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