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Asymmetric Hydroxylation of Enolates with Nsuifonyloxaziridines

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Asymmetric Hydroxylation of Enolates with Nsuifonyloxaziridines Chsm. Rev. 1992, 92, 919-934 919 Asymmetric Hydroxylation of Enolates with NSuifonyloxaziridines Franklin A. Davis' and BangChi Chen oepam01 "fsby. mx.91 m-, mnadelphlb. ~,WYAJ.WI~ 19104 Recsfvd November 8. 199 1 (Flevked Msnusuipt Recsfvd Fsbruary 8. 1992 ) Contents I. Introduction 919 'r 11. Reagents 920 4 f Ill. Mechanism of Oxygen Transfer 921 IV. Transition-State Models 921 V. Asymmetric Hydroxylation of Enolates 922 A. Substrate-Induced Hydroxylations 922 1. Ketones 922 2. Esters 922 ,*"' 3. Lactones 923 :L E. Auxlllary-Induced Hydroxylations 923 1. Ketones 923 2. Aldehydes 923 3. Esters 924 Franklin A. Davis was born in Des Moines. Iowa. He recelved his 4. AmMes 924 B.S. degree in 1962 from the University of WiScOnsln and was granted e Ph.D. degree in 1966 from Syracuse University working 5. CarboximMes 924 under Donald C. Dimer. After two years with Michael J. S. Dewar C. Reagent-Induced Hydroxylations 925 as a Welch PoSMoctoral Fellow at the University of Texas he joined 1. Ketones 925 the facuity at Drexel University in 1968 where he is currently the George S. Sasin Professor of Organic Chemistry. In 1980 he 2. Esters 926 received Drexel University's Research Achievement Award and in 3. Amides 927 1982 the PhiladelDhia ACS Section Award. Dr. Davis' research Interests lie in the area of asymmetric synthesis. molecular 4. Lactones 927 recognMon.and new syntheticmethodspartkularlyrelatedtoenalF 5. Enones 927 tioselective oxidations. me development of new methods for the D. Double Stereodifferentlation 927 regio- and stereoselective fluorination of organic molecules is a recent focus VI. Synthesis of Natural Products 928 A. DlastereOSelectke Hydroxylations 928 1. Ketones 928 2. a.8-Unsaturated Esters 929 3. Lactones 930 4. Lactams 930 5. Carboxlmldes 930 8. fl-Dicarbonyls 931 E. Enantioselectke Hydroxylations 931 1. Ketones 931 2. &Keto Esters 931 VII. Summary 932 VIII. Acknowledgments 932 IX. References 933 BangChl Chen was born in the remote countryslde of Zhejlang Province, PRC in 1964. He received his BSc. degree In chemlsby I. Introductlon In 1984 from Hangzhou University where he was an Instructw. The availability of efficient methods for the con- teaching undergraduate organic chemistry foliowlng graduatkm. During thh time period he conducted research wim Xlan Huang. enantiopure carbonyl struction of a-hydroxy com- In 1987 he began graduate study at Drexel University and. in the pounds [RR'C(OH)C(O)Zl is of considerable current following year, thesis research under the guidance of Frankiln A. interestbecause thisstructuralarray is featuredinmany Davis, Heobtainedthe M.Sc.degreein 1989andthePh.D.in 1991 biologically relevant molecules. Compounds containing and, In #a same year, joined the BrlstoCMeyers Squibb Company In Syracuse New York. He has aUthored and/or co-authored 28 this moiety are also useful auxiliaries' and synthons2 research and review articles. for the asymmetric synthesis of natural products including antitumor agents, antibiotics, pheromones, to the stereogenic carbon because biological activity is and sugars. Of particular concern in these structures often critically dependent upon ita orientation. The is the stereochemistry of the hydroxy group attached significance of the a-hydroxy carbonyl unit in bioac- 0009-2665~92/07926919$10.00~0 0 1992 Amwlcan Chemical Society 920 Chemical Reviews, 1992, Vol. 92, No. 5 Davls and Chen Scheme I. Strategies for the Preparation of versatile method for the preparation of this structural a-Hydroxy Carbonyl Compounds array. Furthermore both enantiomeric forms of a-hy- droxy carbonyl compounds are available by use of the RkR' vmol' RqR' (1) antipodal oxidizing reagents. By using aprotic oxidants 0 0 such as dioxygen,13 dibenzyl pero~ydicarbonate,'~ MoOPH,l5J6 or MoOPD17 to effect the oxidation product, diastereoselectivity is induced by stereodi- Rd\rlR' IR"l+_ RTR' (2) recting groups or chiral auxiliaries in the enolate. 0 0 In 1984, Davis and co-workers18 introduced racemic trans-(A) -2- (phenylsulfonyl)-3-phenyloxaziridine(la) l9 for the direct hydroxylation of enolates. The avail- ability of enantiopure N-sulfonyloxaziridines such as ldY2O 2c,212d,21 and 322-26for the asymmetric hy- droxylation of enolates makes possible control of ste- reoselectivity by the reagent. This article is intended to summarize progress in the asymmetric hydroxyla- tion of enolates using N-sulfonyloxaziridines, with particular emphasis on their applications in natural Scheme 11. Asymmetric Oxidation of Enolates with product synthesis. Both diastereoselective and enan- N-Sulfonyloxaziridines tioselectivehydroxylations of enolates by these reagents will be covered. Some aspects of this topic have been reviewed el~ewhere.~'-~~ 0 I 6) q, Y so* 0 R $6 PI A 2 3 tive compounds and its remarkable versatility as a chi- ral synthon and auxiliary has stimulated the develop- R R Ar 01 R ment of many methods for its synthesis. These methods la Ph Ph 2a 3a H H differ mainly in the substrates used (i.e. the oxidation lb p-MePh Ph 2b Ph 3b CI H state of the carbon atom adjacent to the carbonyl group) IC p-MePh o-MePh 3c McO H and can be categorized as nonoxidative (eqs 1-3) and 3d H Bn 2c + oxidative (eq 4) procedures (Scheme I). 3e H p-MeOBn The classical method for the preparation of optically 3f H p-CF3Bn active a-hydroxy carbonyl compounds involves a sub- stitution reaction using optically active a-amino acids (L = NH2)3and a-halo amides (L = C1, Br)4as starting le Of$ 2-CI-5-@NPh materials (eq 1). Homologation is another approach in which chiral auxiliaries and stereodirecting groups, incorporated into the substrate, are used to induce di- 11. Reagents astereoselectivity (eq 2).5 Addition of hydride or a car- banion to a-dicarbonyl compounds (or their monoketal N-Sulfonyloxaziridines are prepared by the biphasic derivatives) has also been used to prepare chiral a- basic oxidation of the corresponding sulfonimine with hydroxy carbonyl compounds (eq 3). Either chiral n-CPBA or 0x0ne.l~ Oxidation of acyclic chiral sul- reducing reagents! chiral reducing catalysts? chiral or- fonimines affords mixtures of oxaziridine diastereoi- ganometallic reagents,8or chiral auxiliariesghave been somers 1, requiring separation by crystallization (eq employed to induce stereoselectivity. Chiral additives 5).20 Similar results are observed on oxidation of the have also been used in some of these transformations.lO sulfonimines corresponding to 2 which limits their Generally these nonoxidative procedures are limited production on a large scale.21 In contrast the (cam- to the synthesis of acyclic derivatives. phorsulfony1)oxaziridinederivatives 3 [tetrahydro-9,9- Two oxidative methods have been developed for the dimethyl-4H-4a,7-methanooxazirino[3,2-i] [2,1] benz- synthesis of a-hydroxy carbonyl compounds. One, isothiazole 3,3-dioxidel are readily available on multi- known as the Rubottom reaction, requires the prefor- gram and kilogram scales because the endo face of the mation of a silyl enol ether from the carbonyl compound. C-N double bond in 4 is blocked, affording on oxidation The silyl enol ether is then oxidized with reagents such a single oxaziridine isomer (eq 6).22 (Camphorylsul- as m-chloroperbenzoicacid (m-CPBA)llor N-sulfonyl- fony1)oxaziridine(3a) and (8,8-dichlorocamphorsulfo- oxaziridines.12 The past several years have seen the ny1)oxaziridine (3b) [8,8-dichlorotetrahydro-9,9-dim- emergence of the second method, the direct oxidation ethyl-4H-4a,7-methanooxazirino[3,2-i][2,11 benziso- of enolates, as the most widely used method for the thiazole 3,3-dioxidel are commercially available from stereoselective synthesis of the a-hydroxy carbonyl Aldrich Chemical Co. and Fluka, respectively. array (Scheme 11). This protocol affords the target Sulfonimines relating to oxaziridines 1 and 2 are functionality directly and is effective for both acyclic prepared by condensing sulfonamides with aromatic and cyclic substrates. The great diversity of metal eno- aldehydeslg and by reacting lithium reagents with late types makes their oxidation potentially the most saccharin, respectively.21 The (-)-(camphorsulfony1)- Asymmetrlc Hydroxylation of Enolates Chemical Reviews, 1992, Vol. 92, No. 5 921 Scheme 111, Synthesis of the (Camphorsulfony1)- imine Derivatives m-CPBA RS&N=CHAr 4 3 a) X=Y=H, b) X=C1, Y=H, c) X=OMe. Y=H d) X=H, Y=p-2-Bn 4c I- 6 4d imine 4a is the common intermediate for the prepa- ration of the (camphorsulfony1)imine derivatives 4 Scheme IV. General Mechanism for Enolate (Scheme 111) and is prepared in 90 '7% overall yield from Oxidation with N-Sulfonyloxaziridine (1s)-(+)-10-camphorsulfonic acid.22 The (7,7-dichlo- 0 M+ rocamphorsulfony1)imine 4b arises via halogenation of azaenolate 5.23 Carbon electrophiles generally react with 5 at the azaenolate nitrogen atom to give enam- ines which are hydrolytically unstable.25 However, di- anion 6 undergoes smooth monoalkylation a to the sulfonyl moiety to give 1:l mixtures of exo- and endo- sulfonimines 4d.25326 With selenium dioxide 4a gives the (-)-(3-oxocamphorsulfonyl)imine 7 which is trans- formed to the (7,7-dimethoxycamphorsulfonyl)imine 4c in 70% overall yield.24 The antipodal reagents are .A("' available starting from (1R)-(-)-10-camphorsulfonic 0 acid. 10 I I I. Mechanism of Oxygen Transfer aziridines (Scheme IV).S8 The enolate anion attacks at the oxaziridine oxygen atom to give hemiaminal in- Theoretical3135and e~perimental~~studies have sug- termediate 8 which fragments to the sulfonimine 9 and gested an sN2type mechanism for the transfer
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