Highly Diastereoselective Intramolecular Allylation Reactions of Mixed Silyl-Substituted Acetals

Abstract

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 diastereoselectivity. The initial product of the in-situ reaction is an unstable silyl fluoride 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 highly substituted (α-alkoxyalkyl)dimethylallylsilanes by allyl anion displacement of methoxide from silicon is also described. The methyl siloxane derivatives were obtained by ozonolytic cleavage of an unsubstituted allyl group in methanol.

Tandem Intramolecular Silylformylation−Allylsilylation

“…Examples of intramolecular silicon-tethered allylsilane additions to aldehydes, acetoxyazetidinones, and acetals have been reported, but require the use of external Lewis acids. There has been one report of uncatalyzed additions, both inter- and intramolecular, of tetracoordinate allylsilanes to aldehydes and ketones based on the use of strained allylsilacyclobutanes .…”

The Modified Sakurai and Related Reactions

“…Finally, Rychnovsky and Cossrow [33] preferred to use optically pure a-trimethylsilylbenzylether 64 as the chiral auxiliary. In this case, the oxocarbenium intermediate 64b adopts a well-defined conformation, proposed initially by Linderman [34], which provides the maximum b-silyl-effect. The nucleophile then approaches from the opposite side of the TMS group (Scheme 13.27).…”

Design and Evaluation of Nonfluorous CO2-Soluble Oligomers and Polymers

“…Synthesis Procedure for PVMME. 43 A 100 mL, three-necked, round-bottomed flask was equipped with a magnetic stirring bar. The flask was charged with 25 mL DMSO and PVA (0.7 g, 10 mmol in -OH).…”