Diastereoselective Substitution Reactions of Acyclic Acetals Controlled by Remote Participation of an Acyloxy Group.

A remote carbonyl group up to six atoms away from the acetal group can induce 1,2-asymmetric induction in nucleophilic substitution reactions of acyclic acetals. Isolation of a cyclic carbonate under Lewis acidic conditions and computational studies suggested that the remote carbonyl group participated through the formation of a cyclic dioxocarbenium ion intermediate. The stereochemical outcomes depended on the size of the alkyl substituent and that of the nucleophile employed.

Acetal Substitution Reactions: Stereoelectronic Effects, Conformational Analysis, Reactivity vs. Selectivity, and Neighboring-Group Participation.

Acetal substitution reactions can proceed by a number of mechanisms, but oxocarbenium ion intermediates are involved in many of these reactions. Our research has focused on understanding the conformational preferences, structures, and reactions of these intermediates. This Account summarizes our observations that electrostatic effects play a significant role in defining the preferred conformations, and that torsional effects determine how those intermediates react.

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.