Preparation of Monosilyl Acetals from Esters via iBu2AlH Reduction and Trapping with N-(Trimethylsilyl)imidazole. Addition of Allyltrimethylsilane To Yield Homoallylic Alcohols or Ethers

Article abstract of DOI:10.1021/jo00112a039

Alkyl esters were reduced with iBu2AlH or a 1:1 mixture of iBu2AlH and iBu3Al (iBu2AlH*iBu3Al or iBu5Al2H), followed by trapping of the resulting tetrahedral intermediate with TMS-imidazole to produce monosilyl acetals.Reaction of the mixed acetals with allyltrimethylsilane in the presence of Lewis acids (Hosomi-Sakurai reaction) generated homoallylic alcohols or ethers selectively, depending on the substitution of the monosilyl acetal.TMS methoxy acetals (MeO-CH-OTMS) and TMS ethoxy acetals bearing additional complexing groups such as MeO- or Ph2C=N- provided alcohols with 1.5:1 - 9:1 threoselectivity, while simple TMS ethoxy acetals provided only ethyl ethers as products.The monosilyl acetal configuration was easily epimerized or racemized, and the configuration of the Hosomi-Sakurai product was apparently independent of the initial monosilyl acetal reactant configuration.