Diastereoselective allylations and crotylations under phase-transfer conditions using trifluoroborate salts: an application to the total synthesis of (−)-tetrahydrolipstatin

A mild protocol for diastereoselective allylations and crotylations of α- and β-silyloxy substituted aldehydes utilizing potassium allyl/crotyltrifluoroborates is described. The reactions proceed to completion within 30 min at room temperature in biphasic or aqueous media in the presence of a phase transfer catalyst. The resulting homoallylic alcohols are obtained in high yields and moderate to excellent diastereoselectivities. The method was applied to the asymmetric total synthesis of the antiobesity agent tetrahydrolipstatin (orlistat™).