2 Synthetic methods Part (i) Free-radical reactions Clive S. Penkett and Iain D. Simpson School of Chemistry Physics and Environmental Science University of Sussex Falmer Brighton East Sussex UK BN1 9QJ 1 Introduction Sadly Sir Derek Barton one of the great names of radical chemistry passed away during 1998 and his contributions to the .eld will be missed by all. Professor Barton was one of the early pioneers of the use of free radicals in organic synthesis and it is a testament to his lasting in.uence that the .eld remains so strong today. New reagents and catalytic procedures have been developed which dramatically improve chemoand stereoselectivity of radical reactions. There were many examples of organometallic reagents (such as organosamarium -titanium -chromium -iron -manganese and -copper) being used to accomplish complex radical induced reactions suggesting a general move away from the use of toxic organotin reagents.A number of spectacular tandem cascade reactions have been reported which demonstrate the elegance and power of radical cyclisation reactions. In a number of cases acyclic precursors have been transformed very e.ciently into highly complex molecules which have subsequently been converted into natural products. 2 Initiators and reagents This year a number of novel reagents and improvements to existing reaction conditions have become available to allow radical reactions to proceed with improved e.ciency economy and selectivity. Gansa� uer has reported a system for the radical opening of epoxides using a catalytic amount of titanocene dichloride.This o.ers a considerable improvement on the standard conditions which require two equivalents of the titanium reagent. It was found that either manganese or zinc could be used as the co-reductant although manganese often caused fewer side reactions due to the manganese(..) chloride by-product being a weaker Lewis acid than zinc(..) chloride. The reaction conditions were found to tolerate a variety of functionality including ketones halides and silyl ethers and many of the standard reactions of Nugent’s reagent were successfully performed (Scheme 1). 3 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 O HO 5 % Cp2TiCl2 Mn 78 % Collidine.HCl EtO2C CO2 Et EtO2C CO2 Et OEt I O O Scheme 1 Because radical cyclisation reactions generally favour exo ring closure a cyclisation usually leaves a functional or alkyl group adjacent to the point of ring closure.If this group is not desired in the .nal product it must be removed thereby introducing more complexity into the synthesis. One solution to this problem has been developed by Kim, who carried out cyclisations onto aldehydes in the presence of triphenylphosphine. The oxy-radical initially formed is deoxygenated under these conditions to give a carbon radical which ultimately leads to a cycloalkane with no functionality at the position of ring closure (Scheme 2). PPh3 350 nm O Bu3SnH Ph O Ph O OH Bu3SnH Ph Br OEt + 78 % OH Scheme 2 The search for non-toxic hydrogen atom donors to replace tin hydrides remains an important area of research and two new reagents for the Barton—McCombie reduction of xanthates have been described.Jang reported the use of dibutylphosphine while Barton used phosphine—borane adducts to carry out the reaction. The latter combination is particularly interesting since alkyl bromides and chlorides did not react under the same conditions allowing potentially useful selectivity in radical reactions. The Surzer—Tanner rearrangement (1,2-acyloxy shift of -acyloxy radicals) has long been of interest to radical chemists since it has no equivalent in ionic chemistry. The reaction is generally too slow to be of practical use but Renaud has shown that it can be considerably accelerated by the presence of a Lewis acid.For the rates to be useful there must be a nearby hydroxy group to encourage chelation between the Lewis acid and the carbonyl group (Scheme 3). O OH O O O AIBN Additive A B Additive Ratio A B None 3 97 Sc(OTf)3 Lutidine 98 2 Scheme 3 4 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 Initiators which can be used at low temperatures represent another important area of study. The most commonly used methods are photolysis or the use of triethylborane in the presence of oxygen but these procedures are unsatisfactory where photolabile or oxygen-sensitive compounds are involved. Schiesser has reported that 9- borabicyclo[3.3.1]nonane can be used to initiate stannane mediated reductions of a variety of radical precursors at temperatures as low as 78 °C (Scheme 4).The reaction could be carried out under a nitrogen atmosphere allowing the use of air-sensitive compounds though the authors did not exclude the possibility that small traces of oxygen present in the system may be required for the reaction to proceed. Another low temperature initiation system which has recently been described is the use of diethylzinc in air. Bu3SnH + Br 9-BBN PhMe 68 % (0 °C) 32 % (-78 °C) 29 % (0 °C) 67 % (-78 °C) O Scheme 4 Kim has developed a novel method for the generation of alkoxy radicals based on the addition of tin or tris(trimethylsilyl)silane radicals to one of the carbonyl groups of N-alkoxyphthalimides. Cleavage of the nitrogen—oxygen bond follows leaving an oxygen radical which was shown to undergo many of the expected reactions including 1,5-hydrogen abstraction and cyclisation reactions (Scheme 5).The desired N-alkoxyphthalimides were easily prepared by reaction of N-hydroxyphthalimide with either alkyl halides or alcohols. Bu3SnH AIBN O Ph Ph N PhH reflux O 93 % O Scheme 5 Kim has also reported that methyloxalyl chloride can be used for the carboxylation of alkyl radicals. Treatment of a radical precursor with an initiator in the presence of this reagent gives initially a mixture of the corresponding acyl chloride and methyl ester which can be converted to the pure ester on work-up with methanol and triethylamine. This procedure can be combined with a radical cyclisation to give -cycloalkylcarboxylic acids (Scheme 6).3 Intermolecular reactions Intermolecular radical reactions remain relatively under-utilised in comparison with intramolecular cyclisations. However a striking demonstration of the synthetic power of such reactions has been reported by Takai and co-workers. In a single reaction an 5 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 E 2. MeOH Et3N CO2Me I 63 % O I Scheme 6 alkyl iodide a diene and an aldehyde can be coupled as shown in Scheme 7. The mechanism is believed to involve generation of an alkyl radical from the iodide which adds to the diene. The resulting allyl radical is then reduced to the organochromium species which adds to the aldehyde to give the .nal product. Regio- and stereoselectivities were generally good and yields of up to 76% of a single diastereomer could be achieved after puri.cation.CrCl2 DMF H + + 76 % OH CO2Me Scheme 7 Curran and Fuchs have reported the use of allylic tri.uoromethylsulfones (triflones) for selective activation of solvent CH bonds (Scheme 8). The addition of an alkyl radical to the electron-de.cient double bond yields a substituted ole.n after elimination of sulfur dioxide and a tri.uoromethyl radical. The highly electrophilic CF radical then abstracts a hydrogen atom from solvent thereby propagating the chain. AIBN CO2Me + SO2CF3 O 0.016 M O E E RH SO E EE 1. MeOC(O)COCl (Bu3Sn)2 h. 2 + + R R + CF3 R CO2CF3 Scheme 8 Curran has also developed a method for the halogen atom transfer of -bromocarbonyl compounds to enol ethers (Scheme 9).Under standard conditions (neutral benzene as solvent) this class of addition reactions is unsuccessful despite the fact that electronic factors should be in its favour. Reasoning that the failure may be due to the reversibility of the atom transfer step Curran introduced a nucleophile (typically a mixture of an alcohol and triethylamine) to trap the bromoether as its acetal thereby forcing the reaction to proceed in the desid direction. In this manner a variety of -carbonyl esters could be prepared usefully extending the scope of atom transfer addition reactions. The radical mediated addition of aldehydes to ole.ns is an important method of carbon—carbon bond formation but tends to only give good yields when electronde .cient ole.ns are used.Roberts has described the use of thiols as polarity reversal 6 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 O Sn2Me6 hí O O + O EtO Br EtO OBn BnOH Et3N PhH 91 % OBu E OBu E E OBu E ROH Et3N Br + + + OR E E Br OBu Scheme 10 O Scheme 9 catalysts improving the yields for electron-rich and neutral ole.ns and thus providing a valuable extension to the scope of the reaction (Scheme 10). Carbonylation of alkyl radicals is a useful method for chain extension and Ryu has used this method to form -lactones (Scheme 11). Treatment of an alcohol with lead tetraacetate yields initially an oxygen centered radical which rapidly undergoes 1,5-hydrogen atom abstraction.Carbonylation of the resulting alkyl radical followed by ring closure gives the cyclic ester products. CO (80 atm) OH O Pb(OAc)4 51 % O Scheme 11 N B Bu3SnH r N AIBN CO (80 atm) 81 % Scheme 12 Ryu has also reported the use of carbonylation reactions for the generation of acyl radicals for cyclisation onto imines (Scheme 12). Reaction of analogous alkyl radicals often gives poor selectivity between 5-exo and 6-endo cyclisation products but in the 7 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 case of acyl radicals good selectivity is observed due to the matching of opposite polarities between the carbon of the acyl group and the nitrogen of the imino group. Mikami has shown that a samarium() iodide mediated addition of ketones to acrylates can be carried out in an enantioselective manner by the use of the chiral ligand 2,2-bis(diphenylphosphino)-1,1-binaphthyl oxide (BINAPO) (Scheme 13).Though the yields and enantiomeric excesses were variable the procedure represents another addition to the small but growing list of radical reactions which can be manipulated to give nonracemic products. O O POPh O SmI 2 2 tBuOH OMe + Ph POPh2 BINAPO Ph O (R)- BINAPO 46 % yield 67 % ee O HO Scheme 13 SmI2 80 % H HO OH 4 Intramolecular reactions Molander has continued his work in the area of samarium() iodide induced ketyl radical formation. He has shown that the addition of a ketyl radical to a nitrile group can be enhanced by the use of visible light. He also found that the yields for 5- membered ring formation were much better than for 6-membered ring formation (Scheme 14).O CN O O Scheme 14 SmI2 HMPA 50 % THF Scheme 15 In another series of experiments by Molander samarium() iodide was employed in a sequential ketyl¡Xolen coupling/-elimination reaction (Scheme 15). This process causes the stereoselective incorporation of an alkenyl group via an intramolecular delivery system. The advantage of this is that it introduces an alkenyl group stereoselectively and avoids the basic reaction conditions typical of alkenylmagnesium halides and alkenyllithium reagents which are often used to perform similar 8 Annu. Rep. Prog. Chem. Sect. B 1999 95 3¡X17chemical transformations. The high degree of stereocontrol results from the SmI mediated cyclisation between the ketyl radical and the ole.n.Naito has used the unique properties of a samarium(..) iodide induced ketyl radical cyclisation onto an oxime ether to control the relative stereochemistry of the resulting product. Initially .ve-membered ring variants were investigated resulting in the preparation of cyclic trans-amino alcohols which were subsequently used for the formation of 4-purinylpyrrolidin-3-ol nucleoside analogues. In another paper the same technology was used to prepare the potent inhibitor of protein kinase C ()-balanol. In the key step samarium(..) iodide was used to form a seven-membered ring trans-amino alcohol which was subsequently converted to the target molecule (Scheme 16). O SmI2 HMPA NOR THF 53 % N Boc O Scheme 16 Kim investigated the use of samarium(..) iodide for the reductive cleavage of -halo epoxides. The use of hexamethylphosphoramide (HMPA) had a signi.cant bearing on the nature of the products so that cyclopropanols were prepared in the presence of HMPA and allyl alcohols were produced in its absence (Scheme 17).2.2 SmI2 6 HMPA Ph Br 2.2 SmI THF 75 % 2 O THF 70 % Ph PhS O Br N Bn Scheme 17 Zard has discovered that upon treatment with nickel powder in hot acetic acid N-alkenyl trichloroacetamides undergo reversible 4-exo radical closure. This radical can be trapped by elimination of a thio radical leading to the formation of a -lactam. In the absence of such a trap irreversible 5-endo closure occurs leading to a variety of 5—membered ring lactams.In a subsequent paper he showed how the chemistry could be used to prepare erythrina alkaloids (Scheme 18). Ni AcOH iPrOH CCl3 CCl3 Ni AcOH O N i O PrOH N Bn Scheme 18 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 OH OH NHOR NHOR + N Boc N Boc 1 6.6 OH Ph OH Ph Cl Cl BnN 50 % O Cl 42 % Bn 9 Roy reported the stereoselective formation of polysubstituted tetrahydrofuran derivatives by titanium() mediated radical cyclisation of epoxides (Scheme 19). The chemistry was subsequently used for the synthesis of the antitumor antibiotics ()- methylenolactocin and ()-protolichesterinic acid. O HO (i) Cp2TiCl THF (ii) H3O+ Ph Ph 71 % O O Scheme 19 In a very elegant piece of work Rawal showed how strained oxetanes could undergo radical anion induced fragmentation.Lithium di-tert-butylbiphenylide (LDBB) was used to initiate the tandem radical fragmentation/cyclisation reaction to produce the linear triquinane as shown (Scheme 20). HO HO H H H LDBB Et3Al + H THF 54 % O H H H H 10 1 O O OSiMe3 Fe(NO3)3 DMF + Scheme 20 Further to his work on the use of iron() induced radical fragmentation/cyclisation reactions of silyl-protected cyclopropanols Booker-Milburn has reported an example which initially undergoes a transannular cyclisation and then an intramolecular cyclisation to give an angular triquinane model (Scheme 21). H H rt to 60 ¢XC 39 % H CN Scheme 21 A number of research groups have carried out radical cascade reactions with a view to constructing the steroidal type nucleus.Zoretic used manganese() acetate¡Xcopper() acetate to initiate an oxidative radical cascade to give the polycyclic 5-pregnane skeleton (Scheme 22). In so doing seven stereogenic centres were set 1. Mn(OAc)3 Cu(OAc)2 CN 10 % H H 2. TFA O O H Cl 61 % CO2Me MeO2C Cl Scheme 22 10 Annu. Rep. Prog. Chem. Sect. B 1999 95 3¡X17although the cyano group was subsequently converted to an angular hydrogen in a later step. Snider has continued with his investigations into the use of manganese() acetate¡Xcopper() acetate to prepare tetracyclic diterpenes containing both bridged and fused rings. In one synthetic operation the tetracycle shown was prepared (along with various partially cyclised products) which was subsequently converted to both ()- isosteviol and ()-beyer-15-ene-3,19-diol (Scheme 23).Mn(OAc)3 H Cu(OAc)2 O O 35 % H EtO2C EtO2C Scheme 23 Pattenden used his tin hydride promoted acyl radical technology to perform a similar radical cascade to produce an azasteroid starting from the seleno ester shown (Scheme 24).Ac N Ac H N Bu3SnH NAc H H AIBN O 45 % H SePh O O SePh Bu3SnH O Scheme 24 Pattenden also investigated the use of ,-unsaturated selenoesters to form - ketenyl radicals and subsequently used them in a trans annulation/cyclisation sequence which resulted in a formal synthesis of ()-modhephene (Scheme 25). C O O AIBN 59 % Scheme 25 Malacria published a marvellous example of a tandem radical cyclisation to produce linear triquinanes from acyclic precursors. In the absence of an intermolecular radical trap the diquinane was produced in a highly respectable 66% yield.When acrylonitrile was also incorporated into the reaction medium an eleven elementary step process occurred resulting in the formation of a linear triquinane in an impressive 54% yield. A 1,6-H atom transfer step with a vinyl radical to generate an -silylstabilised radical and an unprecedented -elimination of a trimethylsilyl radical were 11 Annu. Rep. Prog. Chem. Sect. B 1999 95 3¡X17amongst the key elementary steps to form this highly complex molecule from an acyclic precursor (Scheme 26). Si Si O Si Si Bu3SnH O 28 % O Bu3SnH AIBN SiMe3 + Si AIBN Br CN CN 66 % SiMe3 Si Si O 54 % H H Scheme 26 Typically one would expect 5-exo- radical cyclisations to be much more likely than 8-endo-cyclisations but Lee has shown that under certain circumstances the opposite can be true (Scheme 27).This arises from the conformational bias of (alkoxycarbonyl) methyl radicals which prefer to sit in the Z- rather than the E-conformation hence disfavouring the 5-exo mode of cyclisation. Bu3SnH MOMO MOMO O AIBN O H H O Br 80 % O R R R Scheme 27 Further radical cascade studies performed by De Kimpe resulted in a one pot reaction for the formation of pyrrolizidines from -bromomethylaziridines. The - methylaziridyl radical fragmented to a nitrogen centred radical which underwent a tandem radical cyclisation to form the bicyclic heterocycle shown (Scheme 28). Bu3SnH N R N AIBN Br 49 - 63 % Scheme 28 Kim and Lee have recently reported the total synthesis of -cedrene via a tandem radical cyclisation.They utilised the unique nature of N-aziridinylimines which can 12 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 Ph O N Ph O SMe I S THPO O 3SnH PhS AIBN Bu N C5H11 C5H11 O O serve as both radical acceptors and donors sequentially and are ideal for setting up complex quaternary centres. In the .nal elementary step there was a mixture of .veand six-membered ring products formed (Scheme 29). Parsons has demonstrated the utility of radical additions to furans. If an all carbon tether is used in conjugation with a radical trap a complex addition/fragmentation sequence occurs which after an electrocyclic reaction and aerial oxidation of the .nal product results in the formation of an aromatic species (Scheme 30).If a heteroatom is used in the tether a Diels—Alder-like reaction occurs presumably via a halogen atom transfer pathway. Scheme 29 [3,3] shift O (Me3Si)3SiH O C5H11 AIBN 26 % Br O O (i) Bu3SnH AIBN + H H (ii) TsOH MeOH 15 % 45 % H O OTHP OTHP C5H11 51 % Br H O O O O + C [ O ] 5H11 C5H11 1 6 Scheme 30 13 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 Crimmins used the unique properties of radicals to cause the fragmentation of a strained ring followed by a ring expansion reaction to form a spirofused 6,5-ring system. The radical product was subsequently converted into ()-lubiminol (Scheme 31). OH O O CO2Me H Bu3SnH AIBN S OH CO2Me 92 % O N OH OH Me (±)-lubiminol O O Bu3SnO HO Bu3SnH then H2O O O Scheme 31 The incorporation of anionic charge is often used to accelerate [3,3]-sigmatropic shifts and Enholm reasoned that radical anion type species may also accelerate these reactions.He showed that tin radical addition to an appropriately functionalised enone led to the formation of a formal [3,3]-Claisen rearrangement product (Scheme 32). Bu3SnO [3,3] Bu3Sn O 74 % PhH + N N reflux CO2Me Scheme 32 Little extended his work on the use of trimethylenemethane-like diyls by coupling them with a vinylcyclopropane and a cyclopropylcarbinyl radical. Upon thermolysis the diazene shown evolved nitrogen and resulted in the formation of both the [6.3.0] and the [4.3.0] adducts.If an enol ether was used instead of an unsaturated monoester only the [4.3.0] adduct was isolated (Scheme 33). MeO2C 26 % 64 % CO2Me Scheme 33 Murphy continues to prove that radical reactions do not have to be initiated by toxic tin reagents by extending the use of the catalytic ‘radical-polar crossover’ 14 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 reaction. In the example shown below tetrathiafulvalene promoted radical cyclisation of a diazonium salt resulted in the formation of a cyclic ether in the .nal step (Scheme 34). NH2 O O 1. NOBF4 2. TTF (cat) O HO 57 % Scheme 34 Bu3SnH AIBN 5 Stereoselectivity Studer investigated a novel 1,5-aryl migration from sulfur and silicon to carbon. Both methods allow for stereoselective arene transfer from the oxygen substituent via a chair-like transition state (Scheme 35).2 S O Ph O OH Ph I 49 % d.r. = 7 1 Ph Ph OH O Si Me3Si (i) Bu3SnH AIBN Ph I (ii) MeLi 70 % d.r. = 10 1 Scheme 35 Continuing the ipso-substitution theme Malacria has reported the results of forming enantioenriched .ve-membered rings by performing a radical cyclisation onto an ole.n which is adjacent to a homochiral sulfoxide. He also found that the stereoselectivity could be reversed if the reaction was performed in the presence of a very bulky Lewis acid such as methylaluminium bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) (Scheme 36). Further to his work on the radical cyclisation of substituted hex-5-enyl radicals Beckwith has investigated the stereochemical in.uences of anomeric e.ects.Typically pseudoaxially substituted transition states are disfavoured but in the presence of an anomeric e.ect pseudoaxial substituents are stabilised and hence give rise to products with the reverse stereochemistry to that normally observed (Scheme 37). 15 Annu. Rep. Prog. Chem. Sect. B 1999 95 3—17 E E E E PhSe E E Bu3SnH 0 °C + MeO MeO MeO O Et3B O2 S pTol 98 2 93 % No Lewis acid 0 94 52 % With MAD Typically... Br Scheme 36 Bu3SnH R R O O AIBN but... Br Bu3SnH EtO EtO O O (tBuON)2 61 % d.r. = 5.8 1 Scheme 37 References 1 A. Gansa� uer M. Pierobon and H. Bluhm Angew. Chem. Int. Ed. Engl. 1998 37 101; A. Gansa� uer H.Bluhm and M. Pierobon J. Am. Chem. Soc. 1998 120 12 849. 2 H. J. Gold Synlett 1999 159. 3 S. Kim and D. H. Oh Synlett 1998 525. 4 D.O. Jang D. H. Cho and D. H. R. Barton Synlett 1998 39. 5 D.H.R. Barton and M. 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