Chiral Synthesis via Organoboranes. 47. Efficient Synthesis of Unsymmetrical Ketones and Enantiomerically Pure Spiroketals Using (±)-Isopinocampheyldichloroborane

Readily prepared and stable (±)-isopinocampheyldichloroborane [(±)-IpcBCl₂] was conveniently used for the stepwise hydroboration of two different alkenes using the in situ reduction−hydroboration protocol to give mixed trialkylboranes, IpcBR¹R². Convenient elimination of α-pinene from these trialkylboranes by treatment with an aldehyde, RCHO, provided the borinate ester, R¹R²BOCH₂R. This intermediate was readily converted into the unsymmetrical ketones, R¹COR², in high yields and purity, by an established method. This methodology was successfully applied to the synthesis of enantiomerically pure spiroketals using optically pure TBS ether protected homoallylic alcohols as the alkenes for stepwise hydroboration.