On the stereoselectivity of γ-lactol substitutions with allyl- and propargylsilanes - Synthesis of disubstituted tetrahydrofuran derivatives

Abstract

Monosubstituted γ-lactols 1a-1c, 3a-3c and 4a-4c, as well as disubstituted γ-lactol 5 and the γ-hydroxy-substituted γ-lactone 6, were transformed into disubstituted tetrahydrofuran derivatives by treatment with allyl- and propargylsilanes in the presence of Lewis acids. The diastereoselectivities were moderate to excellent and are interpreted by application of the Felkin-Anh model to cyclic oxocarbenium ions. The effects of the equilibria between conformers of the intermediates are discussed. The surprisingly high diastereoselectivity of 4-substituted γ-lactols 3 can be explained on this basis.