types of glycosyl Scheme 14 from Scheme 15, the highest P-selectivities are obtained with reactive alcohols at low reaction temperatures. Unfortunately, mannosides give poor anomeric selectivities under these conditions. 2.6 Intramolecular aglycon delivery Recently, Stork2' and Hindsgau126 reported independently the preparation of P-mannosides in a highly stereoselective manner by an intramolecular aglycon delivery approach. In this approach, the sugar alcohol (ROH) is first linked via an acetal or silicon tether (Y = CH2 or SiMe2, respectively) to the C-2 position of a mannosyl donor and subsequent activation of the anomeric centre of this adduct 62 forces the aglycon to be delivered from the p-face of the glycosyl donor (Scheme 16).A silicon tether could easily be introduced as is shown in Scheme 17. Compound 66 was first converted to the corresponding chlorodimethyl silyl ether and subsequent reaction with 67 gave the tethered compound 68. Oxidation of the phenylsulfanyl group of 68 yielded phenyl sulfoxide 69 which on activation by the method of Kahne resulted in the selective formation of P-mannoside 70 in a 61% overall yield.The acetal tethered compound 72 could easily be prepared by treatment of equimolar amounts of 71 with 67 in the presence of a catalytic amount of acid (Scheme 18). Reaction of 72 with N-iodosuccinimide (NIS) in dichloromethane resulted in the formation of P-linked disaccharide 70 in 61% overall yield. In this reaction, no a-linked disaccharide could be detected. It is of interest to note that when this reaction was performed in the presence of methanol, no methyl glycoside was obtained. This experiment indicates that the glycosylation proceeds via a concerted reaction and not a free anomeric carbocation. Recently, Ogawa and co-workers showed2' that an intramolecular acetal can also be introduced by treatment of a mixture of a mannoside, having a methoxybenzyl protecting group at C-2 and an alcohol with DDQ.Intramolecular aglycon delivery has also been used for the preparation of 1,2-ci~-glucosides.~~ Furthermore, glycosyl acceptors have also been tethered via the hydroxy groups C-4 and C-6.29 However, in these cases the anomeric ratios in the glycosylations were rather disappointing. 180 Contemporary Organic SynthesisBnO Bno*OyNH BnO CCI3 BnO BnO BnO CCI, BnO BnO, Bn&+ow BnO Bnr%F BnO BnO BnO BnO O - 4 Bnb I B n ~ ~ o y N , , BnO Cct3 Scheme 15 Bnjn* Brio OMe .;+ Brio OMe Bg++ Brio OMe Bn3n* Brio OMe Me,SiOTf MeCN,-2O0C * Me3SiOTf MeCN, -20 'C NBS hner;~, room templ SiF4 MeCN, 0 "C - Me3SiOTf MeCN,-20°C * MesSiOTf MeCN.-2O0C * Me3SiOTf MeCN, -20 OC * .. T h o , BnO B ' o E $ q p:a = 16:l Brio OMe p:a = 24:l p:a = 1.6:l BnO p:a = 3:l p:a = 16:l BnOBnO BnO yoBn BnO -kL, p:a = 6:l BnbF2% Brio OMe B n : a o , BnO Scheme 16 Bnb AMe 61 62 1 64 63 J 65 3 Glycosylation of 3-deoxy-2-ulopyranoso- nates (3-deoxy 2-ketoaldonic acids) The above discussed methods for stereoselective glycosylations are mainly applicable to aldoses with a substituent at C-2. However, there are other types of glycosides, the preparation of which requires special consideration. ketoaldonic acid which frequently terminates oligosaccharide chains of glycoproteins and N-Acetyl-a-neuraminic acid (NeuSAc) is a cyclic I BnO AMe SPh 67 i. C12SiMe2 ii. Then67 66 I \ / BnO BnO BnO x Bnb AMe 68 X = SPh ( 69 X = S(0)Ph Bn;*o, BnO 70 OMe Scheme 17 Boons: Recent developments in chemical oligosaccharide synthesis 181BnO SEt 71 BnO SEt 72 I NIS BnO O - 7 Bnb I OMe BnO- BnO 70 BIIO I OMe Scheme 18 0 HQ II > HO HO -OH C-OH OH Kacetylneuraminic acid 3-deoxyoct-2-ulopyranosonic acid (KDO) HO YH OH 3-deoxynon-2-ulopyranosonic acid W N ) Figure 4 Some important cyclic ketoaldonic acids glycolipids of cell membranes and plays vital roles in their biological activities (Figure 4).Iy2 The use of derivatives of Neu5Ac as glycosyl donors is complicated by the fact that no C-3 functionality is present to direct the stereochemical outcome of the glycosylations. Furthermore, the electron withdrawing carboxylic acid at the anomeric centre makes these derivatives prone to undergo elimination.Finally, the glycosylation of NeuSAc has to be performed at a tertiary oxy carbonium ion. Silver or mercury salt promoted activation of bromides and chlorides of N , 0-acylated neuraminic acid esters gives, particularly with secondary sugar hydroxy groups as acceptors, only modest yields of the desired a-linked coupling product^.^' Recently, thioglycosides of neuraminic acid derivatives have been used as sialyl donors.31 These compounds are 67 PhSeOTf 73 C02Me A C O v O A c I 7 5 - Scheme 19 74 BnO% + ACO, ,OAc OMe readily available, stable under many different chemical conditions but undergo glycosylation in the presence of a thiophilic reagent [N-iodosuccinimide (NIS), dimet hyl( methylt hio)sulfonium trifluoro- methanesulfonate (DMTST) or phenyl selenyl triflate (PhSeOTf)].For example, PhSeOTf mediated coupling of 73 with 67 in acetonitrile gave mainly the a-linked product 74 (a : p = 82: 18) in a 78% yield (Scheme 19). The P-product predominated when the glycosylation was performed in dichloromethane (63%, a : p = 16:84). Apart from the coupling products glycal 75 was also isolated. Recently, effective sialylations have been reported using phosphites'* or xanthate~~~ as the anomeric leaving group. It is important to note that the anomeric phosphite group can be activated by catalytic amounts of promoter. Some indirect glycosylation methods have been described which take advantage of a temporary stereocontrolling functionality at C-3 of N e ~ 5 A c . ~ ~ For example, Ogawa and co-workers employed glycosyl donors such as 79 and it was expected that during glycosylation an intermediate episelenium ion 80 would be formed, nucleophilic substitution of which should lead to a-glycosides (Scheme 20).Essential for this strategy is the steroselective introduction of a C-3p substituent. The synthesis of 79 was started from the readily available 2,3-dehydro derivative 76. Thus, reaction of 76 with phenylselenyl acetate afforded a mixture of 77 and 78 in which the axial adduct predominated over the equatorial one. Treatment of this mixture of compounds with sodium methoxide resulted in epimerisation at C-3 (77 : 78 = 2 : 1). The epimers could easily be separated by silica gel column chromatography to afford pure 77. The undesired epimer 78 could be equilibrated to a mixture of 77 and 78.Treatment of 77 with DAST afforded the glycosyl donor 79. Silver triflate/tin(Ir) chloride mediated glycosylation in carbon tetrachloride with the secondary sugar alcohol 81 gave clean formation 182 Contemporary Organic Synthesis76 + c-- OBn AcHN OBn OBn 77 X=SePh;Y=H NaoMe c 78 X = H; Y = SePh BnO BnO- - AcHN 79 r 1 1 AgOTf, SnCI2 BuBSnH Scheme 20 82 R=SePh c 83 R = H L 81 J 80 of a-linked product 82. Finally, the phenylselenyl group could easily be removed by reduction with tributyl tin hydride to give the desired disaccharide 83. Despite the fact that this method provides a reliable approach for the preparation of a-sialic acid derivatives, it is hampered by the fact that the synthetic sequence is rather laborious. It is important to note that efficient enzymatic methods have been developed for the glycosylation of sialic acid.35 Other important 3-deoxy-2-ulopyranosonic acids are KDO and KDN (Figure 4).The glycosylation of these compounds is hampered by the same difficulties as for NeuSAc, but for these compounds no enzymatic approaches have been described yet. 4 Formation of 2-deoxy glycosidic linkages The macrolides, anthracyclines, cardiac glycosides and aureolic acids are important classes of glycosylated compounds which share the same feature; i.e. they contain 2,6-dideoxy glycosides. The introduction of a 2-deoxy : P-glycosidic linkage requires special considerations since the absence of a functionality at C-2 excludes neighbouring group assisted glycosylation procedures and furthermore enhances the lability of the corresponding glycosyl donors.36 2-Deoxy glycosyl halides have been employed in glycosidic bond synthesis; however, yields and stereochemical outcomes were often rather disappointing.36 The increased stability, ease of preparation and excellent reactivity of 2-deoxy thioalkyl (or thiophenyl) glycosyl donors makes them an ideal choice to be used as glycosyl donors.37 For example, it has been reported that reaction of thioglycoside 84 with 85 in acetonitrile in the presence of NBS gave disaccharide 86 in a good yield with high /?-selectivity (72%, a : P = 1/9) (Scheme 21).In this case, the P-selectivity probably arises from a solvent effect (participation of acetonitrile). On the other hand, Wiesner and co-workers used a participating 3-0-p- methoxybenzoyl protecting group to obtain P-selectivity (Scheme 22).38 Thus, mercury-ion assisted glycosylation of 87 gave high levels of 0 + '\% OMe Me 84 85 NBS-MeCN 0 86 OMe Scheme 21 - OMe H O W 88 ODig OpMBz ~ M B Z O V : ~ * ODig OpMBz OpMBz pMBz = para-methoxybenzyl Scheme 22 P-selectivity and it was assumed that this glycosylation proceeds via intermediate 89.However, Binkley and co-workers suggested that neighbouring group participation of a C-3 acetoxy functionality is not a major determinant of stereochemical outcome of this type of Boons: Recent developments in chemical oligosaccharide synthesis 183glyco~ylation.~~ Thiem and co-workers showed4" that S-( 2-deoxyglycosyl) p hosp horodit hioat es are also relatively stable donors for 2-deoxy-glycosyl preparation.a temporary directional functionality at C-2.41 For example, treatment of glucal 91 with NIS as an electrophile leads to the formation of intermediate 92 which is favoured by an inverse anomeric effect (Scheme 23). Nucleophilic attack by 93 from the opposite side provided the a-glycosidic linked dimer 94. The iodo derivative could be reduced with tributyl tin hydride to give the corresponding 2-deoxy glycoside 95. Recently, it was observed that treatment of various 2-hydroxy sugars with diethylaminosulfur trifluoride (DAST) resulted in a stereoselective 1,2-rnigrati0n.~~ For example, treatment of 96 with DAST gave the glycosyl fluoride 97 as mixture of anomers. Compound 97 could be used as a glycosyl donor and, depending on the solvent, a- and P-glycosides could be prepared (Scheme 24).The thiophenyl group of a coupling product 99 could be Other reliable approaches are based on the use of r 1 92 A&oBn X I Bu3SnH 94 95 X = H 93 Scheme 23 TPDPSO OH TBDMSO Meo%sph A!:* 98 OMe I OMe removed by reduction with Raney Ni to afford a corresponding 2-deoxy glycoside 100. A similar type of migration was observed when a thioglycoside having a phenoxythiocarbonyl ester on C-2 was activated with iodonium ions.43 For example, treatment of a mixture of 101(a or b) and 104 with NISRfOH gave the formation of 1,2-truns P-glucoside 105 (a or b) (Scheme 25). The reaction proceeds probably via the intermediate 102 (a or b) and 103 (a or b) which are formed after iodonium ion activation of the phenoxythiocarbonyl ester of 101 (a or b).It was noted that the glycosylation with an SEt glycosyl donor is more effective then for the SPh donor (85% and 75% respectively). However, it is well known that Raney nickel mediated desulfurisation of the SEt derivative 105a is less facile (50%, 5 d) than of SPh derivative 105b (81%, 2 h). When in this reaction a glucose-type of glycosyl donor is used then a-linked 2-deoxy glyco- sides can be obtained. tionally rigid glycosyl donors which possess a thioether bridge between the C-2 and C-6 position and these compounds have been used for the stereoselective synthesis of 2,6-dideoxy glycosides (Scheme 26).44 Chemoselective activation of the anomeric thiophenyl moiety of 107 with NIS/ TMSOTf and reaction with the hydroxy group of 108 gave mainly formation of the or-linked dimer 109 in high yield (89%).The chemoselectivity of this reaction is based on the greater reactivity of the 2,6-anhydro-2-thio glycosyl donor 107 compared to that of the 2,6-anhydrosulfenyl sugar 108 (for Toshiba and co-workers have designed conforma- s-l 101a R = E t 102a R = E t b R=Ph b R=Ph 1 [2qR 103a R = E t b R=Ph BnO 105a X = SEt OMe Bu3SnH( 106 X = H Bu3snH( 106 X = H 105b X = SPh Scheme 24 184 Contemporary Organic Synthesis Scheme 25OSE TPSO SPh HO SPh SMe + AcHN OAc TPSO 108 107 TPSO & A d I OTPS O p ? SPh 110 1 cyclothtanol NIS-Me3SiOTf 1 TPSO dk RO NW* I OTPS SPh 109 R = cyclohexyl 112 O R Scheme 26 chemoselective glycosylations see paragraph 7). The sulfoxide moiety of 109 was reduced with lithium aluminium hydride to afford 110 which was glycosylated with cyclohexanol to yield 111.Finally, reductive cleavage of the thioethers of 111 gave the 2,6-dideoxy glycoside 112. In an interesting approach to glycosides of 2-deoxy sugars, Giese and co-workers reported45 that treatment of a 2-O-phosphoryl bromide and a sugar alcohol with Bu3SnH under conditions of photochemical initiation gave the corresponding 2-deoxy glycoside. In this reaction, the phosphoryl bromide rearranges to give a 2-deoxy derivative which undergoes glycosylation. The anomeric ratios in this approach were disappointing. 5 Regioselective glycosylations In most glycosylations, the glycosyl acceptor contains only one free hydroxy group. However, when two or more hydroxy groups differ significantly in reactivity then a regioselective glycosylation can be considered.For example, 115 Scheme 27 iodonium ion promoted glycosylation of the thioglycoside 113 derived from neuraminic acid with trio1 114 in acetonitrile gave dimer 115 in a 61% yield, exclusively as the a-anomer (Scheme 27).4h The regioselectivity of this reaction is due to the greater reactivity of the equatorial alcohol compared to the axial hydroxy group. Furthermore, a C-2 hydroxy group has generally a lower nucleophilicity due to the electron withdrawing effect of the anomeric centre. It is of interest to note that the yield of this reaction is significantly higher than when performed on a galactosyl acceptor having only a free C-3 hydroxy group. It should also be realised that the procedures for the preparation of glycosyl acceptors having several free hydroxyls are often easier to conduct and, hence, may offer shorter routes to oligosaccharides.Recently, Garegg and co-workers reported4’ that glycosidation of the tin acetal of unprotected methyl galactoside gave regioselectively 1,6 linked dimers in yields of 44-81% (Scheme 28). For example, the main product obtained when methyl /?a- galactopyranoside 116 was treated first with dibutyltin oxide and then reacted with 2,3,4,6-tetra- 0-benzyl-a-D-glucopyranosyl bromide 117 in the presence of tetrabutylammonium bromide was the a-linked dimer 118 in a 78% yield. Glycosidation of the same stannylene derivative with thioglycosides 119 and 121 in the presence of dimethyl- (methy1thio)sulfonium triflate (DMTST) as the thiophilic promotor gave the 1,6 linked products 120 and 122, respectively.Owing to the participating group at C-2, the products had the expected /?-configuration. No orthoester formation was observed in these reactions. Furthermore, when the reactions were performed in the absence of stannylene activation, no reaction products or mixtures of oligomers were obtained. Ziegler and co-workers have that 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl (TIPS) protected glycosides can be regioselectively glycosylated by glycosyl fluorides using BF3 - OEt, as the promoter (Scheme 29). For example, treatment of 124 with 2,3,4,6-tetra-O-acetyl-a-~-glucopyranosyl Boons: Recent developments in chemical oligosaccharide synthesis 185B n & + , BnO HO 116 Scheme 28 AcO F 123 B n & $ , BnO Brio Br 117 Bu4NBr, CH2CI2 (78%) m Bu2SnO/ I MeOH 119 * DMTST, CH2C12 (69%) I Bz:* BzO I '*' ;El DMTST, CH2C12 (64%) + BFs.OEt2 AcO BzO TBAF ( 125 R = TIPSF Bzo /JMe 126 R = H AcO A& 128 R = TIPSF TBAF 129 R = H Scheme 29 OMe HO OMe HO HO BzoB% BzO OMe HO HO fluoride 123 in the presence a catalytic amount of BF3 * OEt, in dichloromethane resulted exclusively in glycosylation at the C-6 position to give the P-linked disaccharide 125 in a 71% yield.Apart from the dimer, a small amount of the partial hydrolysed monomer was isolated. Thus, in this reaction the BF3 hydrolyses partially the silyl diether as well as activates the anomeric fluoride. The silyl group at C-4 of dimer 125 could easily be removed by treatment with a catalytic amount tetrabutyl- ammonium fluoride to give 126.When the 2,3-TIPS- protected methyl glycoside 127 was treated with 123 in the presence of BF3 - Et,O the corresponding laminaribioside 128 was solely formed. On the other hand, predominantly C-2 glycosylation was observed when methyl 4,6-O-benzylidene-a-~-glucopyranoside was coupled under various conditions with 2,3,4,6-tetra-O-acetyl-a-~-glucopyranosyl bromide. Thus, in the latter example, the TIPS-protecting group completely reverses the regioselectivity. This observation was explained by the fact that the C-2 position of 127 is sterically very congested forcing the glycosylation to take place at the C-3 position. other approach to invert expected regioselectivity (Scheme 30). They showed that mainly the 1,4-linked disaccharide 132 was formed when the cyanoethylidene glycosyl donor 130 was coupled with the di-tritylated glycosyl acceptor 131 in the presence of a catalytic amount of TrC104.The opposite reactivity of primary and secondary trityl ethers in comparison with the corresponding alcohols could be explained by the formation of an earlier transition state upon glycosylation of trityl ethers compared to that of alcohols. Thus, in such a glycosylation the electron density at the oxygen atom and not steric factors determines the regioselectivity. Recently, Kochetkov and coworkers an 186 Contemporary Organic SynthesisScheme 30 6 Convergent block synthesis Oligosaccharides can be prepared by a linear glycosylation strategy or by block synthesis.In a linear glycosylation strategy, monomeric glycosyl donors are added to a growing saccharide chain. Such an approach is less efficient than when oligosaccharide building blocks are used as glycosyl donors and acceptors (convergent approach). Glycosyl bromides have been used in block synthesis; however, results were often rather disappointing especially with labile bromide^.^" Nowadays a variety of glycosyl donors are available which can be prepared under mild conditions, are sufficiently stable to be purified and stored for a considerable period of time, undergo glycosylations under mild conditions, and by selecting the appropriate reaction conditions give high yields and good a : p ratios. These features allow the preparation of oligosaccharides by efficient block syntheses.acetimidate methodology have been exploited in the block synthesis of the prominent tumour associated dimeric antigen Lewis X (Le’).” The retrosynthetic strategy is depicted in Figure 5. In order to make efficient use of common building blocks, it was decided to disconnect the octasaccharide into two trimeric units and a lactoside residue. The trisaccharide was further disconnected into a fucose The favourable properties of the trichloro- and a lactosamine moiety and the latter was readily available from lactose. Thus, the strategy was designed in such a manner that optimal use could be made of the cheaply available disaccharide lactose. In such an approach, the number of glycosylation steps is considerably reduced. The key building blocks for the preparation of the target compound I were 133, 134 and 135.The azido-lactose building block 134 was prepared by azidonitration of lactal, followed by selective protection. The selectively protected lactoside 135 was readily available from lactose via a sophisticated protecting group interconversion strategy. a-Fucosylation of acceptor 134 with the very reactive fucosyl donor 133, under ‘inverted procedure’ condition^,^^ gave trisaccharide 136 in a 89% yield (Scheme 31). The trisaccharide 136 was converted into the required glycosyl donor 137 and acceptor 138. Thus, removal of the TBDMS protecting group of 136 with TBAF and treatment of the resulting lactol with trichloroacetonitrile in the presence of DBU afforded trichloroacetimidate 137 in a good overall yield.On the other hand, cleavage of the isopropylidene moiety of 136 under mild acidic conditions furnished 138. Coupling of glycosyl donor 137 with acceptor 138 in the presence of BF,-Et20 as catalyst gave the hexasaccharide 139 in a 78% yield. In the latter reaction, the higher acceptor reactivity of the equatorial 3-OH group with respect to the axial 4-OH was exploited. The synthesis of octasaccharide 142 required the repetition of the above described strategy, i.e. conversion of the anomeric TBDMS group into a trichloroacetimidate functionality (139441) and coupling of the trichloroacetimidate 141 with lactoside unit 135 (64%). Finally, target molecule I was obtained by reduction of the azido group of 142, followed by acetylation of the amino group and hydrogenation under acidic conditions.Using a similar approach, spacers containing dimeric and trimeric Lewis X antigens have been synthesised.”“ The described glycosylation strategy is highly convergent and makes optimal use of the common trisaccharide 136. Furthermore, efficient use was made of the commercially available dimer lactose. Finally, the trichloroacetimidates could be prepared Galp~-(l4)-GlcNacp~-(l-3)-Galp~-(l-4)-GlcNacp~-(l-3)-Galp~-(l-4)-Glc a Galpp-(14)-GlcNac + lactose t I Fucpa-( 1-3) t Fucpa-( 1-3) t Fucpa-(1-3) J 11 fucose + lactosamine cc5 135 OBn 134 Figure 5 Boons: Recent developments in chemical oligosaccharide synthesis 187BnO 1 42 i. HzS. PY ii. AczO iii. Pd-c, H2, HOAc I I Scheme 31 in high yield and these donors behaved very well in the glycosylation reactions (high yields and anomeric selectivities).The latter point requires some attention. It should be realised that some types of glycosidic linkages can be constructed rather easily whereas others impose great difficulties. In planning a synthetic scheme, the disconnections should be chosen in such a way that the block assembly will not impose problems. Furthermore, difficult glycosylations should be performed in an early stage of the synthesis. Nicoiaou and co-workers have prepared5' a trimeric LeX antigen exploiting the favourable properties of anomeric phenylthio groups (Scheme 32). As in the approach of Schmidt and co-workers, the target molecule is disconnected into a common trisaccharide (151) and a lactoside building block (152).In this case, the trisaccharide intermediate is assembled from monomers, 143, 144 and 147. Furthermore, the building blocks are assembled in a different order. Thus, coupling of 143 with 144 in the presence of silver perchlorate and tin(I1) chloride gave stereospecifically the P-glycoside 145 in a 72% yield. Selective removal of the ally1 protecting group of 145 furnished glycosyl acceptor 146 which was coupled with the fully benzylated fucosyl fluoride 147 to yield trisaccharide 148 in a 87% yield. Thioglycoside 148 was converted into a glycosyl fluoride 149 by treatment with N- bromosuccinimide (NBS) and DAST which was followed by a protecting group exchange to give glycosyl donor 151 in a 84% overall yield. presence of silver triflate (AgOTf) and HfCp2C12 Coupling of 151 with lactoside 152 in the 188 Contemporary Organic SynthesisOAc Brio 2,$? C l C H 2 C O O m ; & SPh ClCH2COO&F + H;h SPh NPhth CICH2C00 OAc NPhth CICH2C00 OPiv 143 i.H2Ru(PPh ii. TsOH. Me8H ( 146 R = H 145 R = All 144 (All = allyl) C l A c O f l ~ ~ & ~ ClAcO OwoR NPhth OR RO AcpO, py ( 151 R = Ac; X = F A OTf, Hf8p2CI2 I-- + AgC104 + SnCI2 @lBn Brio 147 H o @ i h O R OPiv HO OBZ OBZ 152 R = FC,3H27 N3 BzO, t OR thiourea ( 153 R' = OCOCH2CI 154 R'=H 151, AgOTf, HfCp2C12 r I 1 0 PivO OR OPiv CICH2C00 OAc OAc AcO 155 R = 1 AcO 156 R =2 Scheme 32 occurred regioselectively at the more reactive 3-position to give stereoselectively the penta- saccharide 153 (91%). The chloroacetyl groups of 153 were removed by treatment with thiourea and two reiterations of the coupling and deprotection procedure led to the isolation of undecasaccharide 156.Compound 156 was deprotected to yield the desired trimeric LeX antigen. This described strategy is highly convergent and minimises the number of manipulations which have to be executed at the oligosaccharide stage. Attractive features of the strategy are: (i) the stability of thioglycosides under many different chemical conditions, (ii) the ease of activation of thioglycosides by conversion into glycosyl fluorides, (iii) the high efficiency of glycosyl fluorides in glycosidic bond formation and, (iv) the excellent behaviour of thioglycosides as glycosyl acceptors. Apart from these synthetic approaches, several alternative synthetic routes for LeX have been reported.54 Martin-Lomas and co-workers reported55 a highly convergent approach for the preparation of the fully protected tetragalactoside moiety of the GPI anchor of Trypanosome brucei.In this approach the tetra- saccharide 162 was prepared from the common building block 157 (Scheme 33). Thus, the common building block 157 could be converted into the glycosyl acceptor 158 by base-mediated removal of the acetyl protecting group. Treatment of 157 with MCPBA gave the anomeric sulfoxide 159 which was coupled under the conditions described by Kahne to give dimer 160 (78%, a : p = 86 : 14). Dimer 160 was converted into the glycosyl acceptor 161 which was coupled with 159 to give the tetrasaccharide 162. The phenylthio group of 162 could be oxidised to the corresponding anomeric sulfoxide and is ready for further glycosylation.anomeric leaving group has been used. However, for the successful preparation of complex oligo- In the examples discussed above only one type of Boons: Recent developments in chemical oligosaccharide synthesis 189Thus, coupling of armed donor 163 with disarmed acceptor 164, in the presence of the mild activator iodonium di-collidine perchlorate (IDCP), gave the dimer 165 as an anomeric mixture in a yield of 62% (Scheme 34). Next, the disarmed dimer 165 could be further glycosylated with acceptor 166, using the more powerful activating system N-iodo- succinimide-catalytic triflic acid (NIS-TfOH) to yield the trisaccharide 167 (60%). Thus, this chemoselective glycosylation approach allows the preparation of a trisaccharide without a single protecting group manipulation between the glycosylations. The C-2 acyl protecting group of compound 165 performs neighbouring group participation in the glycosylation and therefore only a 1,2-trans linked product will be formed.When a 1,2-cis glycosidic linkage is required, the acyl group has to be replaced by an ether type protecting group, hence introducing additional manipulations at the oligosaccharide stage. It has also been found that cyclic acetals reduce the reactivity of pentenyl glycoside~.~’ This deactivating effect is large enough to allow a chemoselect ive glycosylation of benzylat ed pent enyl glycosyl donor 168 with cyclic acetal protected glycosyl acceptor 169 to give dimer 170 as an anomeric mixture in a modest 52% yield (Scheme 35). It should be noted that compound 170 has a non-participating C-2 benzyl functionality and coupling of 170 with a glycosyl acceptor will allow the introduction of an a-glycosidic linkage.Deactivation by cyclic acetals reflects presumably the torsial strain inflicted upon the developing cyclic oxy carbonium ion, the planarity of which is opposed by the cyclic protecting group. The pentenyl methodology has been applied to the preparation of several complex oligosaccharides.5R kOo& SPh \ OTBDMS 1 57 \ \ OTBDMS OTBDMS 159 158 n ,OAc SPh RO 160 R = TBDMS ( 161 R = H 0 OAc 162 Scheme 33 saccharides, a range of different leaving groups often need to be examined. 7 C hemoselec t ive glycosy la tions BnO BnO 6 0 pent + A;&&+OPent OAc IDCP 164 disarmed I BnO 163 armed (OBn BnO-0, BnO (OBn Bn\O AcO-opent A& An important requirement of convergent oligosaccharide synthesis is ease of accessibility of oligosaccharide building blocks.Fraser-Reid and co-workers have a chemoselective glycosylation (armed-disarmed glycosylation strategy) which allows the preparation of this type of unit with a minimum of protecting group manipulations. They have shown that pentenyl glycosides having a C-2 ether protecting group can be coupled chemoselectively to C-2 benzoylated pentenyl glycosides. The chemoselectivity relies on the fact that an electron withdrawing C-2 ester deactivates (disarms) and an electron donating C-2 ether activates (arms) the anomeric centre. Scheme 34 OAc 165 NIS/TfOH 14- 167 3 190 Contemporary Organic SynthesisOPent OBn OBn 1 70 Scheme 35 Chemoselective glycosylations have also been developed for other types of glycosides.Van Boom and co-workers showeds9 that, similar to pentenyl glycosides, the reactivity of thioglycosides towards iodonium cations can be modulated by the choice of protecting groups and it was found that a C-2 ether group activates and a C-2 ester deactivates the sulfur atom at the anomeric centre. Thus, iodonium cation mediated coupling of 171 with 172 gave disaccharide 173 mainly as the a-anomer in a 84% yield (Scheme 36). In addition, it was established Bn:*sEt BnO + BzOH&sEt BzO BnO BzO 171 172 I IDCP Bn?&?+, BnO BnO BzO&&,SEt BzO 173 BzO Scheme 36 E3n:&&E, BnO OBn 171 174 BnO, that a disarmed thioglycoside (e.g.173) could be readily activated with the strong thiophilic promotor NISEfOH. It was also found that thioglycosides are more reactive than analogous pentenyl glycosides and give often better or-selectivities. In this case, the chemoselective glycosylation approach was rationalised as follows: the electron density on the anomeric sulfur atom in a 2-0-acyl ethyl thioglyco- side is decreased, due to the inductive effect of the electron withdrawing ester functionality at C-2 and as a result, the nucleophilic complexation of the anomeric thio group with iodonium ions decreases and the thioglycoside can be regarded as disarmed with respect to an armed 2-0-alkyl thi~glycoside.~" Ley and co-workers proposedm that the armed- disarmed glycosylation strategy could gain versatility by tuning the glycosyl donor leaving group ability further.They described that a dispiroketal protecting group (R-R) has a marked effect on the reactivity of the anomeric centre and it was found that a dispiroketal protected thioglycoside (eg. 174, Scheme 37) has a reactivity between an armed C-2 alkylated thioglycoside (eg. 171) and a disarmed C-2 acyl thioglycoside (e.g. 176). The three levels of anomeric reactivity were exploited in the prepara- tion of a protected pentasaccharide unit common to the variant surface glycoprotein of Trypanosome brucei. Thus, iodonium dicollidine perchlorate mediated chemoselective glycosylation of glycosyl donor 171 with dispiroketal protected acceptor 174 gave disaccharide 175 in an excellent yield (82%, LY : /? = 5 : 2).Further chemoselective glycosylation of the torsially deactivated donor 175 with electronically deactivated acceptor 176 in the presence of the more powerful activator NIS-TfOH gave trisaccharide 177 in 63% yield as one isomer. Finally, the pseudo-pentasaccharide 179 was obtained by condensation of glycosyl donor 177 with glycosyl acceptor 178. In the armed-disarmed glycosylation approach, the leaving group ability is controlled by protecting IDCP L BnO, 37 0-Ph 1 79 u- SEt 0 177 Scheme 37 Boons: Recent developments in chemical oligosaccharide synthesis 191groups (ether-dispiroketal-ester). It may, however, be advantageous to control the anomeric reactivity by means of modifying the leaving group itself. Boons and co-workers showed61 that the bulkiness of the anomeric thio group has a marked effect on glycosyl reactivity whereby a new range of differentially reactive coupling substrates could be produced (Schemes 38 and 39).glycosylation of glycosyl donor 171 with glycosyl acceptor 180 gave the disaccharide 181 in an excellent yield of 79% as one anomer. Further chemoselective coupling of sterically deactivated donor 181 with the electronically deactivated glycosyl acceptor 172 in the presence of the more powerful promoter system NIS-TfOH gave trisaccharide 182 in a 82% yield. In both coupling reactions, no self-condensed or polymeric products were detected. These experiments show that the reactivity of a C-2 benzylated dicyclohexylmethyl thioglycoside is of an order of magnitude between the reactivities of ethyl thioglycosides having a fully armed ether and disarmed ester protecting group on c-2.The new method to control the anomeric leaving group mobility allowed the generation of glycosyl donors or acceptors with new reactivities. It was envisaged that the sterically and electronically deactivated glycosyl acceptor 184 should have a lower reactivity than the electronically deactivated Thus, IDCP mediated chemoselective B n O a BnO SEt OBn 171 + BnO Bno*8 1 80 I I IDCP t B n O a BnO BnO 181 I .OH 8 172 + BnO ,o B20% BzO SEt OBz 182 Scheme 38 192 Contemporary Organic Synthesis B z O a BzO SEt + BzO&&S BzO OBZ 183 1 a7 OMe Scheme 39 glycosyl donor 183 (Scheme 39). Indeed, coupling of glycosyl donor 183 with glycosyl acceptor 184 in the presence of NIS-TfOH gave dimer 185 in a 61% yield.Glycosyl donor 185 was coupled with 186 in the presence of NIS-TfOH and trisaccharide 187 was isolated in a good yield. The latter reaction demonstrated that a sterically and electronically deactivated substrate is still a suitable glycosyl donor. This glycosylation approach offers the largest number of reactivity levels published to date. Investigations by Danishefsky and co-workers have revealed6* that chemoselective activation is also applicable to glycals and this methodology opened the way for the efficient preparation of 2-deoxy containing oligosaccharides (Scheme 40). Thus, IDCP mediated chemoselective oxidative coupling of ether protected glycal 188 with the partly acylated glycal 189 stereoselectively gave the disaccharide 190 in 58% yield.The dimer 190 could also be activated with IDCP and reaction with glycosyl acceptor 191 yielded trimer 192 (79%). Radical mediated dehalogenation of 192 afforded the 2-deoxy glucoside containing trisaccharide 193 (94%). In another strategy,63 glycals were activated by epoxidation followed by stereoselective condensation with a partly protected glycal. After protection of the 2-hydroxy group, this procedure could be repeated. The chemoselective glycosylations described in this section allow the facile preparation of di-, tri- and tetra-saccharides. These saccharides can be used in a convergent block synthesis of larger oligosaccharides. Often the monomeric units required for the preparation of the building block can be synthesised from a common unit.It should1 IDCP BzO 190 IDcpl-- Ph3SnH AlBN 0 191 x ( 192 R = I x 193 R = H Scheme 40 be noted also that other glycosylation strategies (e.g latent-active@ and orthog~nal~~ glycosylations) have been developed which allow efficient preparation of oligosaccharide building blocks. 8 Polycondensations and one-pot multi-step glycos yla t ions The glycosylations discussed above utilise a fully protected glycosyl donor which is condensed with an appropriately protected glycosyl acceptor to give a well defined product. On the other hand, the use of a glycosyl donor having a reactive alcohol or ether functionality will result in polymerisation.h6 For example, when an 0-trityl ether (Tr) and a cyano- ethylidene group are present in the same molecule ,OTr CN 1 94 Scheme 41 TrCIOd (e.g.194) then under glycosylation conditions polymerisation will take place. For example, treatment of 3,4-di-O-acetyl-l,2-O-cyanoethylidene- 6-O-trityl-a-~-glucopyranoside (194) with a catalytic amount of tritylium perchlorate (10-20 mol%) resulted in polycondensation giving 195. The reaction gave compounds with a molecular weight of approximately 3000-4000 (Scheme 41). It has been found that for some reactions an increase in the degree of polymerisation can be obtained by applying high pressure. It should be mentioned that also other substrates have been used in this type of polymerisation reaction. glycosylations have been reported. Kahne and co- workers described67 a glycosylation method that is based on activation of anomeric sulfoxides with triflic anhydride (Tf20) or triflic acid (TfOH).Mechanistic studies revealed that the rate limiting step in this reaction is triflation of the sulfoxide; therefore the reactivity of the glycosyl donor could be influenced by the substituent in thepara position of the phenyl ring and the following reactivity order was established OMe > H > NO2. The reactivity difference between a p-methoxyphenyl sulfonyl donor and an unsubstituted phenylsulfonyl glycosyl acceptor is large enough to permit selective activation. In addition, silyl ethers are good glycosyl acceptors when catalytic triflic acid is the activating agent but react more slowly than a corresponding alcohol. These features opened the way for a one-pot synthesis of a trisaccharide 200 from a mixture of monosaccharides 196,197 and 198 (Scheme 42).6* Thus, treatment of this mixture with triflic acid resulted in the formation of trisaccharide 200 in a 25% yield.No other trisaccharides were isolated and the only other coupling product was dimer 199. The products of the reaction indicate that the glycosylation takes place in a sequential manner. First, the most reactive p-methoxyphenylsulfenyl glycoside 197 was activated and reacts with alcohol 198 and not with the silyl ether 197. In the second stage of the reaction, the less reactive silyl ether of disaccharide 199 reacts with the less reactive sulfoxide 196 to give trisaccharide 200. The phenylthio group of trisaccharide 200 could be oxidised to a sulfoxide which was used in a subsequent glycosylation.The obtained trisaccharide Recently, more controlled methods for sequential HO AcO A&-:-]" ~ OAc AcO OH AcO OAc 195 (n = 15-20) Boons: Recent developments in chemical oligosaccharide synthesis 193W i P h W i G 0 . e wSph HO OBn 198 197 TMSO OBn O0 196 Slow HSPh - e0 0 wo 2oo Scheme 42 B n O w s E t OBn OBn &HsEt 201 OMe 202 OMe &H OMe 203 NISTTfOH 203 205 Scheme 43 is part of the natural product ciclumycin 0 and despite the relatively low yield of the coupling reactions, this methodology provides a very efficient route for this compound. It has, however, to be proven whether this methodology is applicable to a wide ranger of glycosyl donors and acceptors. Ley and co-workers reported" a facile one-pot two-step synthesis of a trisaccharide unit 205 which is derived from the common polysaccharide antigen of a group B Streptococci.The trisaccharide was assembled from the beiizylated rhamnoside 201 and the cyclohexane-1,Zdiacetal (CDA) protected rhamnosides 202 and 203 (Scheme 43). The preparation of 205 is based on the armed-disarmed glycosylation strategy and exploits the fact that the activated thioglycoside 201 is more reactive than the torsially deactivated CDA protected rhamnoside 202. Thus, NIS-TfOH mediated chemoselective coupling of 201 with 202 gave dimer 204. Next, the second acceptor 203 was added to the reaction mixture and the disaccharide 204 could be activated by the addition of another equivalent of NIS and a catalytic amount of triflic acid to afford the trisaccharide 205 in an excellent overall yield of 62%.It is of interest to note that a stepwise preparation of 205 resulted in a lower overall yield. This glycosylation approach was also employed for the preparation of a tetramannoside. Recently, one-pot multi-step glycosylations have been reported in which glycosyl donors and acceptors having different types of anomeric groups were used.70 The described glycosylation strategies allow the construction of several glycosidic linkages by a one- 194 Contemporary Organic Synthesispot procedure. It should, however, be realised that this type of reaction will give only satisfactory results when all the glycosylations are high yielding and highly diastereoselective. For example, it is generally known that rhamnoside donors often give very high a-selectivities.Furthermore, by exploiting neighbouring group participation it is easy to form 1,2-trans-glycosides. Other type of glycosidic linkages may impose problems. 9 Solid-phase oligosaccharide synthesis Inspired by the success of solid-phase peptide and oligonucleotide syntheses, in the early seventies several research groups attempted to develop methods for solid supported oligosaccharide ~ynthesis.~' However, since no powerful methods for glycosidic bond formation were available, the success of these methods were limited and only simple di- and tri-saccharides could be obtained. solid supported synthesis of a D-galactofuranosyl heptamer. The synthetic approach which was followed is illustrated in Scheme 44. The selectively protected L-homoserine 206 was linked to the Merrifield polymer chloromethyl polystyrene (PS = polystyrene) 207 to give the derivatised polymer 208.The loading capacity of the polymer was 0.5 mmol g-' resin. Acid hydrolysis of the trityl group of 208 gave 209 and coupling of the chloride In 1987, van Boom and co-workers reported7' the 206 207 (PS = polystyrene) 210 with the immobilised 209 under Koenigs-Knorr conditions afforded the polymer linked homoserine glycoside 211. It was observed that the coupling reaction had not gone to completion and to limit the formation of shorter fragments, the unreacted hydroxy groups were capped by treatment with acetic anhydride in the presence of pyridine and N,N-dimethylaminopyridine (DMAP). Elongation of 211 was performed as follows: the levulinoyl (Lev) group of 211 was removed by treatment with a hydrazine-pyridine-acetic acid mixture, the released alcohol was coupled with chloride 210 and the unreacted hydroxy groups were capped by acetylation.After repeating this procedure five times (n = 6), the heptasaccharide 216 was released from the resin by basic hydrolysis. Under these conditions also the benzoyl and pivaloyl (Piv) protecting groups were removed. Finally, cleavage of the benzyloxycarbonyl ( Z ) group by hydrogenolysis over Pd-C gave 217 in an overall yield of 23%. Kahne and co-workers described73 the solid supported synthesis of oligosaccharides using anomeric sulfoxides as donors. In the procedures of van Boom and Kahne, the anomeric centre of a saccharide is linked to the solid support and glycosyl donors are added to the growing chain.Recently, Danishefsky rep~rted'~ an inverse approach in which the incoming sugars are glycosyl acceptors. The basic strategy involves NHZ 208 R = Trit - c 209 R = H P 0 212 n = 2 213 n = 3 214 n = 4 215 n =5 216 n = 6 21 6 I (ii, iii, i) r 1 n times n t I NHZ w r i v OBz I r 1 Reagents : (i) Hs(CN)~, HgBr,, 144; (ii) Ae0, pyridine, DMAP; (iii) NH2NH2, HOAc, pyridine Z = benzyloxycarbonyl; Lev = levulinoyl; Piv = pivaloyl Scheme 44 Boons: Recent developments in chemical oligosaccharide synthesis 195attachment of a glycal to a polymer support, followed by epoxidation to provide a 1,Zanhydro- derivative. This polymer-bound glycosyl donor is then treated with a solution of a protected glycal, acting as a glycosyl acceptor, to give a polymer- bound disaccharide.Reiteration of this reaction sequence provides larger oligosaccharides which ultimately are retrieved from the support (Scheme A commercially available 1 % divinylbenzene- styrene copolymer was employed and the glycal was attached to this resin using a diisopropylsilyl ether linker. Such a linker is stable under the employed reaction conditions but can be cleaved by fluoride ion treatment. In previous studies a diphenyl- dichlorosilane linker was used; however, it was shown that this linker was inferior to the diisopropylsilyl linker.74b Lithiation of the copolymer followed by quench- ing with diisopropyldichlorosilane provided 218. The silylated polymer was reacted with a solution of galactal 219 in dichloromethane and Hunig's base to give the corresponding dialkylsilyl linked polymer construct 220.The loading of the solid support was 0.9 mmol g-' resin. The double bond of the polymer bound glycal 220 was activated by epoxidation with 45). i. BuLmEDA 21 8 1% divinylbenzene- styrene copolymer 0 OH 1 222 i. !A. 3,3-dimethyldioxirane and the epoxide 221, thus obtained, was reacted with a tetrahydrofuran solution of 219 in the presence of ZnC12 to give the polymer bound dimer 222. The glycosidation procedure required a 6-10 fold excess of solution based glycosyl acceptor and 2-3 equivalents of promoter. However, in some reactions less acceptor and shorter reaction times have been applied. It should also be noted that no glycosylation at the 2-position was observed.Twice repetition of this two-step procedure (epoxidation, glycosylation) provided a polymer bound tetrasaccharide 225 which was released from the solid support by treatment with tetrabutyl- ammonium fluoride (TBAF). The method allowed the preparation of the tetrasaccharide 226 in a 74% overall yield. An advantageous aspect of this solid supported approach is that no capping step is required because any unreacted epoxide will hydrolyse in the washing procedure. On the other hand, in the case of a very difficult glycosylation step, most of the solid supported linked glycosyl donor may decompose lowering the overall yield. In the procedures of van Boom and Kahne, excess of donor can be used to achieve acceptable yields in difficult glycosylation reactions. OSiPr',Ph(S) 0 21 9 220 t 21 9 ZnCh 4 C - ii. ZnC12.THF 224 221 '0 0 OH 1 BnO 225 R = SiPr12S 226 R = H Scheme 45 196 Contemporary Organic SynthesisPhthN ~0% BzO OMe OMe 230 OSuPEG 232 Scheme 46 The rate of reactions on a solid support are generally reduced compared to solution based methods. Krepinsky and co-workers addressed this problem by a polymer-supported solution synthesis of oligosaccharides (Scheme 46).75 This strategy is based on the fact that a polyethylene glycol polymer supported saccharide is soluble under conditions of glycosylation but insoluble during work-up. Poly(ethy1ene glycol) monomethyl ether (PEG) was coupled through a succinic (Su) ester linkage to a carbohydrate hydroxy group. When PEG is bound to a carbohydrate, a glycosylation reaction can be driven to completion by repeated addition of the glycosylating agent.For example, in the silver ion mediated coupling of 230 with 231 to give 232, several portions of the bromide were added until the reaction had gone to completion. The progress of the glycosylation could be monitored by NMR spectroscopy. After the reaction had finished, the PEG-bound product was precipitated by the addition of diethyl ether. Subsequently, the crude polymer was recrystallised from ethanol and after drying was used in the next synthetic step. The PEG-succinimide linkage could be cleaved by DBU- catalysed methanolysis in dichloromethane. Recently, PEG was bound to the anomeric centre of a saccharide via an a, a’-dioxyxylyl gly~oside.~~ This linkage is stable under many chemical conditions including glycosylation but can be cleaved by hydrogenolysis.The PEG-based methodology has been used for the preparation of a heptaglucoside having phyto-alexin elicitor and other oligosa~charides.~~~-~ eliminate time-consuming work-up procedures and purification steps. However, despite recent advances, only relatively simple oligosaccharides have been prepared by these methods and the glycosidic linkages of these oligosaccharide were 1,2-trans linked. The polymer support based glycosylation methods 10 Concluding remarks Nowadays a range of glycosyl donors, such as trichloroacetimidates, thioglycosides and fluorides, can be prepared by standard procedures and these compounds have a reasonable shelf-live and can be used confidently in glycosylations.Several methods have been developed to control the anomeric selectivity of glycosylation reactions. These approaches are in many cases unreliable and often modest CI : p selectivities are obtained. It should be realised that often many reaction conditions need to be examined to achieve acceptable results. Nevertheless, contemporary carbohydrate chemistry makes it now possible to execute multistep synthetic sequences that give complex oligosaccharides. Convergent strategies have been developed which make efficient use of common building blocks. In this respect, chemoselective glycosylations allow the preparation of saccharide building blocks without extensive protecting group manipulations. Several polymer support based glycosylation methods have been reported.However, only relatively simple oligosaccharides have been prepared by these methods and the glycosidic linkages of these oligosaccharide were often 1,Ztrans linked. Problems associated with basic glycosylation methodology need first to be addressed before solid oligosaccharide synthesis can be introduced as a general method. An attractive alternative for solid-phase oligosaccharide synthesis may be one-pot multi-step glycosylations. This type of methodology allows each glycosylation step to be monitored but reduces time-consuming work-up and purification steps. In order to overcome problems associated with chemically based methods, combined enzymatic approaches have been developed. 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