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 O-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.