Davis s oxaziridine

David-Thieffiry oxidation 542 Davis s (camphorsulfonyl)-oxa-ziridine 725,728 Davis s oxaziridine 459 DCBI 697 f. 

During the highly stereoselective total synthesis of epothilone B by J.D. White and co-workers, the stereochemistry of the alcohol portion of the macrolactone was established by applying Davis s oxaziridine oxidation of a sodium enolate. ... 

Despite high yield, the Rubottom oxidation is limited by the necessity for synthesis of the requisite silane ethers. The direct oxidation of enolates has thus emerged as the preferred method for the stereoselective formation of a-hydroxy carbonyl compounds because of the method s effectiveness for both acyclic and cyclic substrates. Davis s oxaziridine reagents have proved to be ideally suited for the one-step enolate hydroxylation process. The following chiral oxaziridine reagents have been utilized effectively in this protocol and will be showcased throughout the chapter. 

The enantioselective chemical and enzymatic oxidations of sulfides [86, 94] have also received many interesting developments. High e.e. values have been obtained independently by Kagan [103,104] and Modena [105] via modified Sharpless reagents and by Davis s group [106], which used various chiral oxaziridines. 

Oxidation by chiral oxaziridines. For more than a decade, Davis s group49,71 76 has been working on the stoichiometric asymmetric oxidation of prochiral sulfides. In a series of elegant and important papers, they have demonstrated that their approach is one of the best methods in the synthesis of chiral sulfoxides. This research has yielded four generations of chiral oxaziridines 41- 44 exhibiting different stereoselectivities as a result of their dissimilar active-site structures (Fig. 5). 

R " = H, alkyl, aryl, substituted alkyl and aryl SIRs = SIMes, SiMe2(t-Bu), SIEts solvent CHoCH. pentane, toluene n = 1-3 chiral oxidant Davis chiral oxaziridine, Shi s D-fructose derived ketone/Oxone, (Salen)manganese(lll)-complexes/NaOCI or PhIO... 

Davis s group has looked at the metallation reactions of several types of N-sul-fonyl imines in order to produce new oxaziridine reagents (vide infra). For example, cyclic sulfonyl imine 268 could be converted to a mono anion using LDA, but this species could not be successfully alkylated [91] [Eq. (63)]. However, a dianion 269 formed from 268 did C-alkylate with benzyl bromide and ethylene oxide, and gave endo/exo mixtures of products 270 and 271. 

Scheme 1.21 Davis s first chiral oxaziridine for asymmetric epoxidation...

A large proportion of Davis s work has been involved in the elucidation of the transition state employed in the transfer of the oxygen from the oxaziridine to the olefin substrate. Davis favoured the planar transition state and was at the time supported by theoretical calculations however, more recent calculations favour the spiro transition state [6]. Davis has also described the asymmetric oxidation of enolate anions by chiral oxaziridines, which led to a-hydroxyketones with enanti-oselectivities of up to 95% ee [56, 57], Silyl enol ethers have also been reported to give epoxides when treated with oxaziridines, but the instability of these compounds is too great to allow isolation [37,58,59], To date, only Davis has reported successful isolation of a-silyloxy epoxides [60],... 

Davis, F. A., Harakal, M. E., Awad, S. B. Chemistry of oxaziridines. 4. Asymmetric epoxidation of unfunctionalized alkenes using chiral 2-sulfonyloxaziridines evidence for a planar transition state geometry. J. Am. Chem. Soc. 1983, 105, 3123-3126. 

Davis, F. A., Hague, M. S. Oxygen-transfer reactions of oxaziridines. Adv. Oxygenated Processes 1990, 2, 61-116. 

Asymmetric hydroxylation of etiolates. Davis and Chen1 have reviewed this reaction using in particular (R,R)- and (S,S)-2-phenylsulfonyl)-3-phcnyloxaziridene (1) and (camphorylsulfonyl)oxaziridine (2). Of these reagents, 1 and ( + )- and (—)-2, derived from (lR)-lO-camphorsulfonic acid, provide highest enantioselectivity and in addition are easy to prepare. They are effective for hydroxylatation of ketones, esters, /2-keto esters, amides, lactones, and lactams. 

Davis and coworkers have recently found that asymmetric oxidation of sulfides with (+ )-(camphorsulfonyl)oxaziridine (58) affords sulfoxides with 8-73% enantiomeric excess. In contrast, (— )-a,a-(dichlorocamphorsulfonyl)oxaziridine (59) in CHC13 affords uniformly high stereoselectivity (66-95% eeof(S)-configuration of sulfoxides). The solvent... 

Davis et al. used a chiral oxaziridine for the asymmetric oxidation of sulfides to sulfoxides.Oxidation of isopropyl- -tolyl sulfide (427) with oxaziridine 428, for example, gave 60.3% ee (S) of 429 (at -78°C in chloroform).5 0 The absolute configuration of the sulfide is determined by approach of the sulfide to the oxaziridine oxygen, as illustrated by 430. Steric factors appear to be the primarily reason for the chiral recognition. 5 1 In this model, attack by sulfur minimizes the Rl and Rs interactions with the oxaziridine... 

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