96 research outputs found
Stereoelectronic Model To Explain Highly Stereoselective Reactions of Seven-Membered-Ring Oxocarbenium-Ion Intermediates
Nucleophilic attack on seven-membered-ring oxocarbenium ions is generally highly stereoselective. The preferred mode of nucleophilic attack forms the product in a conformation that minimizes transannular interactions, thus leading to different stereoselectivity as compared to that of reactions involving six-membered-ring oxocarbenium ions
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
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