Enantioselectivity Davis oxaziridine reagents

Several examples exist in the literature in which cyclic ketone enolates are enantioselectively hydroxylated by chiral, nonracemic Davis oxaziridine reagents. In contrast to their acyclic counterpart, the enolate geometry is fixed in cyclic systems. During the preparation of enantiomerically pure (-)-blebbistatin, the enolate of the quinolone 61 was reacted with the Davis reagent 5 to afford the optically enriched 62 with 82% yield and 86% ee.54 The related reagent 6 was used in the synthesis of (-t-)-o-trimethylbrazilin, which was... 

Enantioselective oxidation using Davis chiral oxaziridine reagents is not as well developed as its diastereoselective counterpart. However, a number of simple enolates have been... 

The utility of Davis chiral oxaziridine reagents has been more recently applied to the synthesis of optically active a-hydroxy phosphonates. Two groups have been largely responsible for developments in this area. Principally, Wiemer and co-workers have demonstrated the highly enantioselective hydroxylation of a series of benzyl phosphonates. As shown below for 69, hydroxylation makes use of oxaziridine 6, proceeding in moderate... 

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. 

Acetalization of thiochroman-3-one gives a 1 1 diastereomeric mixture and subsequent oxidation with Davis reagent, W-(phenylsulfonyl)(3,3-dichlorocamphoryl)oxaziridine, yielded the sulfoxides each with a 4 1 enantioselectivity. Chiral chromatographic separation of the diastereomers preceded isolation of the major enantiomers. (3-Elimination and isomerization of the double bond then produced the individual thiochromene 1-oxide diastereomers. The generation of an a-sulfinyl carbanion effects the cleavage of one of the acetal C-O bonds with the protected diol released in a final ozonolysis step. The stereochemical results indicate that it is the C-O bond syn to the sulfoxide function that is cleaved (Scheme 63) <1996TA29>. 

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. 

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