Synthesis of Fused Cyclobutanes via [2+2] Cationic Cycloadditions

Significance

Four Lewis and Brønsted acids were found to be competent at mediating an intramolecular version of Gassman’s original cyclo­butane synthesis (Tetrahedron Lett. 1990, 31, 6489). The reaction was shown to proceed by stepwise formation of cationic intermediates, as monocyclic intermediates were isolated in some cases. The reaction could be made catalytic by using an iron salt as the Lewis acid. A synthetic application was demonstrated in the formal synthesis of natural product raikovenal and its epimer.