Hydroboration of Enol Acetates

The directive effect of an acetoxy substituent in the hydroboration of olefins was examined. Cyclohexenyl acetate and 2-cholesten-3-ol acetate (7) yielded the expected diols, but in addition a considerable amount of monols was obtained. Cyclopentenyl acetate and 2-cholesten-3-ol benzoate gave only monols and none of the expected diol products. ∆¹⁶-17-Acetoxy steroid 6 gave a trans diol on hydroboration. The yield of monol from cyclohexenyl acetate was raised with increasing temperature and increasing amount of diborane. Pyridine borane-aluminum chloride gave solely cyclohexanol. It was shown that the monol derived from 7 did not result from an elimination-rehydroboration reaction. Deuterium-labeling experiments were performed and the reaction mechanism is discussed.