Highly Diastereoselective Intramolecular Allylation Reactions of Mixed Silyl-Substituted Acetals

The reaction of preformed mixed acetals derived from (α-hydroxyalkyl)dimethylallylsilane with a number of aromatic and aliphatic aldehydes in the presence of Lewis acids results in a highly diastereoselective intramolecular allylation reaction. The reaction proceeds through a cyclic synclinal SE‘ addition of the allylsilane to an intermediate oxocarbenium ion. The reaction occurs exclusively by an intramolecular process as determined by means of a cross-over experiment. The relative stereochemistry was determined by the conversion of one of the allylation products to a known (stereodefined) aldol-type product. A greater degree of diastereoselectivity is obtained by in-situ formation of an oxocarbenium ion from (α-hydroxyhexyl)dimethylallylsilane and an aldehyde in the presence of boron trifluoride etherate. The diastereoselectivity of the in-situ allylation reaction typically exceeds 100:1 in favor of the syn adduct. However, reactions with electron rich aryl aldehydes resulted in a diminished degree of d...