A concise protocol for the synthesis of 2-alkenylindoles through [4 + 1] annulation of aminobenzyl phosphonium salts with acrylaldehydes

A [4 + 1] annulation protocol is developed for the synthesis of various 2-alkenylindole derivatives starting from aminobenzyl phosphonium salts and cinnamaldehydes. In this process, the reaction proceeds efficiently without the usage of any metal catalysts or base, and features available starting materials, a broad substrate scope, excellent yields and simple reaction conditions. A concise and efficient protocol for the synthesis of various 2-alkenylindole derivatives is developed through [4 + 1] annulation starting from aminobenzyl phosphonium salts and cinnamaldehydes.