[2+2] Photocycloaddition of 3-Alkenyloxy-2-cycloalkenones: Enantioselective Lewis Acid Catalysis and Ring Expansion

The construction of heterocyclic compounds from activated cyclopropane derivatives offers an alternative strategy for the preparation of molecules that may be of interest from a structural or biological standpoint. Several newly developed methods provide access to densely functionalized heterocycles in a manner that can be considered useful for both diversity- and target-oriented synthetic approaches. This tutorial review focuses on the latter, describing recent developments and applications of cyclopropane ring-expansion reactions in natural product synthesis.