10 Enzyme Chemistry By A. G. SUTHERLAND School of Applied Chemistry University of North London London N7 8DB 1 Introduction The use of isolated enzyme and whole cell catalysis in synthetic organic chemistry has become ever more widespread reflected by the advent of an ‘Organic Syntheses’ style publication on the subject.’ Perhaps unusually for a developing area impetus is provided as much by successful industrial application as by academic research.2 This area has been comprehensively reviewed in monograph form3 which given the current rate of publication in the primary literature is likely to be the last time this is possible. The application of biotransformations to the provision of chiral building blocks has also been ~overed.~ The use of lipases and esterases in the kinetic resolution of secondary alcohols through hydrolysis or formation of esters is crossing the threshold to become a routine technique that any chemist involved in asymmetric synthesis should be able to consider.Accordingly in contrast to previous year^,^.^ the present report will devote less attention to this topic in order to cover the many developments in other areas more deeply. 2 Hydrolysis and Condensation Reactions Complex Alcohols.-The kinetic resolution and desymmetrization of racemic and nteso (or prochiral) alcohols respectively by enzyme catalysed acylation has been re~iewed.~ The use of these techniques -and the corresponding hydrolytic procedures -to provide low molecular weight enantiomerically pure chiral synthons continues to be the main thrust of this area.8-’ Anecdotal evidence has suggested that few tertiary alcohol esters are subject to ’Preparative Biotransformations’ ed.S. M. Roberts K. Wiggins and G. Casy Wiley England. 1992. ‘Chirality in Industry’ ed. A. N. Collins G. N. Sheldrake and J. Crosby Wiley England 1992. K. Faber ‘Biotransformations in Organic Chemistry’ Springer-Verlag Berlin 1992. E. Santaniello P. Ferraboschi P. Grisenti and A. Manzocchi Chem. Rec.. 1992 92 1071. A.G. Sutherland Annu. Rep. Prog. Chem. Sect. B Org. Chem. 1991 88 263. N. J. Turner Annu. Rep. Prog. Chem. Sect. B Org. Chem. 1990. 87 333. ’ K. Faber and S. Riva Synthesis 1992 895. H.-J. Gais H. Henmerle and S. Kossek Synthesis 1992 169. B. Herradon Tetrahedron Asymmetry 1992 3 209.lo M. Mekrami and S. Sicsic Tetrahedron Asymmetry 1992 3 431. S. Takano T. Yamane M. Takahashi and K. Ogasawara SYNLETT 1992 410. C.R. Johnson A. Golebiowski and D.H. Steensma J. Am. Chem. Soc. 1992 114. 9414. 28 1 A. G. Sutherland enzymatic hydrolysis and that only low enantiomeric excesses are accessible.' However O'Hagan has reported a reasonable enantioselectivity in the hydrolysis of the trifluoromethyl substituted acetate (1) (Scheme l) which may stimulate new interest here. l4 87%e.e. 75%e.e. Reagents i Candida cylindracea lipase 40% conversion Scheme 1 The particular difficulties involved in resolving 1,2-diols have received attention but as yet no common solution has become apparent. Thus transesterification acylation and hydrolytic procedures have been m~oted.'~-'~ The last of these methods highlighted the hitherto unrecognized potential of enzymes to hydrolyse carbonates (Scheme 2),' which coincides with a report on the analogous use of vinyl carbonates as acylating agents.' 40% 78% e.e.Reagents i Pig liver esterase DMSO pH 7 buffer Scheme 2 The potential for the use of enzymes in the separation of diastereoisomers has been realized in the treatment of (2) with porcine pancreatic lipase when only the anti-acetate was hydroly~ed.'~ The elements of diastereoisomer separation kinetic resolution and desymmetrization have all been combined in the Pseudornonas sp. lipoprotein lipase catalysed acylation of the diol mixture (3). Thus the (R,R)diol was converted to the diacetate the (S,S) diol was recovered unreacted and the rneso-diastereoisomer was enantioselectively monoacetylated at the (R) alcohol centre.20 l3 I.C. Cotterill S. M. Roberts and A. G. Sutherland; K. Faber unpublished observations. l4 D. O'Hagan and N. A. Zaidi J. Chem. SOC.,Perkin Trans. 1 1992 947. D. Bianchi A. Bosetti P. Cesti and P. Golini Tetrahedron Lett. 1992 33 3231. l6 A. Bosetti D. Bianchi P. Cesti P. Golini and S. Spezia J. Chem. Soc. Perkin Trans. I 1992 2395. P. Barton and M.I. Page Tetrahedron 1992 48 7731. l8 M. Pozo R. Pulido and V. Gotor Tetrahedron 1992 48 6477. l9 J. Mulzer S. Greifenberg A. Beckstett and M. Gottwald Liebigs Ann. Chem. 1992 1131. '' S.J. Wallace B. W. Baldwin and C. J. Morrow J. Org.Chem. 1992 57 5231. Enzyme Chemistry A detailed attempt to understand the relationship between choice of organic solvent and enantioselectivity in lipase catalysed acylation reactions has failed to find any obvious correlation suggesting the possible significance of bound solvent molecules within the active site.21 It is likely that much more progress will have to be made in the understanding of the structure and function of active sites before further advances are made in this field.22-25 Complex Acids.-In marked contrast to the above work with lipases Klibanov has reported a marked correlation between solvent hydrophobicity and enantioselectivity in the transesterification of the phenylalanine ester (4)catalysed by Aspergillus oryzae protease.26 In polar solvents the L-enantiomer reacted selectively.This was rationaliz- ed in terms of the aromatic group being bound in a hydrophobic pocket in an orientation that favoured the selective transformation of that enantiomer. It was suggested that in hydrophobic solvents the molecule would show a greater tendency to bind with the side chain exposed to the solvent and that the orientation in the active site arising from this mode explained the consequent reversal to D-selectivity. The hydrolysis of neopentyl esters has been shown to provide good access to enantiomerically pure quaternary centres. Thus the regio- and enantioselective hydrolysis of dihydroisoxazole (5) at the more hindered ester moiety followed by chemical modification provided access to esters of the otherwise inaccessible (R)-citramalic acid (6).2’ Similarly an esterase obtained from a crude Candida lipolytica lipase preparation was used to resolve a range of apdialkyl amino acids (e.g.7) by ester hydrolysis.’* 0 AcNHfiOCH2CH2Cl EtO,C C02Et CH2Ph 0-N (4) 21 F. Secundo S. Riva and G. Carrea Tetrahedron Asymmetry 1992 3 267. 22 P.G. Hutlin and J.B. Jones Tetrahedron Lett. 1992 33 1399. 23 Z. Vimmer Tetrahedron 1992 48 8431. 24 C. Exl H. Honig G. Renner R. Rogi-Kohlenprath V. Seebauer and P. Seufer-Wasserthal,Tetruhedron Asymmetry 1992 3 1391. 25 M.-J. Kim and H. Cho J. Chem. SOC.,Chem. Commun. 1992 1411. 26 S. Tawaki and A.M. Klibanov J. Am. Chem. SOC. 1992 114 1882. 27 S. Yang W. Hayden K. Faber and H. Griengl Synthesis 1992 365.28 C. Yee T.A. Blythe T.J. McNabb and A.E. Watts J. Org. Chem. 1992 57 3525. A. G. Sutherland 0 Me*<C02Me Me02C OH H2N CH3 An unusual application of ester hydrolysis was demonstrated in the lipase mediated resolution of the isotopically labelled ester (8). Subsequent reduction of the recovered ester and introduction of a guanine residue led to the antiviral agent (9) in an isotopically chiral form that could be used to examine stereorecognition in the biosynthesis of the corresponding triph~sphate.~~ Sih has explored the effect of chemical modification of an enzyme on enantioselectiv- ity with some success. Thus nitration of the tyrosine residues of Candida cylindracea lipase with tetranitromethane resulted in a marked increase in efficiency in the resolution of a number of esters (e.g.0 Regioselective Ester Reactions.-There has been a marked increase in activity in the study of regioselective hydrolysis or formation of esters of homochiral polyols particularly in the carbohydrate field. Most instances of this procedure to date have involved selective reaction at a primary alcohol in the presence of one or more secondary groups and examples of this continue to appear.31 The scope of these reactions has been extended by the increased realization that subsequent induced acyl migration to the primary centre from a neighbouring secondary ester can be highly selective. This was elegantly exploited in the synthesis of the antifungal phospholipid lysofungin (1 1) from a readily available starting material (Scheme 3),32 and has also seen application in carbohydrate ~ynthesis.~ It has also been clearly demonstrated that it is feasible to discriminate between secondary alcohols in a variety of carbohydrate systems even in the context of vicinal diequatorial die~ters.~~-~’ Similarly the selective acylation of the secondary alcohol in 29 J.T.Sime R. D. Barnes S. W. Elson R. L. Jarvest and K. J. O’Toole J. Chem. SOC. Perkin Trans. I 1992 1653. 30 Q.-M. Gu and C. J. Sih Biocatalysis 1992 6 115. 31 0. Kirk F. Bjorkling S. E. Godtfredsen and T.0.Larsen Biocatalysis 1992 6 127. F. VanMiddlesworth M. Lopez M. Zweerink A. M. Edison and K. Wilson J. Org. Chem. 1992,57,4753. D. Chaplin D.H. G. Crout S. Bornemann D. W. Hutchinson,and R. Khan J.Chem.Soc.,Perkin Trans. I 32 33 1992 235. Enzyme Chemistry 0 ROA li roHO lii 0 ROYo* Reagents i Rhizopus arrhizus lipase pH 6.5; ii pH 8.5 buffer Scheme 3 deoxynucleosides has been shown to be a~hievable.~**~~ Selective enzymatic hydrolysis has also been successfully applied to the glycopeptide field. The alkaline protease from Thermoactinomyces vulgaris has been shown to cleave C-terminal t-butyl esters without competing chain hydr~lysis,~' while the near quantitative hydrolysis of the heptyl ester in (12) is a striking example of the degree of regio- and chemoselectivity available in these proce~ses.~~ Amide Hydrolyses and Condensations.-The kinetic resolution of amino acids via the hydrolysis of an N-acyl derivative of the L-enantiomer using aminoacylase from pig kidney or Aspergillus oryzae is well e~tablished.~' Wang and co-workers have 34 M.J. Chinn G. Iacazio D. G. Spackman N. J. Turner and S. M. Roberts J. Chem. Soc. Perkin Trans. I 1992 661. 35 C. Vogel B. Liebelt W. Steffan and H. Kristen J. Carbohydr. Chem. 1992 11 287. 36 F. Nicotra L. Panza G. Russo and L. Zucchelli J. Org. Chem. 1992 57 2154. 37 M. Marek I. Raich K. Kefurt J. Jary and I. M. Rouwenhart Biocatalysis 1992 6 135. 38 F. Moris and V. Gotor J. Org. Chem. 1992 57 2490. 39 V. Gotor and F. Moris Synthesis 1992 626. 40 P. Braun H. Waldemann and H. Kunz SYNLETT 1992 39. 41 M. Schultz P. Herman and H. Kunz SYNLETT 1992 37.42 H. K. Chenault J. Dahmer and G. M. Whitesides J. Am. Chem. Soc. 1989 111 6354. A. G. Sutherland however reported the use of an enantiocomplementary D-aminoacylase from Al-caligenesfaecalis which may have operational advantages in certain contexts but does not appear to hydrolyse as wide a range of substrates with so near complete enantio~electivity.~~ None of these enzymes have been found to hydrolyse N-alkyl-N-acyl amino acids so the discovery that the N-acyl-L-proline acylase from Cornarnonas testosteroni does accept these substrates (e.g. 13) highlights a useful entry to enantiometrically pure N-alkyl amino acids that should see wider appli~ation.~~ CH3 F3 CH3 Proline acylase -.NJ\C02H + H 0 The use of proteases in peptide synthesis continues to prove fruitful.Thus alcalase (a subtilisin preparation) has been shown to catalyse the formation of a wide range of dipeptide~.~’ The repeated application of this type of reaction was applied to a more ambitious synthesis of N-acyl enkephalin amides (Scheme 4).46 Cbz-Tyr-OEt + Gly-Gly-OEt Phe-OEt + Met-NH2 li li Cbz-Tyr-Gly-Gly-OEt Phe-Met-NH2 lii 89% 89% Cbz-Tyr-Gly-Gly-Phe-Met-NH2 60% Reagents i Chymotrypsin; ii Proteinase K Scheme 4 43 H.-P. Chen S.-H. Wu Y.-C. Tsai Y.-B. Yang and K.-T. Wang Bioorg. Med. Chem. Lett. 1992 2 697. 44 U. Groeger K. Drauz and H. Klenk Angew. Chem. Int. Ed. Engl. 1992 31 195. 45 S.-T. Chen S.-Y. Chen and K.-T. Wang J. Org. Chem. 1992 57 6960. 46 I. Gill and E.N. Vulfson J.Chem. Soc.. Perkin Trans. I 1992 667. Enzyme Chemistry 287 A novel approach to the synthesis of peptide C-terminal amides was reported by Green and Marg~lin.~~ The condensation of an alkyl ester of the appropriately protected peptide with the benzyl amine (14) was catalysed by papain. Subsequent acid-catalysed removal of the protecting groups resulted in N-benzyl cleavage and formation of the required primary amide linkage. OMe (14) Papain 1 OMe BocMet -Met-Leu-NH2 Me0 OMe 76% Nitrile Hydrolysis.-The chemoselectivity and enantioselectivity of isolated enzyme systems from Rhodococcus sp. which convert nitriles to the corresponding amide under nitrile hydratase catalysis and thence to the carboxylic acid utilizing an amidase have been critically examined.Griengl and co-workers have demonstrated that a wide range of aliphatic alicyclic and heterocyclic nitriles are accepted as substrates and that for the most part other functional groups present were not hydr~lysed.~~,~~ The enantioselectivity available from these processes has been largely5' attributed to the amidase and has been applied to the resolution or desymmetrization of both racemic and meso-substrates (Scheme 5).50*5 The use of a nitrilase which catalyses direct nitrile to acid conversion from Rhodococcus rhodochrous in the conversion of a-amino nitriles to the appropriate amino acids was reported. The enzyme showed high enantioselectivity producing L-leucine in ca. 97% e.e.52 G1ycosidations.-The syntheses of a wide range of disaccharides using a variety of commercially available glycosidases and transferases have been reported 3-5 while 41 J.Green and A. L. Margolin Tetrahedron Lett. 1992 33 7759. 48 A. de Raadt N. Klernpier K. Faber and H. Griengl J. Chem. Soc. Perkin Trans. I 1992 137. 49 N. Klempier A. de Raadt H. Griengl and G. Heinisch J. Heterocycl. Chem. 1992 29 93. 50 M. A. Cohen J. S. Parratt and N. J. Turner Tetrahedron Asymmetry 1992 3 1543. 51 J. A. Crosby J. S. Parratt and N.J. Turner Tetrahedron Asymmetry 1992 3 1547. 52 T.C. Bhalla A. Miura A. Wakamoto Y. Ohba and K. Furuhashi Appl. Microbiol. Biotechnol. 1992,37 184. 53 B. Sauerbrei and J. Theim Tetrahedron Lett. 1992 33 201. 54 Y. Nishida T. Wiemann and J. Theim Tetrahedron Lett.1992 33 8043. 55 D. H.G. Crout S. Singh B. E. P. Swoboda. P. Critchley and W.T. Gibson J. Chem. Soc. Chem. Comrnun. 1992 704. A. G. Sutherland 22%,> 98%e.e. 3 1% 90%e.e. OCH Ph OCH2Ph ~ NC&CN i NCAC02H 73% 83% e.e. Reagents i amidase and nitrile hydratase from Rhodococcus sp. pH 7 30 "C Scheme 5 the specificity of P-mannohydrolase in the synthesis of alkyl mannopyranosides has been explored in The synthesis of glycolipids presents an attractive target for this area of enzyme chemistry. The potential in this field has been realized by Flitsch in the synthesis of a glycosphingolipid (15) where a sequence of transferase catalysed reactions were utilized to construct the carbohydrate portion with the lipid moiety in place through the whole reaction sequence (Scheme 6)." Reagents i galactosyl transferase UDP-glucose UDP-glucose epimerase; ii sialyl transferase CMP- Neu-5-Ac Scheme 6 3 Reduction Reactions Whole Cell Ketone Reductions.-The enantioselective reduction of /3-keto esters to the corresponding P-hydroxy esters remains the most active field of whole cell reduction 56 N.Taubken and J. Theim Synthesis 1992 517. 57 B. Guilbert T. H. Khan and S.L. Flitsch J. Chem. Soc. Chem. Commun. 1992 1526. Enzyme Chemistry work. Although the majority of these reductions are still performed with bakers’ yeast (Saccharomyces cerevisiae) as a consequence of availability and ease of handling the advantages of using other microorganisms to transform these substrates are being emphasized.Thus while S. cerevisiae can be used to reduce ethyl 30-0x0-3-phenylpropanoate to the corresponding (S)-alcohol with reasonable efficiency higher yields and enantioselectivities are available through the use of other fungi (Scheme 7).58 Similarly the P-keto ester (16) is reduced by S. cerevisiae with moderate (R)-selectivity but higher optical purities of either enantiomer are available through the use of different species of the genus Clostridia.” uoEt Microorganism ph ~ Ph Microorganism Yield/% e.e./% Saccharomyces cerevisiae 63 93 Beauveria sulfurescens 72 96 Geotrichium candidum 64 298 Scheme 7 However bakers’ yeast was found to be the organism of choice in the reduction of a range of N-protected amino P-keto esters.The prochiral substrate (17) was reduced enantioselectively as part of a high yielding synthesis of the anticonvulsant (R)-GABOB (18).60 High diastereoselectivity was observed in the reduction of the enantiomers (19) and (20) of the y-methyl substituted analogue although surprisingly the erythro dias-tereoisomer was formed in both cases (Scheme 8).60 The reduction of the similar a-keto ester (21) was also successfully performed with Saccharomyces sp. again leading to a GABA inhibitor after deprotection (Scheme 9).61 Tremendous stereoselectivity was displayed in a rare example of a reduction of an enolizable /I-diketone bearing both endocyclic and exocyclic carbonyl groups (22). The rapid rate of epimerization of the substrate allowed formation of only one R.Chenevert G. Fortier and R. B. Rhlid Tetrahedron 1992 48 6769. 59 M. Christen D. H.G.Crout R. A. Holt J.G. Morris and H. Simon J. Chem. SOC.,Perkin Trans. 1 1992 491. 6o S. Hashiguchi A. Kawada and H. Natsugari Synthesis 1992 403. K. J. Harris and C. J. Sih Biocatalysis 1992 5 195. A. G. Sutherland BocN. P O M e Bo:..? OMe (19) 87% 94% d.e. 1 OMe -B o c g POMe (20) 86%,99% d.e. Reagents i S. cereuisiae sucrose EtOH H,O Scheme 8 (21) 54% 88% e.e. (22) 66%. 100%d.e.,97%e.e. (23) 51% Reagents i Saccharomyces sp. Edme glucose; ii S. cereuisiae sucrose; iii Ra-Ni MeOH Scheme 9 diastereoisomer with high enantioselectivity. Desulfurization of the resultant thianol gave ready access to the weevil pheromone sitophilure (23) Scheme 9.62 Isolated Enzyme Ketone Reductions.-In an upsurge of interest in the use of isolated enzymes for carbonyl reduction the use of alcohol dehydrogenases from Pseudomonas SP.,~~ and pig liver6’ has been described.All show considerable Lactobacillus I~ejir,~~ potential in asymmetric synthesis and representative reactions are depicted in Scheme 10. Although the use of D-and L-lactate dehydrogenases (from Staphylococcus epidermis and rabbit muscle respectively) has been shown to give high chemical and optical yields 62 T. Fujisawa B.I. Mobele and M. Shimizu Tetrahedron Lett. 1992 33 5567. 63 C. W. Bradshaw H. Fu G.4. Shen and C.-H. Wong J. Org. Chem. 1992 57 1526. 64 C. W. Bradshaw W. Hummel and C.-H.Wong J. Org. Chem. 1992 57 1532. 6s Y. Hirose M. Okutsu M. Anzai K. Naemura and H. Chikamatsu J. Chem. SOC.,Perkin Trans. 1 1992 317. Enzyme Chemistry 29 1 of a-hydroxy acids of predictable stereochemistry the narrow substrate specificity of these enzymes has rather limited their application.66 OH 0 “AOMe “aoMe 76%. 98% e.e. 46%. > 97% e.e. 0 H+ iii OCOBu‘ OCOBu‘ 86%.95% e.e. Reagents i Pseudomonas sp. dehydrogenase NADH; ii Lactobacillus kejr dehydrogenase NADPH; iii Pig liver dehydrogenase NADPH Scheme 10 The range of possible substrates has been extended by the use of the enzyme from Bacillus stearothermophilus which also accepts y-monoalkyl P,y-unsaturated a-keto acids.67 Even bulkier substrates (e.g.24) can be reduced after modification of the enzyme active site by site directed mutagenesis.68 The isolation of a new (R)-2-hydroxyisocaproate dehydrogenase (R-HicDH) from Lactobacillus casei with a wide substrate selectivity should provide enantiocomple- mentary activity.69 The synthetic applicability of this enzyme was illustrated in the high yielding enantioselective reduction of the ‘parent’ substrate (25). The bugbear of the use of isolated enzymes in reductions has tended to be the need to recycle the enzyme cofactor. No universally accepted method has yet been found for NADPH regeneration while formate dehydrogenase recycling of NADH is rather limited by expense. It has been reported that both problems may be surmounted by the 66 (a)M.-J.Kim and J.Y.Kim,J. Chem.Soc. Chem. Commun. 1991,326;(6)M.-J. Kim and G. M. Whitesides J. Am. Chem. SOC.,1988 110 2959. 67 G. Casy T.V. Lee and H. Lovell Tetrahedron Lett. 1992 33 817. G. Casy T.V. Lee H. Lovell B.J. Nichols R.B. Sessions and J.J. Holbrook J. Chem. Soc.. Chem. Commun. 1992,924. 69 H. K.W. Kallwas Enzyme Microb. Technol. 1992 14 28. A. G. Sutherland B. steurothermpWus D C02H lactate. dehydrogenase CO2H 91% 99% e.e. MCOIH Lactobacillus R-HicDHcasei flC02H 88%. > 99% e.e. common sdution of non-enzymic cofactor regeneration. Thus [(C,Me,)-Rh(bpy)(H,O)]Cl can be used to catalytically regenerate either cofactor by utilizing sodium formate as a source of hydride. The efficacy of the procedure was illustrated in the stereoselective reduction of 4-phenylbutan-2-one by a range of both NADH and NADPH utilizing enzymes.70 Other Reductions.-The bakers' yeast reduction of thiophenpropenal (26) provides a neat general entry to (S)-2-methylalkanols.Reduction of both alkene and aldehyde moieties gives the corresponding alcohol in high enantiomeric excess. Subsequent Friedel-Crafts acylation Huang-Minlon reduction and Raney Nickel desulfuriz- ation/hydrogenation then leads to the required products (Scheme 1 1).71 H -40% > 98% e.e. (26) Scheme 11 Asymmetry is also induced by reduction of the alkene moiety of butenolides (27) and (28) giving the corresponding (S)and (R)enantiomers respectively in moderate yield and high optical purity (Scheme 12).72,73 The most unusual reduction of the year is the conversion of the a-chloro keto ester (29)to the corresponding dechlorinated alcohol.The halogen reduction would appear to be an enzyme catalysed process as higher substrate concentrations than 1 gl-' inhibit the dechlorination reaction although the carbonyl reduction still occurs.74 70 D. Westerhausen S. Herrmann W. Hummel and E. Steckham,Angew. Chem..Int. Ed. Engl. 1992,31,1529. 71 H. E. Hagberg E. Hedenstrom J. Fagerhag and S. Servi J. Org. Chem. 1992 57 2052. 72 K. Takabe M. Tanaka M. Sugimoto T. Yameda and H. Yoda Tetrahedron Asymmetry 1992,3 1385. 73 K. Takabe H. Hiyashi H. Sawada M. Tanaka A. Miyazaki T. Yamada T. Katagiri and H. Yoda Tetrahedron Asymmetry 1992 3 1399. 74 0.Cabon M. Larcheveque D. Buisson and R. Azerade Tetrahedron Lett.1992 33 7337. Enzyme Chemistry 4 Oxidation Reactions Cyclohexadienedio1s.-The synthesis and chemistry of the cyclohexa-3,5-dien- 1,2-diols obtained through the oxidation of arenes by mutant strains of Pseudomonas putida has been reviewed in depth.75 Most of the work in this field to date has utilized mono- or unsubstituted substrates however Hudlicky and co-workers have demonstrated that more complex systems can be biotransformed. Thus o-chlorostyrene can be converted to the diol (30) in enantiomerically pure form.76 0 0 PhCHzO -PhCH2O ,, (27) 34% 95% ex. 0 0 (28) 4 1% 99% ex. Reagents i S. cerevisiae sucrose Scheme 12 S. caevisiac * Ph HOJ" Ps.putido * (ca.l:2) Recent efforts to provide ever more direct approaches to the conduritols from these starting materials would appear to have been topped by Carless who reported the conversion of the chlorobenzene derived diol (31) to (-)-conduritol C in three steps 75 H.A. J. Carless Tetrahedron Asymmetry 1992 3. 795. 76 T. Hudlicky E.E. Boros and C.H. Boros SYNLETT 1992 391. A. G. Sutherland (Scheme 13).77 Given that a two-pot modification of this procedure is possible a shorter route would seem to be hard to find! __c OH OH OH OH OH (31) 61% 90% 70% Reagents i MCPBA; ii H,O cat. CF,CO,H; iii Na NH Scheme 13 Researchers in this field now seem to be taking on more challenging targets. The alkaloid (+)-lycoricidine (32) has been prepared in a short synthesis from the bromobenzene diol via a Heck-type coupling proced~re,~ while model studies toward morphine incorporating all of the required chiral centres have been reported.” OH 0 Sulfoxidation Reactions.-The use of commercially available enzymes to catalyse the oxidation of sulfides by hydrogen peroxide has attracted some interest.Horse radish peroxidase was found to have only limited use in terms of asymmetric synthesis. An enantiomeric excess of 68% was reported for the oxidation of p-tolyl methyl sulfide but all the other substrates examined were transformed with lower or no enan tioselectivi ty . O However the chloroperoxidase from Caldariomyces fimago displays much greater potential. Colonna and co-workers reported the oxidation of a series of aryl alkyl sulfides to the (R)-sulfoxide.B1 The majority of methyl sulfides were converted with enantiomeric excesses greater than 85% although poorer optical purities were obtained with larger alkyl groups.The main problem with this chemistry appears to be in minimizing the background non-catalysed and therefore racemic oxidation. Colonna approached this by slow 77 H. A.J. Carless J. Chem. Soc.. Chem. Commun. 1992 234. 78 T. Hudlicky and H. Olivo J. Am. Chem. SOC.,1992 114,9694. 79 T. Hudlicky C. H. Boros and E. E. Boros Synthesis 1992 174. S. Colonna N. Gaggero G. Carrea and P. Pasta J. Chem. SOC. Chem. Commun. 1992 357. S. Colonna N. Gaggero L. Casella G. Carrea and P. Pasta Tetrahedron Asymmetry 1992 3 95. Enzyme Chemistry addition of hydrogen peroxide to the enzyme and substrate.Wong and colleagues found that by adding separately both substrate and oxidant the enantioselectivity could be markedly improved for many substrates (Scheme 14).82 Reagents i C. fumago chloroperoxidase addition of H,O to substrate and enzyme (98% 91 % e. e.); ii C. fumago chloroperoxidase addition of H,O and substrate to enzyme (92% 98% e.e.) Scheme 14 Catalytic Deracemization.-Two reports have emerged which couple an enzyme catalysed enantioselective oxidation with a chemical non-selective reduction. The net outcome of these processes is the accumulation of the enantiomer of starting material that is not a substrate for the enzyme i.e. a catalytic deracemization with a theoretical yield of 100% as opposed to a kinetic resolution (maximum yield of 50%).Moiroux and co-workers performed a deracemization of lactic acid by coupling the L-lactic acid dehydrogenase oxidation of the parent substrate to pyruvic acid with an electrochemical reduction. In this manner D-lactic acid was obtained in >97% yield (Scheme 15).83 aMde NAD+ -NADH Scheme 15 Similarly Huh et al. coupled the D-amino acid oxidase catalysed conversion of proline with the sodium borohydride reduction of the resultant A’-imine to give L-proline in near quantitative yield.84 Other Oxidations.-The enantioselective Baeyer-Villiger oxidation of a series of rneso-bicyclic ketones (e.g. 33) with the isolated monooxygenase from Acinetobacter calcoaceticus has been e~plored.~’ Furstoss has investigated the similar oxidation of H.Fu H. Kondo Y.Ichikawa G.C. Look and C.-H. Wong J. Org. Chem. 1992 57 7265. 83 A.-E. Biade C. Bourdillon J.-M. Laval G. Mairesse and J. Moiroux J. Am. Chem. Soc. 1992 114,893. 84 J.W. Huh K. Yokiogawa N. Esaki and K. Soda J. Ferment. Bioeng. 1992 74 189. 8s M. J. Taschner and L. Peddada J. Chem. Soc.. Chem. Comrnun. 1992 1384. A. G. Sutherland the enantiomers of a series of monoterpenes such as dihydrocarvone (34) using a whole cell preparation of Acinetobacter (Scheme 16).86 0 (33) 70%. > 98% e.e. 0 / 95% / 66% Reagents i Acinetobacter NC1B 987 1 monooxygenase; ii Acinetobacter TD63 Scheme 16 The oxidation of methyl groups on a range of aromatic heterocycles to the corresponding carboxylic acids using Pseudornonas putida has been investigated.The oxidations are generally high yielding and show good selectivity dimethylheteroaro- matics are converted to monocarboxylic acids with no dicarboxylic analogues being detected. The process was shown to be amenable to scale-up with 2,s-dimethyl- pyrazine being converted in excellent yield on a 25 kg scale (Scheme 17).87 Ps.purida ATCC 33015 pH7 xylene > 95% Scheme 17 86 V. Alphond and R. Furstoss Tetrahedron Asymmetry 1992. 3 379. '' A. Keiner Angew. Chem.. Int. Ed. Engl. 1992 31 114. Enzyme Chemistry 5 Carbon-Carbon Bond Forming and Cleaving Reactions The past year has seen considerable activity in the isolation and use of enzymes capable of catalysing carbon-carbon bond forming reactions which although characterized in biochemical terms have seen little application in synthetic chemistry.0 OH 0 8,+ HO,),,O@ Aldolase R OH Scheme 18 As last year’s report predicted the use of the fourth stereocomplementary dihydroxyacetone phosphate utilizing aldolase (Scheme 1S) namely tagatose 1’6-bisphosphate aldolase has been reported.88 While the enzyme was shown to be of use in preparing the otherwise inaccessible sugar that is the natural substrate (3S,4S stereochemistry) non-natural aldehydes reacted to give fructose (3S,4R)stereochemis-try with only moderate diastereoselectivity. The deoxyribose-5-phosphate aldolase from Escherichia coli was found to utilize ketones as well as aldehydes as nucleophilic (‘donor’) components of the reaction.The potential of the reaction in utilizing a range of azido aldehyde ‘acceptors’ as a route to new aza-sugars was explored (Scheme 19).89 66% OH 93% Reagents E. co[i deoxyribose-5-phosphate aldose; ii Pd/C H (50psi) MeOH Scheme 19 The utility of two pyruvate aldolases 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolasegO and 4-hydroxy-2-ketoglutarate (HKG) aldolase,’ has been examined. While both enzymes are useful preparative catalysts for the natural reactions (Scheme 88 W.-D. Fessner and 0.Eyrisch Angew. Chem.. lnt. Ed. Enyl. 1992 31 56. 89 L. Chen D. P. Dumas and C. H. Wong J. Am. Chem. SOC. 1992 114 741. 90 S.T. Allen G.R. Heintzelman and E.J. Toone J. Org. Chem. 1992 57 426. 91 N.C.Floyd M. H. Liebster and N. J. Turner J. Chem. Soc. Perkin Trans. I 1992 1085 A. G.Sutherland 20)’ only KDPG aldolase was found to accept a range of aldehyde donors -HKG aldolase being highly substrate specific. ii -0 OH 0 Reagents i KDPG aldose; ii HKG aldose Scheme 20 The use of transketolase has come under greater scrutiny. The enzymes from both bakers’ yeast and spinach were examined of which the former was found to have greater appli~ability.~~ This enzyme was shown to accept a wide range of 2-hydr~xyaldehydes~**~~ to give products not readily accessible by aldolase chemistry (Scheme 21).” OH 0 OH 0 +o + HoJ(coi i+OH OH OH 60% Reagents i S. cereuisiae transketolase Scheme 21 Enzymatic carbon-carbon bond cleavage procedures are a less than obvious approach to inducing chirality in a molecule.Nonetheless Ohta has demonstrated that incubation of a range of a-substituted-a-aryl malonic acids (35) with Alcaligenes bronchosepticus results in enantioselective decarboxylation (Scheme 22).94 The reaction was shown to proceed with inversion of stereochemi~try~~ and to be intolerant of ortho-substituted aryl groups. (35) 5699%. >95% e.e. R,X = CH3 H; CH3 m-F; CH3 p-CF3; F H Scheme 22 92 Y. Kobori D. C. Myles and G. M. Whitesides J. Org. Chem. 1992 57 5899. 93 F. Efknberger V. Null and T. Ziegler Tetrahedron Letr. 1992 33 51 57. 94 K. Miyamoto S.Tsuchiya and H. Ohta J. Fluorine Chem. 1992 59 225. 95 K. Miyamoto S. Tsuchiya and H. Ohta J. Am. Chem.SOC. 1992 114 6256.