[3+1] Synthetic Route Toward Synthesis of Cyclobutane/butene Derivatives Based on the Large α-Cation-Stabilizing Effect of Cyclopropane¹

The large α-cation-stabilizing effect of cyclopropyl group was utilized in the efficient two-step synthesis of cyclobutane and cyclobutene derivatives:The first step is the Lewis acid-promoted carbonyl ene reaction, where vinylcyclopropane 2 acts as a particularly efficient ene donor to give alcohol 3 in high yield. Treatment of this ene adduct 3 with triflic anhydride in the presence of collidine led to the formation of cyclobutane derivatives in high yield. Furthermore, we found that treatment of the reaction mixture of the triflate formation with Et₃N cleanly effected elimination of the triflic acid to give cyclobutene in high yield.