HIGHLY DIASTEREOSELECTIVE INTRAMOLECULAR ALLYLATION REACTIONS OF MIXED SILYL-SUBSTITUTED ACETALS

The reaction of preformed mixed acetals derived from (alpha-hydroxyalk yl)dimethylallylsilane with a number of aromatic and aliphatic aldehyd es in the presence of Lewis acids results in a highly diastereoselecti ve intramolecular allylation reaction. The reaction proceeds through a cyclic synclinal S-E' 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 relati ve stereochemistry was determined by the conversion of one of the ally lation products to a known (stereodefined) aldol-type product. A great er degree of diastereoselectivity is obtained by in-situ formation of an oxocarbenium ion from (alpha-hydroxyhexyl)dimethylallylsilane and a n aldehyde in the presence of boron trifluoride etherate. The diastere oselectivity of the in-situ allylation reaction typically exceeds 100: 1 in favor of the syn adduct. However, reactions with electron rich ar yl aldehydes resulted in a diminished degree of diastereoselectivity. The initial product of the in-situ reaction is an unstable silyl fluor ide which is readily hydrolyzed to a silanol derivative upon reaction with methanolic potassium hydroxide. The overall yield of the two-step process is greater than 80%. A method for the synthesis of more highl y substituted (alpha-alkoxyalkyl)dimethylallylsilanes by allyl anion d isplacement of methoxide from silicon is also described. The methyl si loxane derivatives were obtained by ozonolytic cleavage of an unsubsti tuted allyl group in methanol.