To complete our studies, we examined the influence of
substitution adjacent to the acetal carbon. An alkyl substituent
confers
little stereochemical differentiation between the two sides
of the intermediate oxocarbenium ion (eq 6), consistent with
observations on 2-methyl-substituted tetrahydrofuran acetals.22 The
reaction of the 2-alkoxy acetal 13b, on the other hand, was cisselective.
23