PREPARATION OF ENANTIOMERICALLY ENRICHED (2R,3R)-TRANS-2,3-DIARYLOXIRANES OR (2S,3S)-TRANS-2,3-DIARYLOXIRANES VIA CAMPHOR-DERIVED SULFONIUMYLIDES

Easily available D-(+)-camphor-derived sulfides 3, 4, 6, and 7 were em ployed for enantioselective epoxidation via an ylide route. When benzy lated or methylated sulfides were used as reagents or mediators for be nzylidene transfer, stoichiometric and catalytic epoxidations were rea lized, respectively. Opposite asymmetric induction was achieved only w hen sulfides containing exo- (3 and 4) and endo- (6 and 7) alkylthio g roups were used. That is, both (+)- and (-)-trans-diaryloxiranes could be obtained in excellent yields and moderate to good ee values under extremely mild conditions from the same chiral pool-derived reagents. A nonbonded interaction between the free OH in the ylides from sulfide s (3, 6, and 7) and the carbonyl group of aldehydes controls the appro ach of the substrates to the ylidic carbon preferentially at one speci fied face and therefore leads to a more efficient asymmetric induction than that in the case of the ylide from methyl-protected hydroxylated sulfides 4, which cannot cause such an interaction. The same opposite asymmetric induction was also observed in the catalytic reaction with methyl-protected hydroxylated sulfide 4b and unprotected hydroxylated sulfide 3b.