Photochemical, Thermal, and Base‐Induced Access to Hydroazulene Derivatives via Two‐Carbon Ring‐Enlargement Reactions of Condensed Electrophilic Cyclobutenes.

Abstract

Enamines, silyl enol ethers, and β-keto ester anions derived from bicyclo[3.3.0]octan-2-one efficiently underwent a formal [2 + 2] cycloaddition reaction with DMAD and ethyl propynoate leading to a large variety of electrophilic cyclobutenes. The latter were transformed into polyfunctionalized bicyclo[5.3.0]decane (or hydroazulene) ring systems in high yields by fragmentation of the cyclobutene moiety. These two-carbon ring-enlargement reactions were utilized as a synthetic tool for the construction of a polyfunctionalized hydroazulene derivative that represents a potential precursor of the tricyclic framework of ingenol.