CLAISEN REARRANGEMENTS OF ENANTIOMERICALLY PURE C3-(ACYLOXY)-(E)-VINYLSILANES

The Ireland ester Claisen rearrangement of chiral (R)- and (S)-C3-(acyloxy)-(E)-vinylsilanes gives access to a wide range of alpha-chiral-beta-silyl-(E)-hexenoic acids with useful levels of diastereoselectivity for both the 2,3-syn and 2,3-anti diastereomers as illustrated in eqs 1 and 2. The vicinal diastereoselectivities for simple propionate esters rac-1b, (S)-1e, and (R)-1g can be varied from 1:12 to 16:1 (syn:anti) depending on reaction conditions. The employment of appropriately protected glycolate esters yields Claisen products with diastereoselectivities ranging from 23:1 (syn:anti), resulting from chelation control of enolate geometry, to 1:3.6 (syn:anti) via the use of a nonchelating, silicon-protecting group. The reaction of alpha-azido acetate (R)-1i is the first example of a Claisen rearrangement involving an alpha-azido ester and results in a new approach toward the asymmetric synthesis of alpha-amino acids. The assignment of relative stereochemistry based on the vicinal coupling constant data has been well documented.