Protecting Group-Dependent Synthesis of Densely Substituted Dihydropyrroles v/s Pyrroles via 5-Exo-trig Cascade Radical Cyclization to Alkynyl Vinylogous Carbamates

Abstract

Densely substituted dihydropyrroles could be synthesized with excellent diastereoselectivity via 5-exo-trig cascade radical cyclization to alkynyl vinylogous carbamates. N-Alkyl/acyl protected alkynyl vinylogous carbamates upon radical cyclization using thiophenol gave substituted pyrroles as against dihydropyrroles, which were formed with N-sulfonyl protecting groups. This enabled a rare example wherein both dihydropyrrole and pyrrole rings are assembled in the same reaction. This strategy could be used for the synthesis of an unprecedented adjacent polyheterocyclic system having a furan-thiophene-pyrrole motif. When vinylogous carbamate is embedded in the isoindole moiety, a pyridoisoindole derivative was formed with excellent diastereoselectivity, instead of the expected pyrroloisoindole product.

Modular Synthesis of Tetrasubstituted Pyrroles through a Four-Component Cyclization Strategy Using Ammonium Salt as the Nitrogen Source

“…Some convergent reaction strategies were also used to synthesize pyrroles, such as multicomponent reactions, electrochemical synthesis, photocatalyzed reactions, and transition-metal-catalyzed reactions . Despite significant efforts that have been devoted to the synthesis of pyrrole skeletons, efficient construction of all-carbon substituted pyrroles (tetrasubstituted pyrroles) is still appealing, especially using easily accessible starting materials, low-cost catalysts, and simple operation procedures …”

Cascade radical cyclization on 3-propargyl-2-alkenyl indole gives stereoselective access to cyclohepta[b]indole over carbazole

“…It was only recently that Yang disclosed a synthesis of cyclohepta[ b ]indole via visible-light-induced photocatalytic [2+2] cycloaddition/retro-Mannich-type reaction of enaminones. 11 b In continuation of our interest in the synthesis of heterocycles, 12 herein, we report the serendipitous synthesis of cyclohepta[ b ]indole (over the corresponding carbazole) via cascade radical cyclization to 3-propargyl-2-alkenyl indole dictated by the protecting group.…”