Nonracemic oxaziridines, Davis oxaziridine reagents

Further examples of acyclic stereocontrol in related amino acid systems make use of chiral and nonracemic oxaziridine reagents to induce high levels of stereocontrol. In 1992, Davis and coworkers synthesized the methyl ester of the Taxol C13 side chain using this method.30 Following enolization, the dianion of 44 was reacted with the Davis reagent 5 to yield the a-hydroxy (5-amino acid 45 in 49% yield. While the yield was marginal in this particular example, the 86 14 ratio of stereoisomers produced is impressive in this acyclic system. 

Davis oxaziridine reagents such as 1 have exhibited ample synthetic utility as oxidizing agents for the hydroxylation of enolates to provide a-hydroxy carbonyl compounds, such as 2 with superb yield. When the oxaziridine is chiral and nonracemic, the hydroxylation has been shown to proceed with high stereoselectivity.1... 

Chiral auxiliaries have found abundant use in providing a template for efficient and highly diastereoselective enolate reactions. Concurrent with the development of chiral and nonracemic Davis oxaziridine reagents, the use of chiral auxiliaries to direct the stereoselectivity in these systems has been generalized. 

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... 

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