HIGH DIASTEREOFACIAL SELECTIVITY IN THE CONJUGATE ADDITION OF PHENYLMAGNESIUM BROMIDE TO ACYCLIC ALPHA-ENONES WITH AN ASYMMETRIC CARBON AT THE GAMMA-POSITION

Moderately high diastereofacial preference has been observed in the conjugate addition of phenylmagesium bromide to the acyclic alpha-enones with an asymmetric carbon at the gamma-position (4E,6RS)-2,2,7,7-tetramethyl-6-phenyloct-4-en-3-one (1), (2E,4RS)-5,5-dimethyl-1,4-diphenylhex-2-en-1-one (2), and (4E,6RS)-2,2-dimethyl-6-phenylhept-4-en-3-one (3). The RR,SS selectivity is practically unaffected by the addition of copper(1) salts. These results are in qualitative agreement with the selectivity predicted by a Felkin-like transition state model. The method is shown to be useful for the stereoselective preparation of acyclic ketones having two chiral centres.