Asymmetric reduction of prochiral cycloalkenones. The influence of exocyclic alkene geometry

The asymmetric reduction of a series of prochiral enones of general structu re 1 using the Corey oxazaborolidine 2, leading to enantiomerically enriche d allylic cycloalkanols 3 is described. The influence of alkene geometry on both the sense (R vs. S) and efficiency (% ee) of the asymmetric reduction process has been probed for two systems, (E)- and (Z)-4 and (E)- and (Z)-7 , based on cyclohexanone and cyclopentanone respectively. The absolute ster eochemistry of the cyclopentyl derivative (E)-8 has been established by X-r ay crystallographic analysis of carbamate 10. The ability to assign an abso lute configuration to allylic alcohols 3, based on the NMR methods describe d earlier by Riguera, has been evaluated.