OEt O O R OEt O R PPh3Br + – 62–90% (7 examples) 1 i Scheme 1 Reagents i BuLi Et 2 O 0 °C 2 Synthetic methods Part (iv) Heteroatom methods By PATRICK J. MURPHY Department of Chemistry University of Wales Bangor Gwynedd UK LL57 2UW 1 Introduction This report focuses on the organic chemistry of phosphorus arsenic sulfur silicon selenium and tellurium. The heterocyclic free radical and transition metal chemistry of these areas has been largely ignored as this will be covered in other reports. 2 Organophosphorus and organoarsenic chemistry In this the centenary year1 of the birth of Georg Wittig the reaction that bears his name is still of considerable interest to synthetic chemists and continues to find application in many synthetic procedures. Indeed a review by Nicolaou et al. on the applications of the Wittig and related reactions in natural product synthesis has appeared highlighting many landmark achievements of this reaction.2 Several applications of tandem processes involving Wittig reactions have appeared for example the use of a tandem Michael–Wittig reaction for the formation of substituted cyclohexadienes 1 which are potential A-ring precursors for taxol3 (Scheme 1).The conjugate addition of crotonate arsonium ylide to a,b-unsaturated carbonyl compounds has also been reported.4 In general the reactivity of arsonium ylides was found to be higher than in the corresponding phosphorus ylide which also forms the tandem product however considerable quantities of direct Wittig condensation byproducts were also isolated. The first example of cyclopropane formation by ylides of this type was also reported in the same paper.The same worker also reported the synthesis of highly functionalised cyclohex-2-enonedicarboxylates from a novel Michael–Wittig reaction of methyl 3-oxo-4-(triphenylarsoranylidene)butanoate 2 and substituted 2H-pyran-5-carboxylates.5 Reaction of two equivalents of 2 with al- 51 O COSEt Ph3As CO2R O O COSEt CO2Me HO HO CO2R R¢ CO2R + R = Me 2 i 48% Scheme 2 Reagents i 0.5 equiv. R@CHO; R@\aryl alkyl R\Pr* Et Me P Ph Ph R OEt O P+ O– EtO Ph Ph R EtO R PPh2 O 3 5 examples 37–73% R = alkyl aryl 4 Scheme 3 Reagents i reflux 16 h dehydes has been shown to give cyclohepta-1,2,5-trienes in moderate yields (6 examples 8–53%) (Scheme 2).6 The reaction of the cyclic phosphonium ylide 3 with a,b-unsaturated esters leads to the formation of the seven-membered cyclic enol ether derivatives 4 via a tandem Michael–intramolecular Wittig reaction with the ester group (Scheme 3).7 A tandem Michael–HWE (Horner–Wittig–Emmons) procedure for the one-pot synthesis of d-substituted a,b-unsaturated esters has been reported in which the HWE reagent is deprotonated in situ by the enolate formed from a conjugate addition of a cuprate or organolithium species to an enone; yields of 15–92% were quoted (23 examples) with E:Z ratios of 1 1\4 1 being typical.8 In a series of papers9–11 Akiba has reported the use of the pentacoordinate spirophosphorane unit 5 in Wittig style reactions; for example reaction of the phosphorane (R\CH 2 CO 2 Et) under Wittig conditions with a range of aldehydes gave excellent Z-selective olefin formation (7 examples 73–83% yield[96 4 Z:E) (Scheme 4).9 High Z-selectivity has been observed in the olefination of base-sensitive chiral b-hydroxy-a-amino aldehydes using the HWE reaction; this was achieved under mild conditions using bis(trifluoroethyl)phosphonates and LiCl–DBU in THF.12 Nishizawa et al.have reported an indirect Wittig reaction which proceeds via the low temperature isolation of the intermediate 1,2-hydroxyphosphonium salts; elimination of water from these to give the required alkene was achieved by treatment with DBU. The reaction displayed identical stereoselectivity to the corresponding direct 52 P. J. Murphy R1 CO2Et P R O O F3C CF3 CF3 F3C i 5 Scheme 4 Reagents i R1CHO KOBut 3h N Ph2(O)P CHO H 7 (racemic) (RO)2P O O O 6 N CO2R* Ph2(O)P H N CO2R* Ph2(O)P H N Ph2(O)P H N Ph2(O)P H CO2R* CO2R* + Scheme 5 Wittig reaction for aliphatic aldehydes and unstabilised ylides.13 Microwave heating has been shown to accelerate the rate of the Wittig reaction of stabilised phosphoranes with ketones under solvent-free conditions; reasonable yields (36–85%) were obtained with a range of ketones over reaction times of 15–20 min however E:Z ratios were generally poor.14 Nagao has reported a Sn(OSO 2 CF 3 ) 2 and N-ethylpiperidine catalysed variant of the HWE reaction that is thought to proceed via a tin enolate species; high levels of Z or E selectivity have been observed using aryl ketones or aldehydes respectively.These observations are explained using a tin-chelated 6-membered transition state for the addition of the carbonyl species to the enolate.15 Phosphonates 6 have been used in the kinetic resolution of the diphenylphosphinylprotected a-amino aldehyde 7 and it was found that by using di§erent bases and phosphonate ester groups any of the four possible diastereomeric products can be obtained.16 Geometric selectivities from 66 34 to 98 2 and diastereomeric ratios between 93 7 and 99 1 were reported (Scheme 5).A similar kinetic resolution has been employed in the synthesis of the C1–C11 subunit of the macrocyclic marine meta- 53 Synthetic methods Part (iv) Heteroatom methods But O + EtO2P CN O 9 RNH Me Ph OLi i 8 77–96% But CN 17–52% e.e. Scheme 6 Reagents i PhMe [78 °C 3 h; R\H Me Pr* CH 2 But CH 2 CHPh 2 CH 2 -(1-adamantyl) O Ph3As O Ph 10 O R1 11 R1 R O O Ph + i Scheme 7 Reagents i [78 °C THF; R1\Ph Me Et But bolites the iejimalides.17 Chiral lithium 2-amino alkoxides 8 were applied as chiral bases for the enantioselective HWE reaction between the achiral phosphonates 9 and 4-tert-butylcyclohexanone; ees of up to 52% were obtained.18 It was demonstrated that the formation of the lithium aldolate intermediate is reversible and it is not this step that is responsible for the asymmetric induction (Scheme 6).An asymmetric Wittig-type olefination of 4-substituted cyclohexanones 11 with the 8-phenylmenthol-derived chiral arsonium ylide 10 gave the alkene product in 58–69% yield and 47–80% de which is an improvement on results reported previously for the corresponding chiral phosphonate (Scheme 7).19 An alternative to traditional Wittig procedures has been reported by Ledford and Carreira who used a combination of N 2 CHCO 2 Et a catalytic (1%) amount of ReOCl 3 (PPh 3 ) 2 and (EtO) 3 P in the olefination of a range of aldehydes.Yields of 70–95% (11 examples) and E/Z selectivities of 3 1 to 20 1 were observed.20 The olefination of dialkyl squarates by Wittig and HWE reactions has also been reported; limited and generally E-stereoselectivity was observed with stabilised phosphoranes whereas high levels of Z-selectivity (2 1 to 19 1) were observed under HWE conditions. 21 The formation of N-styrylformamides by the reaction of N,N-diformylamines with arylmethylenephosphoranes under mild conditions has been detailed.22 The transformation of one or both of the methoxycarbonyl groups of a substituted porphyrin into isopropenyl groups was e§ected by treatment with excess methylenetriphenylphosphorane.23 In an elegant new approach to the synthesis of aristolactams 24 Couture and co-workers have e§ected the conversion of the phosphine oxide 12 into the enamine product 13 via an intermediate benzyne intermediate (Scheme 8). Warren and co-workers have investigated the configurational stability of lithiated phosphine oxides by studying their addition to phenylalanine-derived aldehydes (the Ho§mann test); they concluded that lithiated diphenylphosphine oxides are not con- figurationally stable in THF at [78 °C.25 Warren has also reported a range of 54 P. J. Murphy R1 R2 N Br P O R3 O Ph Ph 12 R1 R2 N P O R3 O Ph Ph R1 R2 N O R3 K P O Ph Ph K R1 R2 N O R3 I 13 ii i 71–81% R1 H H CH2—O—CH2 OMe R2 H H OMe R3 Me Bn 4-MeOC6H4CH2 4-MeOC6H4CH2 Scheme 8 Reagents i KHDMS,[78 °C to[30 °C THF; ii o-iodobenzaldehyde Ph2P R O OH 14 Ph2P R O OH Me anti syn 2:1–4:1 i Ph2P R1 O OP 15 Ph2P R1 O OP SiPhMe2 anti syn 70:30–95:5 ii Scheme 9 Reagents i methylcuprates; ii (PhMe 2 Si) 2 CuLi 2 diastereoselective reactions of optically active c-substituted vinyl phosphine oxides;26 addition of dimethylcuprate to substrates 14 (R\Me Bun Ph) proceeds with 2 1 to 4 1 anti syn stereoselectivity whilst the addition of (TBDMS) 2 CuLi 2 to substrates 15 (R\Bun Ph P\TBS MOM) proceeds with 70 30 to 95 05 anti syn stereoselectivity (Scheme 9).The factors governing the e¶ciency of the asymmetric dihydroxylation of allylic phosphine oxides under Sharpless conditions have also been investigated by Warren; the use of the monomeric DHQD-CLB ligand was found to give the best results (42–68%ee).27 The direct conversion of easily prepared and air-stable phosphine–borane complexes directly into phosphonium salts (some chiral) by reaction with an alkyl (or aryl) halide in the presence of an alkene has been reported; the reaction proceeds in reasonable yields (50–92% 11 examples) and sometimes requires the use of high pressure.28 An interesting route to vinyl phosphonium iodides 16 by treatment of a,b-epoxysilanes sequentially with LiPPh 2 and methyl iodide has also been reported (Scheme 10).29 55 Synthetic methods Part (iv) Heteroatom methods O R SiPhR1R2 PPh2MeI– R + i ii 16 Scheme 10 Reagents i LiPPh 2 ; ii MeI; R\Bu Ph; R1\Me But; R2\Me Ph OM P RO RO O S p-Tol N Ar H •• + OM P RO RO N S Tol- p H Ar O •• i 17 Scheme 11 Reagents i THF,[78 °C; R\Et Pr*; M\Li Na p-Tol S N Ph O H 18 i p-Tol S N O Ph P(OMe)2 O H Scheme 12 Reagents i (MeO) 2 P(O)CH 2 Li Several addition reactions to sulfinimines have been reported for example the asymmetric synthesis of a-amino phosphoric acids via the addition of phosphites to enantiopure sulfinimine 17 has been reported to give the addition products in de of 84–97% (Scheme 11).30 Similarly addition of the a-phosphonate carbanions to (S)- sulfinimines 18 gives N-sulfinyl-b-aminophosphonates with diastereoselectivity of up to 10 1 (Scheme 12).31 In a similar but enantiodivergent approach Mikolajczyk has reported that the addition of either dialkyl phosphite or diamido phosphite anions to 18 leads to either the R- 19 or S-isomer 20 at the carbon centre predominating; hydrolysis of these adducts o§ers a new convenient synthesis of a-aminophosphonic acids (Scheme 13).32 The addition of diethylaluminum cyanide and the lithium enolate of methyl a-bromoacetate to enantiopure sulfinimines leads to the formation of a-amino nitriles and N-sulfinylaziridines respectively.33 A TMSOTf-promoted 1,4- addition of silyl phosphites prepared in situ from dialkyl phosphites and N,Obis( trimethylsilyl)acetamide to cyclic enones leading to b-keto phosphonates in 20–98% has also been reported.34 Several organophosphorus-related reviews have appeared in 1997 including a volume of the Journal of Organometallic Chemistry devoted to current trends in organophosphorus chemistry.35 An account detailing several synthetic strategies for the preparation of chiral hydroxy phosphine ligands,36 and reviews on the synthetic methods available for the synthesis of non-racemic phosphonates37 and dialkyl a- halogenated methylphosphonates38 have also appeared.3 Organosulfur chemistry A comprehensive review on the preparation and reactions of chiral sulfonium ylides and related species has appeared which includes details of their use in asymmetric 56 P. J. Murphy p-Tol S NH C P(OMe)2 O H Ph O p-Tol S NH C P(NEt2)2 O H Ph O p-Tol S N Ph O H i ii 18 19 20 94:6 90:10 Scheme 13 Reagents i (MeO) 2 POLi; ii (Et 2 N) 2 POLi Me 2-Py +S p-Tol –O Me 2-Py +S p-Tol –O 2-Py S ButMe2SiO Me p-Tol OSiMe2But OMe ( S) ( S) ( R) ( S) 21 22 or 23 24 Scheme 14 epoxidation cyclopropanation aziridination olefination and rearrangement.39 The Pummerer reaction has been the topic of several reports indeed a detailed review highlighting the applications of the Pummerer reaction to the preparation of complex carbocyclic and heterocyclic ring systems has been published by Padwa et al.40 Kita et al.have reported an enantioselective Pummerer-type rearrangement of enantiopure a-substituted sulfoxides with O-silylated ketene acetals. For example diastereomeric sulfoxides 21 or 22 both undergo rearrangement to sulfide 23 in [99%ee on treatment with the O-silyl ketene acetal 24 (Scheme 14).41,42 Similarly Shibata has reported that Pummerer-type reactions induced by ethoxyvinyl esters are an alternative to the generally used acid anhydrides and that high levels of asymmetric induction are possible with ees as high as 84%.43 The chemistry of sulfoxides continues to be of interest for example chiral b-hydroxy sulfoxides (e.g.25) have been shown to be excellent proton sources for the enantiofacial protonation of prochiral lithium enolates with ees as high as 97% being observed for the reaction with lithium enolates of cyclohexanone derivatives.44 This methodology was used for the protonation of enolate 26 in 82%ee yielding ketone 27 a synthetic precursor of ([)-epibatidine (Scheme 15).45 c-Hydroxy sulfoxides have been prepared by a highly stereoselective sulfoxide-directed reduction of c-keto sulfoxides 28 using 57 Synthetic methods Part (iv) Heteroatom methods O O OLi N Cl O O O N Cl S O F3C OH 25 i 2.5 26 27 Scheme 15 Reagents i [90 °C to [60 °C R¢ S O p-Tol R¢ S O p-Tol R¢ S O p-Tol O OTBS OTBS OH OTBS OH 28 29 30 Syn-6 yield > 90% de > 90% Anti-6 yield > 90% de > 93% i ii Scheme 16 Reagents i DIBAL THF [78 °C; ii DIBAL ZnI 2 THF [78 °C; R\Me Ph allyl vinyl DIBAL or DIBAL–ZnI 2 leading to the syn- 29 or anti-products 30 respectively (Scheme 16).46 A short stereocontrolled synthesis of 31 an unusual amino acid component of ustiloxins A and B has been achieved; the key step being the RBINOL –Ti(OPr*) 4 –ButOOH mediated oxidation of sulfide 32 with [50 1 6Sstereoselectivity.The use of S-BINOL gave the 6R-sulfoxide with 16 1 stereoselectivity (Scheme 17).47 TheN-sulfinylsultam 33 has been utilised as a new sulfinyl transfer agent; reaction of this conveniently prepared reagent with Grignard reagents led to the formation of the corresponding sulfoxide 34 in 91–98% yield and 99–99%ee.48 In addition reaction with LiHMDS led to the formation of the intermediate 35 which can be converted into the corresponding sulfinimine ([98%ee) via a modification of the Davis procedure (Scheme 18).Davis et al. have reported full details of this method for the asymmetric synthesis of sulfinimines together with two other less e¶cient methods for their synthesis by either the asymmetric oxidation of sulfenimines (Ar–SN––CR 2 ) with chiral oxaziridines or by the reaction of metal aldimines (RCH––NM) prepared from nitriles with (R)- or (S)-menthyl toluene-p-sulfinate were also reported.49 The asymmetric sulfimidation of sulfides to sulfimides (R 2 S––NTs) using TsN–– IPh in the presence of a catalytic amount of CuOTf and a chiral bis-oxazoline ligand has been reported to proceed with a wide range of sulfides in yields of 50–83% with ees of up to 71%.58 P. J. Murphy O O CO2H S Ph O OH H2N O BocHN BocHN O S Ph O i steps 32 31 SPh 75% Scheme 17 Reagents i But OOH Ti(OPr*) 4 (R)-BINOL S R O S N SiMe3 SiMe3 O S N R¢ O S N SO2 33 34 35 O iii 65–84% i ii •• •• •• •• Scheme 18 Reagents i RM THF [78 °C 1 h; ii LiHMDS; ii R@CHO; R\alkyl allyl vinyl aryl thienyl furyl pentynyl; M\MgBr ZnBr; R@\allyl aryl a,b- unsaturated Sulfonamides are produced via a [2,3]-sigmatropic rearrangement when the reaction is applied to allylic sulfides; ees of up to 58% are obtained.50 The reaction of lithiated chiral non-racemic methyl p-tolyl sulfoxide with imidoyl chlorides [RC(––NR)Cl] has been reported as a general synthetic method for the synthesis of N-substituted fluorinated b-imino sulfoxides.51 The first example of a Claisen rearrangement stereocontrolled by a sulfinyl group has been reported; the thermal rearrangement of ketene dithioacetals 36 occurs over 5–45 hours at room temp.giving product 37 in 40–65% yield as 93 7 to 99 1 mixtures of diastereoisomers (Scheme 19).52 The ability of sulfur-containing functional groups to mediate carbanion formation continues to generate new synthetic methodology. The 1,4-dithiin 38 is easily deprotonated leading to the lithiated species 39 which can be reacted with a range of electrophiles including alkyl halides epoxides and aldehydes. Desulfurisation of the products obtained leads to the formation of substituted cis-allylic alcohols e§ectively illustrating the use of 38 as an allylic alcohol anion equivalent (Scheme 20).53 Simpkins has reported the enantioselective rearrangement of three-membered ring sulfoxides (episulfoxides) into alkenyl sulfoxides using chiral lithium amide bases.For example the episulfoxide 40 was converted (over two steps) to the sulfone 41 in 59 Synthetic methods Part (iv) Heteroatom methods S SMe R2 R1 S O 36 SMe S S R1 O R2 37 [3.3] CH2Cl2 ca 20 °C •• Scheme 19 R1\Me But Pr* c-C 6 H 11 ; R2\Me H S S H MPMO S S MPMO – Li+ 39 S S OH OBn MPMO MPMO OBn OH 70% iii HO – 38 º i ii 88% O OBn Scheme 20 Reagents i BuLi THF; ii Ti(OPr*) 4 ; iii Raney Ni N N Ph Ph Ph Ph H H OBn S+ O– i 65% OBn H SOMe OBn H SO2 Me 4 R ii 87% 40 41 Li Li 42 4 R Scheme 21 Reagents i THF,[78 °C MeI; ii Oxone' MeOH H 2 O 85–88%ee using the base 42 (Scheme 21).54 Full details of the synthesis anionic substitution conversion to alkenes and ring-opening rearrangement of cyclic three membered sulfones (episulfones) have also been described by the same group55 as well as the conjugate addition reaction of a metallated 2-methoxypyridine to an azabicyclic alkenyl sulfone as the key step in the synthesis of racemic epibatidine.56 Magnus has reported that only two out of a possible four diastereomers are formed from the reaction of the dilithiated sulfone 43 with benzaldehyde.It is felt that the aggregate 43 (n\1–4) is responsible for this long range asymmetric induction the highest level being observed when n\2 where the erythro threo ratio was 2.75 1 (Scheme 22).57 A novel tandem conjugate addition–Ramberg–Ba� cklund rearrangement process has been reported by Taylor and Evans.58 This involved the conjugate 60 P.J. Murphy PhO2S NH N Me Me O Ph H ( ) n ( n = 1,2,3,4) i 43 Me N Li PhO2S N OLi Me Ph ( ) n 44 ii PhO2S NH N Me Me O Ph H ( )n Ph OH H H Scheme 22 Reagents i BuLi THF,[70 °C; ii PhCHO,[100 °C S O2 Br 45 SO2 Ph Br BnS SO2 Ph Bn BnS H – – i BnS Ph 46 77% E Z 93:7 Scheme 23 Reagents i BnSH KOBut But OOH DCM 15 h rt addition of a range of oxygen nitrogen sulfur and carbon nucleophiles to the brominated vinyl sulfone 45 which after proton exchange rearranges and eliminates under the conditions of the reaction to give the Ramberg–Ba� cklund product for example the allyl sulfide 46 (Scheme 23). The Jacobsen epoxidation of dienyl sulfones has been investigated and it has been observed that symchiral (salen)Mn(III)CI complexes catalyse the epoxidation of 2- sulfonyl-cyclic-1,3-dienes with high enantioselectivity (68–99%) and that the incorporation of the sulfone moiety increases the enantioselectivity by up to 30% when compared with the unsubstituted cyclic-1,3-diene.59 The preparation and reactions of trithiocarbonate oxides (sulfines) has been reported.60 These interesting intermediates undergo addition of alkyllithium species to form trithioorthoesters which are unstable and undergo rearrangement to give a disulfide and a thioester as illustrated in the intramolecular example leading to 47. The intermediate lithiated trithioorthoesters can be used in conjugate addition reactions to enones and enals to give 48 which rearrange over a few hours to the ketene thioacetals 49 which are hydrolysed to give thioesters 50 in reasonable overall yield (37–55%) (Scheme 24).Several organosulfur related reviews have appeared in 1997 including a Tetrahedron 61 Synthetic methods Part (iv) Heteroatom methods S S S O S S S O Me i ii S S O SMe Me S S S H O 47 R1S SR1 S O R1S SR1 S O Me – Li+ i R2 R3 O R1S S O Me R3 O R2 R1S 48 R1S R3 O R2 R1S 49 O R3 O R2 R1S 50 then ii –MeSOH ii Scheme 24 Reagents i MeLi THF [78 °C; ii H 2 O; R1\Me Et But Bn; R2\Me Et; R3\H Me specialist periodical report on ‘Recent aspects of S Se and Te chemistry’.61 Others include those on synthetic applications of N-sulfonyl imines,62 xanthates,63 chiral acetylenic sulfoxides,64 and on the chemistry of thioacylsilanes;65 the preparation and asymmetric reactions of chiral sulfinyl-1,3-dienes has also been reviewed.66 4 Organoselenium and organotellurium chemistry There has been continued interest in the asymmetric oxyselenylation of alkenes indeed the area has been reviewed67 and several new reagents have been reported during 1997.A range of chiral ferrocenylselenium reagents were applied to asymmetric methoxyselenylation of alkenes and the amine-derived reagent 51 was found to be the most e§ective with ees in the range of 15–96% being reported.68 This reagent was also applied in the selenation of silyl enol ethers with modest success. A similar series of optically active selenium reagents having a pendant chiral tertiary amino group have been reported.69 Reagent 52 was found to give the best asymmetric induction in the methoxyselenylation of (E)-phenylpropene where a de of 97% was obtained; the presence of a strong Se–N interaction was inferred to explain the results.The C 2 - symmetric reagent 53 has also been reported to give excellent diastereoselectivities sometimes as high as 98% in the selenoalkoxylation reactions of alkenes (Scheme 25).70 In a new twist to selenoxide eliminations it has been reported that antibodies have been elicited that catalyse the selenoxide elimination of several racemic benzylic selenoxides at a k#!5/k6/#!5 of up to 2200; chiral discrimination between individual 62 P. J. Murphy NMe2 Me H SeOTf Fe N O O O O Ph SePF6 Ph 51 52 53 O O SeOTf Scheme 25 OAc C3H7 i Se Ar OH O ii O C3H7 54 Scheme 26 Reagents i MeLi ZnBr Et 2 O 0 °C to RT; ii,[100 °C enantiomers of the substrate selenoxide was also observed.71 The enantiofacial protonation of cyclic enol acetates with chiral c-hydroxyselenoxides 54 (and analogous sulfoxides) in the presence of MeLi and zinc bromide has also been described; ees as high as 88% have been reported when Ar\4-MeOC 6 H 4 (Scheme 26).72 The application of the Sharpless AD-mix oxidation to allyl selenides has been reported; chemoselectivity for dihydroxylation of the double bond was only observed when o-nitrophenylallylselenides were employed and selectivity of 93 7 was observed for both a-ADmix and b-ADmix.73 The synthesis characterisation and stability of selenothioic acid S-esters 55 has been reported; a general method involving the treatment of a terminal alkyne with BuLi and selenium followed by addition of an alkyl thiol results in their formation in 23–83% yield (Scheme 27).74 Treatment of selenocyanates (RSeCN) with either one molar equivalent of LiH NaH or lithium triethylborohydride or 0.25 equivalents of lithium or sodium borohydrides led to the formation of diselenides in good yields (52–96% 14 examples); the reaction proceeds via the intermediate selenol or selenolate.75,76 Sodium phenylseleno(triethyl)borate complex M[NaPhSeB(OEt) 3 ] prepared by reduction of (PhSe) 2 with NaBH 4 in EtOHN and benzeneselenol (PhSeH) generated in situ from this complex by addition of acetic acid have been demonstrated to serve as excellent reducing agents for a,b-epoxy ketones and esters yielding b-hydroxy carbonyl compounds in good to excellent yields.77 An e¶cient stereocontrolled strategy for the cyclopropanation of a,b-unsaturated ketones with semi-stabilized telluronium ylides has been reported which a§ords cis-2- vinyl-trans-3-substituted cyclopropyl ketones 56 with hitereoselectivity (generally [95 5) and in good yield.Conversely the same enones gave trans-2-vinyl-trans-3- substituted cyclopropyl ketones 57 when the corresponding arsonium ylides were employed (Scheme 28).78 63 Synthetic methods Part (iv) Heteroatom methods RC CH i-iii R SR¢ Se 55 Scheme 27 Reagents i BunLi Et 2 O; ii Se; iii R@SH; R\Me 3 Si Ph 3 Si Me n-C 4 H 9 Ph; R@\allyl aryl Bui 2Te+ R1 Br– R R2 O ii i COR2 H R R1 H H > 95:5 Ph3As+ R1 R R2 O ii iii COR2 H R H R1 H > 87:13 56 57 Scheme 28 Reagents i KHMDS THF; ii chalcone; iii LiBr KOBut or i; R\Ph aryl; R1\Ph vinyl CH––CHTMS CH––CHMe CH–– CHPh; R2\Ph But The preparation reactivity and synthetic applications of vinylic selenides and tellurides has been reviewed covering the literature from 1983 to 199679 as has the synthesis and asymmetric applications of optically active selenium and tellurium compounds.80 5 Organosilicon chemistry Fleming Barbero and Walter have co-authored a review entitled ‘Stereochemical control in organic synthesis using silicon-containing compounds’.81 This significant piece of work covers all aspects of stereocontrol involving silicon and will no doubt become an essential reference work for all researchers interested in the organic chemistry of silicon.Other reviews of interest include one focusing on the temporary silicon-connection tethering strategy in synthesis particularly applications involving radical cyclisations cycloadditions nucleophilic delivery and hydrosilation.82 Per- fluoroalkylation with organosilicon reagents83 and the application of fluorotitanium compounds to the addition of allylsilanes to aldehydes84 have also been reviewed.The chemistry of allylsilanes continues to be of considerable interest. Some new computational and experimental evidence for the mechanism of the Sakurai–Hosami reaction the BF 3 catalysed addition of allyltrimethylsilane to an aldehyde has suggested that the reaction proceeds through an eight-membered cyclic transition state deriving from C–C and Si–F bond formation and synclinal (gauche) disposition of the reacting double bonds.85 Several examples of the Sakurai–Hosami reaction in which allylsilanes with chiral non-racemic silyl substituents are utilised have been reported. Barrett et al. have reported the enantioselective synthesis of homoallylic alcohols using (E)-but-2-enyl- 64 P.J. Murphy SiCl3 + ArCHO + N N O Ar OH Me i 58 Scheme 29 Reagents i DCM,[78 °C 4 h N P N N O H O O Si O O OBn O O Si R¢ CO2Pri CO2Pri 59 60 61 Scheme 30 R@\Me Ph Pr* Cl OBut trichlorosilane in the presence of chiral pyridinyloxazolines 58; excellent anti-diastereoselectivity ([99%) and good enantioselectivity (36–74%) were observed (Scheme 29).86 A similar asymmetric allylation or crotylation of aromatic aldehydes catalyzed by chiral phosphoramides e.g. 59 prepared from (S)-proline has been shown to proceed in ees as high as 88%.87 The enantioselective allylation of aldehydes using tartrate ester-modified allylsilanes 60 gave homoallylic alcohols in 63–93% and ees of up to 80%,88 whereas allysilanes containing arabinose-derived chiral substituents for example 61 gave alcohols in 36–45%ee and 54–72% yield (5 examples) (Scheme 30).89 Panek has continued to investigate the applications of chiral-non-racemic crotylsilanes in synthesis reporting a stereodi§erentiating crotylation reaction of a-amino aldehydes 62 where reaction with the R E-crotylsilane 63 leads to predominantly anti-stereoselectivity (2 1 to 30 1) whereas the S E-crotylsilane 64 leads to the synproduct (1 1 to 5 1) (Scheme 31).90 Other work from this group includes the Lewis acid-promoted C-glycosidation reactions of activated glycals with crotylsilanes for example 65 (R\CH 2 CO 2 Me) the reaction being highly a-selective and highly diastereoselective; a factor which was dependent on the chirality of the silane.91 The asymmetric synthesis of (E)-olefin dipeptide isosteres from 65 can also be achieved either by reaction with nitronium tetrafluoroborate (R\CH 2 CO 2 Me) to give eventually 66 or by CuOTf enantioselective aziridination (R\CH 2 CH 2 OH).92 Furthermore Lewis acid-mediated carbocyclization can also be e§ected with excellent stereoselectivity (Scheme 32).93 Panek has also reported a synthesis of the C1–C17 polypropionate fragment and the C19–C34 spiroketal fragment of the macrocycle rutamycin B via a series of syn-selective additions of E-crotylsilanes to aldehydes.94,95 Akiyama et al.have demonstrated that it is possible to modify the outcome of the Sakurai–Hosami reaction by changing its stoichiometry. Reaction of allyl-tert-butyldimethylsilane with an aldehyde in a 2 1 ratio in the presence of SnCl 4 led to the 65 Synthetic methods Part (iv) Heteroatom methods R H NHBoc O R BocHN OH R¢ CO2Me Me anti syn 2:1–30:1 R BocHN OH R¢ CO2Me Me syn anti 1:1–5:1 Me R¢ CO2Me PhSiMe2 Me R¢ CO2Me PhSiMe2 63 64 i i Scheme 31 Reagents i BF3 ·OEt 2 ; R\Bn TBDPSOCH 2 CH3 Me2 CHCH 2 ; R@\H Me O OAc OAc H MeO2C Me CbzNH Me OMe O 41% >30:1 80% >13:1 Me 65 R SiPhMe2 O OAc OAc OAc R = CH2CO2Me R = CH2CO2Me i ii–iv TsNH Me OH 65% >30:1 Me TBDPSO 90% >30:1 v R = CH2CH2OH R = CH2CHO vi vii 66 Scheme 32 Reagents i 2BF 3 ·OEt 2 CH 3 CN [30 °C; ii NO 2 BF 4 ; iii HCl Zn dust; iv CBzCl; v PhI–– NTs Cu(I)OTf CH 3 CN rt; vi TiCl 4 DCM,[78 °C; vii imidazole DMF TBDPSCl 66 P.J. Murphy O R H Si Ph SiMe2But O SiMe2But O O R R SiMe2But + 67 68 i ii 1:2 2:1 64% 13–73% Scheme 33 Reagents i SnCl 4 CH2 Cl 2 8 min [78 °C; R\PhCH 2 CH 2 ; ii BF 3 ·OEt 2 CH 2 Cl 2 15 min,[78 °C; R\alkyl O O SiMe3 Me CO2Me H H Me 69 (80% 2 steps) 10:1 trans cis 70 i ii Scheme 34 Reagents i 2.0 equiv.SnCl 4 ,[78 °C to[45 °C CH 2 Cl 2 12 h; ii CH 2 N 2 ketone 67 in 64% yield whereas when the aldehyde was in excess and BF 3 ·OEt 2 was employed the acetal 68 was formed in 13–73% yield (Scheme 33).96 The formation of [2]2]- and [2]3]-cycloadducts has been reported to be an alternative outcome of the Sakurai reaction of allylsilanes with quinones catalysed by Me 2 AlCl.97 The Yb(OTf) 3 catalysed allylation of the hydrates of a-keto aldehydes and glyoxylates with allylsilanes in yields of 65–83% has been reported; examples employing chiral substrates and an allylsilane containing a menthoxy substituent gave low de in the final product.98 Bismuth bromide has also been reported to be an e¶cient and versatile catalyst for the cyanation and allylation of aldehydes ketones and acetals with organosilicon reagents leading to good yields of alcohols and cyanohydrins or ethers in the case of acetals.99 The Lewis acid-promoted intramolecular addition of allylsilanes to b-lactones has been shown to proceed smoothly to give variously substituted cyclopentanes.For example allylsilane 69 was converted to cyclopentane 70 in 80% yield on treatment with tin tetrachloride (Scheme 34).100 The intermolecular trialkylsilylallylation of an iminium species generated from 71 led to the formation of oxazinones via trapping of the incipient b-silyl carbocation by the N-Boc-protecting group; several other examples were reported (Scheme 35).101 The trans-allylsilylation of unactivated alkynes is catalysed by Lewis acids with HfCl 4 giving the best results.The reaction leads to silylated 1,4-dienes in a regio- and stereo-selective manner in yields of 10–97% (18 examples) (Scheme 36).102 Yamaguchi has described the allylation of alkynes with allyltrimethylsilanes in the presence of 67 Synthetic methods Part (iv) Heteroatom methods N O O OMe N O O N O O SiMe3 SiMe3 + SiMe3 64% i 71 Scheme 35 Reagents i TiCl 4 DCM,[78 °C R2 R1 R5 R3 R4 SiMe3 R2 SiMe3 R1 R3 R5 R4 + i Scheme 36 Reagents i HfCl 4 CH 2 Cl 2 0 °C; R1\allyl aryl H R2\H Me Et TMSi R3,R4,R5\H Me Si O OH OH But But 73 O Si But But 72 O Si But But O O iii 62% 74 i ii Scheme 37 Reagents i Oxone' NaHCO 3 acetone,H 2 O; ii SiO 2 ; iii NBS acetone H 2 O,[23 °C to 0 °C GaCl 3 (allylgallation) leading to the formation of 1,4-dienes after treatment with methylmagnesium bromide.103 In an interesting application of allylsilanes 72 was found to undergo ring expansion on sequential treatment with oxone and silica gel to give the six-membered silyl ether 73 or on NBS–acetone–water treatment the seven-membered silyl ether 74 (Scheme 37).104 The chemistry of silanols has attracted attention in 1997 for example the reaction of the dimetallated allyldiphenylsilanol 75 with a range of electrophiles (E\aldehydes ketones alkyl iodides TMSCl ethylene oxide D 2 O) has been shown to be E-c- regioselective ([97 3 E:Z and [90 10 c a) the best results being obtained when a potassium allylsilanolate is deprotonated with butyllithium (Scheme 38).105 A basepromoted preparation of alkenylsilanols from allylsilanes has been reported for example treatment of allyl-tert-butyldiphenylsilane with ButOK and 18-crown-6 in DMSO at room temperature led to isomerization of the olefinic double bond and subsequent substitution of a phenyl group by a hydroxy group.Eight further examples of this reaction were reported with yields between 52–97% (Scheme 39).106 In two communications107,108 the group of Bruckner has reported an interesting application of the retro-[1,4]-Brook rearrangement. For example treatment of a 68 P. J. Murphy SiPh2 O–K+ SiPh2 O–K+ – Li+ 75 SiPh2 OH E i ii Scheme 38 Reagents i BunLi THF HMPA,[45 °C; ii electrophile (E) SiPh2But SiPh2ButOH i Scheme 39 Reagents i But OK DMSO 18-crown-6 15 min Ph OTBDPS SPh 76 Ph OH SiPh2But 77 i 98% 96:4 Scheme 40 Reagents i 2.2 equiv.K naphthalenide THF,[78 °C 50 min S S TBS (2.6 equiv.) 78 BnO O i ii iii BnO S S OTBS – Li+ Cl O (1 equiv.) (2.6 equiv.) 79 OBn BnO TBSO OH OTBS S S S S 80 iv Scheme 41 Reagents i 2.6 equiv. ButLi [78 °C to [45 °C Et 2 O 1 h; ii [78 °C to [25 °C 1 h; iii 0.3 equiv. HMPA,[78 °C 5 min syn-anti-mixture of sulfide 76 with potassium naphthalenide led to the formation of allylsilane 77 as a 96 4 mixture of the anti-trans syn-trans isomers (Scheme 40). In all the examples quoted there was a strong preference for anti-selectivity and in the majority the formation of the trans-alkene predominated. In a demonstration of the synthetic potential of the [1,4]-Brook rearrangement the reaction of lithiated silyl dithianes 78 with epoxides leads to the formation of the intermediate 79 via a solventcontrolled Brook rearrangement; when an excess (2.6 equiv.) of this intermediate was treated with ([)-epichlorohydrin the highly functionalised product 80 was isolated in 69 Synthetic methods Part (iv) Heteroatom methods Br O Al O Br Me O SiPri 3 OSiPri 3 SiPri 3 CHO 81 i ii 74% 93% Scheme 42 Reagents i 0.2 equiv.81 DCM rt 30 min; ii 2 equiv. 81 DCM,[40 °C 30 min O Ph OH Ph SiMe2Ph SiMe2Ph Ph i ii 90% Ph iii 95% Scheme 43 Reagents i PhMe 2 SiLi PhMe [78 °C 2 h; ii SOCl 2 Py rt 4 h; iii BF 3 ·2AcOH DCM 1–5h rt an impressive 66% overall yield (Scheme 41).109 Two diverse rearrangement pathways to give either a-silyl aldehydes or silyl enol ethers have been described using the bulky Lewis acid 81 by using either 2 equivalents of the reagent or a catalytic amount respectively.It was found that bulky silyl groups were essential for high yields to be obtained (Scheme 42).110 Fleming has reported a new method for the overall reductive conversion of esters and ketones into alkenes. Treatment of ketones with PhMe 2 SiLi followed by dehydration and protodesilylation of the resultant vinylsilanes gives alkenes in good overall yields (Scheme 43).111 A similar process utilising 2 equivalents of PhMe 2 SiLi can be used to convert esters and lactones into terminal alkenes. Singer et al. have studied the conjugate addition of Me 2 PhSiLi to a,b-unsaturated carbonyl compounds mediated by sub-stoichiometric quantities of dimethylzinc (as low as 10 mol%) and have found that good to excellent yields of b-silylated products are obtained.112 The catalytic behaviour is most prevalent when the Me 2 Zn employed is generated in situ from the addition of methyllithium to ZnI 2 .Yamamoto and Fleming have reported a novel route to allylsilanes via a conjugate 1,6-addition of PhMe 2 SiLi to aromatic carbonyl complexes of bulky Lewis acid aluminium tris(2,6-diphenylphenoxide).113 Shimizu has reported that the reaction of bromo(tert-butyldimethylsilyl)fluoromethyllithium (ButMe 2 SiCBrFLi) with aldehydes and ketones yields 1-fluoro-1-silyl oxiranes in good yields (73–98% yield 6 examples); the reagent can also be alkylated with a range of alkyl halides alkyl triflates and TMSCl in good yield (71–90% 9 examples).114 Woerpel has reported115,116 the stereo- and regio-selectivity of reactions of siliranes with aldehydes ketones and imines for example the reaction of cis-82 with aldehydes leads to the formation of cyclic silyl ethers 83a,b which are potential precursors of 1,3-diols.A similar reaction occurs with formamides for example trans-82 reacts with formamide 84 to give 85 as a single isomer which after conversion to the acetate derivative 86 reacts with silyl enol ethers in high yield and with high levels of stereoselectivity (Scheme 44). 70 P. J. Murphy cis – 82 Si But But Me Me O Si But But Ph Me Me i + O Si But But Ph Me Me 83a 83b 69 30 Si But But Me Me trans – 82 ii N H O / 93% 84 O Si But But N Me Me O Si But But OAc Me Me 85 iii,iv 100% 86 Me OSiMe3 Me v O Si Me O But But Me H Me Me 100% > 92 8 selectivity Scheme 44 Reagents i PhCHO 18-crown-6 0.1 equiv.KOBut; ii hexanes 120 °C; iii HOAc H 2 O THF; iv Ac 2 O Py; v CH 2 Cl 2 [78 °C SnBr 4 OSiMe2But 87 + 2 O CO2Me OH OH 0.1 equiv. / Cl2Ti(OPri)2 i MeO2C CO2 Me OH OSiMe2But OH MeO2C CO2 Me O OH HO 89 88 ii Scheme 45 Reagents i CH 2 Cl 2 0 °C 3 h; ii HCl MeOH Mikami has reported the first example of a tandem two-directional asymmetric Mukaiyama aldol reaction; addition of 2 equivalents of methyl glyoxylate to silyl enol ether 87 in the presence of a binaphthol-derived chiral titanium complex gave the silyl enol ether 88 in 77% yield; subsequent hydrolysis gave the diol 89 in 99%ee and 99%de (Scheme 45).117 The isolation of a,a-difluoroketene silyl acetal 90 and its application in asymmetric aldol reactions has been reported; addition of 90 to aldehydes in the presence of Masamune’s catalyst 91 or the analogous Kiyooka’s 71 Synthetic methods Part (iv) Heteroatom methods RCHO + F F OTMS OEt i R OEt OH O F F 90 Me O B N O Ts H Pri N B O H p-NO2C6H4SO2 O But 91 92 Scheme 46 Reagents i 20 mol%91 or 92 EtNO 2 [78 °C or[45 °C catalyst 92 gave the aldol products (8 examples) in good yields (85–99%) and ees (81–98%) (Scheme 46).118,119 Similar results were reported for an analogous bromo- fluoroketene silyl acetal with catalyst 91.120 Kiyooka has applied catalysts similar to 92 to the synthesis of either syn- or anti-1,3-diols from b-silyloxy aldehydes with complete stereoselection controlled by the choice of the absolute stereochemistry of the catalyst.121 Organotin perchlorates have been found to be e§ective and mild catalysts for the Mukaiyama reaction of ketene silyl acetals; they also appear to be chemoselective in competition reactions between aldehydes and acetals.Similarly enals react with ketene silyl acetals in preference to the corresponding alkanal in the presence of organotin perchlorates whilst the presence of an electron-donating group will increase the reactivity of an aldehyde in aldehyde–aldehyde competition reactions. An additional factor is that silyl enol ethers derived from ketones are not activated by organotin perchlorates.122 Kobayashi and Nagayama have studied the competition reaction between aldimines and aldehydes towards nucleophilic addition and have reported that an unprecedented change in their normally assumed reactivity is observed. Preferential reaction of aldimines over aldehydes in nucleophilic additions using lanthanide salts [Yb(OTf) 3 in particular] as catalysts for the addition of silyl enol ethers ketene silyl acetals allyltributylstannane or cyanotrimethylsilane was observed with selectivity as high as 99 1.123,124 The high pressure induced Mukaiyamatype aldol reaction of bis-trimethylsilyl ketene acetals with benzaldehyde has been reported to give an overall syn-selectivity (ca.2 1 to 4 1) for the formation of silylated aldol products.125 A study on the indium trichloride catalysed Mukaiyama-aldol reaction in water has been reported a rationale for reaction rate and stereoselectivity based on the internal pressure e§ect of water is described.126 The Sc(OTf) 3 catalysed aqueous aldol reactions of silyl enol ethers with aldehydes has been successfully carried out in the presence of a small quantity of a surfactant; the reactions are organic solvent free and were found to be sluggish if the surfactant is omitted.127 72 P.J. Murphy References 1 J. Emsley Chem. Brit. 1997 33 43. 2 K.C. Nicolaou M.W. Harter J. L. Gunzner and A. Nadin Liebigs Ann. Chem. 1997 1283. 3 J. S. Yadav and D. Srinivas Tetrahedron Lett. 1997 38 7789. 4 C.M. Moorho§ Synlett 1997 126. 5 C.M. Moorho§ Tetrahedron 1997 53 2241. 6 C.M. Moorho§ Tetrahedron Lett. 1997 38 4157. 7 T. Fujimoto Y. Kodama I. Yamamoto and A. Kakehi J. Org. Chem. 1997 62 6627. 8 O. Piva and S. Comesse Tetrahedron Lett. 1997 38 7191. 9 S. Kojima R. Takagi and K. Akiba J. Am. Chem Soc. 1997 119 5970. 10 S. Kojima K. Kawaguchi and K. Akiba Tetrahedron Lett. 1997 38 7753. 11 S. Kojima and K.Akiba Tetrahedron Lett. 1997 38 547. 12 F. Rubsam A. M. Evers C. Michel and A. Giannis Tetrahedron 1997 53 1707. 13 M. Nishizawa Y. Komatsu D. M. Garcia Y. Noguchi H. Imagawa and H. Yamada Tetrahedron Lett. 1997 38 1215. 14 A. Spinella T. Fortunati and A. Soriente Synlett 1997 93. 15 S. Sano K. Yokoyama M. Fukushima T. Yagi and Y. Nagao Chem. Commun. 1997 559. 16 R. Kreuder T. Rein and O. Reiser Tetrahedron Lett. 1997 38 9035. 17 M.T. Mendlik M. Cottard T. Rein and P. Helquist Tetrahedron Lett. 1997 38 6375. 18 T. Kumamoto and K. Koga Chem. Pharm. Bull. 1997 45 753. 19 W.M. Dai J. L. Wu and X. Huang Tetrahedron Asymmetry 1997 8 1979. 20 B. E. Ledford and E. M. Carreira Tetrahedron Lett. 1997 38 8125. 21 K. Hayashi T. Shinada K. Sakaguchi M. Horikawa and Y. Ohfune Tetrahedron Lett.1997 38 7091. 22 M.C. A. vanVliet G. J. Meuzelaar J. Bras L. Maat and R. A. Sheldon Liebigs Ann. Chem. 1997 1989. 23 P. K. Malinen A. Y. Tauber P. H. Hynninen and F. P. Montforts Tetrahedron Lett. 1997 38 3381. 24 A. Couture E. Deniau P. Grandclaudon and S. Lebrun Synlett 1997 1475. 25 P. O’Brien H. R. Powell P. R. Raithby and S. Warren J. Chem. Soc. Perkin Trans. 1 1997 1031. 26 J. Clayden A. Nelson and S. Warren Tetrahedron Lett. 1997 38 3471. 27 A. Nelson and S. Warren J. Chem. Soc. Perkin Trans. 1 1997 2645. 28 J. Uziel N. L. Riegel B. Aka P. Figuiere and S. Juge Tetrahedron Lett. 1997 38 3405. 29 P. Cuadrado and A. M. Gonzalez-Nogal Tetrahedron Lett. 1997 38 8117. 30 I. M. Lefebvre and S. A. Evans Jr. J. Org. Chem. 1997 62 7532. 31 M. Mikolajczyk P. Lyzwa and J. Drabowicz Phosphorus Sulfur Silicon Relat.Elem. 1997 120 357. 32 M. Mikolajczyk P. Lyzwa and J. Drabowicz Tetrahedron Asymmetry 1997 8 3991. 33 F. A. Davis P. S. Portonovo R. E. Reddy G. V. Reddy and P. Zhou Phosphorus Sulfur Silicon Relat. Elem. 1997 120 291. 34 I. Mori Y. Kimura T. Nakano S. Matsunaga G. Iwasaki A. Ogawa and K. Hayakawa Tetrahedron Lett. 1997 38 3543. 35 F. Mathey J. Org. Chem. 1997 529 1. 36 J. Holz M. Quirmbach and A. Borner Synthesis 1997 983. 37 D. F. Weimer Tetrahedron 1997 53 16 609. 38 R. Waschbusch J. Carran A. Marinetti and P. Savignac Synthesis 1997 727. 39 A. H. Li L. X. Dai and V. K. Aggarwal Chem. Rev. 1997 97 2341. 40 A. Padwa D. E. Gunn and M.H. Osterhout Synthesis 1997 1353. 41 Y. Kita N. Shibata S. Fukui M. Bando and S. Fujita J. Chem. Soc. Perkin Trans. 1 1997 1763.42 Y. Kita Phosphorus Sulfur Silicon Relat. Elem. 1997 120 145. 43 N. Shibata M. Matsugi N. Kawano S. Fukui C. Fujimori K. Gotanda K. Murata and Y. Kita Tetrahedron Asymmetry 1997 8 303. 44 H. Kosugi K. Hoshino and H. Uda Tetrahedron Lett. 1997 38 6861. 45 H. Kosugi M. Abe R. Hatsuda H. Uda and M. Kato Chem. Commun. 1997 1857. 46 G. Solladie G. Hanquet and C. Rolland Tetrahedron Lett. 1997 38 5847. 47 C. A. Hutton and J. M. White Tetrahedron Lett. 1997 38 1643. 48 W. Oppolzer O. Froelich C. Wiaux-Zamar and G. Bernardinelli Tetrahedron Lett. 1997 38 2825. 49 F. A. Davis R. E. Reddy J. M. Szewczyk V. G. Reddy P. S. Portonovo H. M. Zhang D. Fanelli R. T. Reddy P. Zhou and P. J. Carroll J. Org. Chem. 1997 62 2555. 50 H. Takada Y. Nishibayashi K. Ohe S. Uemura C. P. Baird T. J. Sparey and P.C. Taylor J. Org. Chem. 1997 62 6512. 51 S. Fustero A. Navarro and A. Asensio Tetrahedron Lett. 1997 38 4891. 52 C. Alayrac L. C. Fromont L. P. Metzner and N. T. Anh Angew. Chem. Int. Ed. Engl. 1997 36 371. 53 R. Caputo A. Guaragna G. Palumbo and S. Padatella J. Org. Chem. 1997 62 9369. 54 A. J. Blake S. M. Westaway and N. S. Simpkins Synlett 1997 919. 55 A. P. Dishington R. E. Douthwaite A. Mortlock A. B. Muccioli and N. S. Simpkins J. Chem. Soc. Perkin 73 Synthetic methods Part (iv) Heteroatom methods Trans. 1 1997 323. 56 G.M. P. Giblin C. D. Jones and N. S. Simpkins Synlett 1997 589. 57 N. Magnus and P. Magnus Tetrahedron Lett. 1997 38 3491. 58 P. Evans and R. J. K. Taylor Synlett 1997 1043. 59 M. F. Hentemann and P. L. Fuchs Tetrahedron Lett. 1997 38 5615. 60 C.Leriverend P. Metzner A. Capperucci and A. DeglInnocenti Tetrahedron 1997 53 1323. 61 R. S. Glass and R. Okazaki Tetrahedron 1997 53 12 067. 62 S. M. Weinreb Top. Curr. Chem. 1997 190 131. 63 S. Z. Zard Angew. Chem. Int. Ed. Engl. 1997 36 673. 64 L. W. M. Lee and H. Chan Top. Curr. Chem. 1997 190 103. 65 B. F. Bonini and M. Fochi Rev. Heteroat. Chemistry 1997 16 47. 66 M. C. Aversa A. Barattucci P. Bonaccorsi and P. Giannetto Tetrahedron Asymmetry 1997 8 1339. 67 K. Fujita Rev. Heteroat. Chemistry 1997 16 101. 68 S. Fukuzawa K. Takahashi H. Kato and H. Yamazaki J. Org. Chem. 1997 62 7711. 69 K. Fujita K. Murata M. Iwaoka and S. Tomoda Tetrahedron 1997 53 2029. 70 R. Deziel L. E. Malenfant C. Thibault S. Frechette and M. Gravel Tetrahedron Lett. 1997 38 4753. 71 Z. S. Zhou N. Jiang and D.Hilvert J. Am. Chem. Soc. 1997 119 3623. 72 T. Takahashi N. Nakao and T. Koizumi Tetrahedron Asymmetry 1997 8 3293. 73 A. Krief C. Colaux and W. Dumont Tetrahedron Lett. 1997 38 3315. 74 T. Murai K. Kakami A. Hayashi T. Komuro H. Takada M. Fujii T. Kanda and S. Kato J. Am. Chem. Soc. 1997 119 8592. 75 A. Krief C. Delmotte and W. Dumont Tetrahedron Lett. 1997 38 3079. 76 A. Krief C. Delmotte and W. Dumont Tetrahedron 1997 53 12 147. 77 M. Miyashita T. Suzuki M. Hoshino and A. Yoshikoshi Tetrahedron 1997 53 12 469. 78 Y. Tang Z. Y. Huang L. X. Dai J. Sun and W. Xia J. Org. Chem 1997 62 954. 79 J. V. Comasseto L. W. Ling N. Petragnani and H. A. Stefani Synthesis 1997 373. 80 T. Shimizu and N. Kamigata Org. Prep. Proced. Int. 1997 29 603. 81 I. Fleming A. Barbero and D. Walter Chem.Rev. 1997 97 2063. 82 L. Fensterbank M. Malacria and S.McN. Sieburth Synthesis 1997 813. 83 G. K. S. Prakash and A. K. Yudin Chem. Rev. 1997 97 757. 84 R. O. Duthaler and A. Hafner Angew. Chem. Int. Ed. Engl. 1997 36 43. 85 A. Bottoni A. L. Costa D. DiTommaso I. Rossi and E. Tagliavini J. Am. Chem. Soc. 1997 119 12 131. 86 R.M. Angell A. G.M. Barrett D. C. Braddock S. Swallow and B. D. Vickery Chem. Commun. 1997 919. 87 K. Iseki Y. Kuroki M. Takahashi S. Kishimoto and Y. Kobayashi Tetrahedron 1997 53 3513. 88 L. C. Zhang H. Sakurai and M. Kira Chem. Lett. 1997 129. 89 T. K. M. Shing and L. H. Li J. Org. Chem. 1997 62 1230. 90 J. S. Panek and P. Liu Tetrahedron Lett. 1997 38 5127. 91 J. S. Panek and J. V. Schaus Tetrahedron 1997 53 10 971. 92 C. E. Masse B. S. Knight P. Stavropoulos and J.S. Panek J. Am. Chem. Soc. 1997 119 6040. 93 C. E. Masse L. A. Dakin B. S. Knight and J. S. Panek J. Org. Chem. 1997 62 9335. 94 N. F. Jain and J. S. Panek Tetrahedron Lett. 1997 38 1345. 95 N. F. Jain and J. S. Panek Tetrahedron Lett. 1997 38 1349. 96 T. Akiyama M. Nakano J. Y. Kanatani and S. Ozaki Chem. Lett. 1997 385. 97 W. S. Murphy and D. Neville Tetrahedron Lett. 1997 38 7933. 98 Y. Yang M. W. Wang and D. Wang Chem. Commun. 1997 1651. 99 N. Komatsu M. Uda H. Suzuki T. Takahashi T. Domae and M. Wada,Tetrahedron Lett. 1997 38 7215. 100 C. X. Zhao and D. Romo Tetrahedron Lett. 1997 38 6537. 101 S. Brocherieux-Lanoy H. Dhimane J. C. Poupon C. Vanucci and G. Lhommet J. Chem. Soc. Perkin Trans. 1 1997 2163. 102 E. Yoshikawa V. Gevorgyan N. Asao and Y. Yamamoto J. Am.Chem. Soc. 1997 119 6781. 103 M. Yamaguchi T. Sotokawa and H. Hirama Chem. Commun. 1997 743. 104 K. Tanino N. Yoshitani F. Moriyama and I. Kuwajima J. Org. Chem. 1997 62 4206. 105 K. Takaku H. Shinokubo and K. Oshima Tetrahedron Lett. 1997 38 5189. 106 T. Akiyama and S. Imazeki Chem. Lett. 1997 1077. 107 C. Gibson T. Buck M. Noltemeyer and R. Bruckner Tetrahedron Lett. 1997 38 2933. 108 D. Goeppel and R. Bruckner Tetrahedron Lett. 1997 38 2937. 109 A. B. Smith and A.M. Boldi J. Am. Chem. Soc. 1997 119 6925. 110 T. Ooi T. Kiba and K. Maruoka Chem. Lett. 1997 519. 111 A. Chenede N. Abj.Rahman and I. Fleming Tetrahedron Lett. 1997 38 2381. 112 B. L. MacLean K. A. Hennigar K. W. Kells and R. D. Singer Tetrahedron Lett. 1997 38 7313. 113 S. Saito K. Shimada H. Yamamoto E. M. deMarigorta and I.Fleming Chem. Commun. 1997 1299. 114 M. Shimizu T. Hata and T. Hiyama Tetrahedron Lett.,1997 38 4591. 115 P. M. Bodnar W.S. Palmer B. H. Ridgway J. T. Shaw J. H. Smitrovich and K. A. Woerpel J. Org. Chem. 1997 62 4737. 74 P. J. Murphy 116 J. T. Shaw and K. A. Woerpel Tetrahedron 1997 53 16 597. 117 K. Mikami S. Matsukawa M. Nagashima H. Funabashi and H. Morishima Tetrahedron Lett. 1997 38 579. 118 K. Iseki Y. Kuroki D. Asada and Y. Kobayashi Tetrahedron Lett. 1997 38 1447. 119 K. Iseki Y. Kuroki D. Asada M. Takahashi S. Kishimoto and Y. Kobayashi Tetrahedron 1997 53 10 271. 120 K. Iseki Y. Kuroki and Y. Kobayashi Tetrahedron Lett. 1997 38 7209. 121 S. Kiyooka T. Yamaguchi H. Maeda H. Kira M.A. Hena and M. Horiike Tetrahedron Lett. 1997 38 3553. 122 J. X. Chen and J.Otera Tetrahedron 1997 53 14 275. 123 S. Kobayashi and S. Nagayama J. Am. Chem. Soc. 1997 119 10 049. 124 S. Kobayashi and S. Nagayama J. Org. Chem. 1997 62 232. 125 M. Bellassoued E. Reboul and F. Dumas Tetrahedron Lett. 1997 38 5631. 126 T. P. Loh J. Pei K. S. V. Koh G. Q. Cao and X. R. Li Tetrahedron Lett. 1997 38 3465. 127 S. Kobayashi T. Wakabayashi S. Nagayama and H. Oyamada Tetrahedron Lett. 1997 38 4559. 75 Synthetic methods Part (iv) Heteroatom methods mmmm