Involvement of an Oxonium Ion Intermediate in Controlling the Diastereoselectivity of Nucleophilic Substitution Reactions of Septanoses.

The alkoxy substituents at C4 and C2 of septanoses control the stereochemical outcomes of -glycosylation reactions of these seven-membered-ring intermediates. Isolation of a bicyclic acetal byproduct in some substitution reactions suggests that the C4 benzyloxy substituent engaged in long-range participation, stabilizing intermediates by the formation of an oxonium ion intermediate. Inductive destabilization of the carbocationic intermediate provided by the C2 substituent is crucial to the participation of the remote alkoxy group.