STEREOSPECIFIC PREPARATION OF TRISUBSTITUTED ALLYLIC ALCOHOLS BY ALKENE TRANSFER FROM BORON TO ZINC

(Z)-Trialkenylboranes, (R1HC = CR2)3B, easily prepared by hydroboration of the corresponding alkynes with BMS in THF, rapidly react with commercially available diethylzinc in hydrocarbon solvents, in either a 1:3 or 2:3 ratio of organoborane to organozinc, to yield an equilibrium mixture, favoring the migration of alkenyl groups from boron to zinc. Addition of an aldehyde to the 2:3 reaction mixture, in the presence of 10 mol % N-methylpiperidine, results in the exclusive transfer of both alkenyl groups from zinc, furnishing diastereomerically pure trisubstituted double bonds in allylic alcohols in good to excellent yields.