ALPHA-HYDROXY ESTERS AS CHIRAL AUXILIARIES IN ASYMMETRIC CYCLOPROPANATIONS BY RHODIUM(II)-STABILIZED VINYLCARBENOIDS

The use of several alpha-hydroxy esters as chiral auxiliaries for asym metric cyclopropanation with rhodium-(II)-stabilized vinylcarbenoids i s presented. Use of either (R)-pantolactone or (S)-lactate allowed ent ry into both series of enantiomeric vinylcyclopropanes with predictabl e absolute stereochemistry. Steric and electronic modifications of the chiral auxiliary as well as catalyst structure were shown to have maj or effects on the asymmetric induction. These results were rationalize d on the basis of an interaction between the carbonyl oxygen of the ch iral auxiliary and the carbenoid carbon. By combining the asymmetric c yclopropanation with a subsequent Cope rearrangement, an enantioselect ive entry into hydroazulenes was achieved. The potential of the asymme tric cyclopropanation was illustrated by a short synthesis of (1R,2R)- 2-phenylcyclopropane amino acid 5.