Cyclization vs. cyclization/dimerization in o-amidostilbene radical cation cascade reactions: the amide question

Electrochemically Mediated Oxidative Transformations of Substituted 4-Methoxystilbenes: Effect of Ortho-Substituted Nucleophilic Groups

A systematic study was undertaken to determine the influence of ortho'-substituted nucleophilic groups (OH, NH₂, or NHR) on the reactivity of anodically generated 4-methoxy- and 3,4-dimethoxystilbene cation radicals. The results showed that when ortho-substituted nucleophilic groups such as OH and NHR are present in the other ring, both direct and crossover intramolecular cation-nucleophile reactions occur to give bisbenzofurans/bisindoles or fused bisbenzopyrans/bisquinolines, respectively. Where an additional 3-methoxy substituent is present, bridged oxocine/azocine products are formed in addition to the bisbenzopyrans/bisquinolines and bisbenzofurans/bisindoles. Mechanistic rationalization of the observed behavior is presented based on a generalized pathway involving fast cation radical dimerization following electron transfer, followed by direct and crossover trapping of the benzylic cations by the ortho-substituted oxygen and nitrogen nucleophilic groups. In the instances where an additional 3-methoxy group is present, the bridged oxocine/azocine products are also formed as a result of competing aromatic substitution (Friedel-Crafts reaction). The results have shed further light and provided additional clarification on the reactivity of anodically generated stilbene cation radicals.

The radical cation mediated cleavage of catharanthine leading to the vinblastine type alkaloids: implications for total synthesis and drug design

Comparison of the Boger and Kutney mechanistic interpretations for the FeCl₃ promoted oxidative coupling of vindoline and catharanthine.