Diastereoselective cyclopropanations of alpha,beta-unsaturated ketals derived from chiral hydrobenzoin

Abstract

2-Cycloalken-1-one-(-)-erythro-hydrobenzoin ketals undergo efficient and diastereoselective cyclopropanation when treated with an excess of Simmons-Smith reagent. The resulting crystalline products yield enantiomerically pure cyclopropyl compounds upon recrystallization. For example, 2-cyclohexen-1-one-(-)-erythro-hydrobenzoin ketal gave, in 90% yield, a 19:1 mixture of diastereomeric cyclopropanes as established by 62.9 MHz ¹³C NMR spectroscopy. Upon one recrystallization, diastereomerically pure cyclopropane was obtained in 78% recovery. Hydrolysis gave (+)-norcaranone of greater than 99% ee. Three other examples are presented which demonstrate the advantages of hydrobenzoin ketals over previously used homochiral ketals, as well as several techniques for the synthesis of chiral hydrobenzoin.