| art A. Several optically active thiolanes were synthesized and converted into sulfonium salts. These were subsequently transformed into the corresponding sulfur ylides which in turn reacted with carbonyl compounds to produce optically active epoxides. trans-2,3-Diaryloxiranes have been obtained in up to 83% enantiomeric excess via the reaction of S-benzyl ylides prepared from optically active trans-2,5-disubstituted thiolanes. The complete stereofacial transfer of a benzylidene group to a substituted benzaldehyde was successfully accomplished using S-benzyl ylide derived from (1R,2S,3R,5S)-2,3-dibenzyl-1,8,8-trimethyl-3-thianiumbicyclo (3.2.1) octane perchlorate 123 (prepared from 1R,3S-(+)-camphoric acid). trans-Stilbene oxides were produced with ee's approaching enantiomeric purity. These results represent the first example of the preparation of epoxides in nearly enantiomerically pure form by a sequence which involves both a carbon-carbon and a carbon-oxygen bond rather than only carbon-oxygen bond formation. A rationale for the high chiral induction in the latter case based on the control of facial selectivity at the ylide will be discussed. The facial selectivity at the carbonyl is believed to play a minor role. The design of potentially more efficient chiral sulfide auxiliaries and their use in the synthesis of epoxide is also presented.;Part B. Erythro- and threo-1,2-diphenyl-2-butylsulfinylethanol 154 and 158 were prepared from the condensation of the... |
