A NEW PROTOCOL FOR REGIOCONTROLLED AND STEREOCONTROLLED ALDOL REACTIONS THROUGH THE CONJUGATE ADDITION OF DIALKYLBORANES TO ALPHA,BETA-UNSATURATED KETONES

A one-pot, two-step procedure, consisting of the 1,4-addition of dialkylboranes to beta-substituted (E)-alpha,beta-unsaturated ketones followed by the reaction of the resulting configurationally pure (Z)-(vinyloxy)boranes with aldehydes, is reported. The overall process corresponds to a regio- and stereocontrolled aldol addition of an unsymmetrical ketone to an aldehyde. A concerted 1,4-addition mechanism accounts for the stereochemical outcome of the hydroboraton reaction; cyclic enones do not undergo conjugate addition, while (Z)-beta-substituted or beta,beta-disubstituted alpha,beta-unsaturated ketones still react in a 1,4-fashion, but with a slower rate and a lower degree of chemoselectivity with respect to beta-substituted (E)-alpha,beta-unsaturated ketones. In the cases of alpha,beta-disubstituted alpha,beta-unsaturated ketones and (E)-(S-phenylthio)cinnammate, which react with dicyclohexylborane to give a mixture of E and Z enolates, an alternative mechanism is proposed.