Nucleophilic substitution of acetals

The conclusion from spectroscopic evidence that dioxocarbenium ion with an alkoxymethyl group at C5 adopts a pseudoequatorial conformation is consistent with reactivity of the related oxocarbenium ion. Lewis acid-catalyzed nucleophilic substitution of acetate 45 afforded the 1,5-trans product 46 with high diastereos-electivity (Fig. 4.18). This result is inconsistent with the product ratio (70 30) previously reported for this reaction [84] (Fig. 4.18). In that paper, a minor product observed in the unpurified reaction mixture was assumed to be a diastereomer of 46. It was later found that the minor product formed in the reaction mixture is not the cis isomer. This result can be explained by stereoelectronically controlled addition to the half-chair conformer analogous to 38 (or even 40) in which the alkoxymethyl... 

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