Thermally-induced intramolecular [2 + 2] cycloaddition of allene-methylenecyclopropanes: expedient access to two separable spiropolycyclic heterocycles

Abstract

In this paper, a thermally-induced intramolecular [2 + 2] cycloaddition of allene-methylenecyclopropanes has been developed for the rapid construction of nitrogen-containing spiro[bicyclo[3.2.0]heptane-6,1′-cyclopropane] and spiroheptane-2,1′-cyclopropane polycyclic heterocycles in excellent total yields with good functional group compatibility under relatively mild conditions. These two heterocycles are separable and formed at the same time in the absence of any catalyst or additive. The reaction mechanism is clarified by control experiments and DFT calculations, and it was found that the reaction proceeded through a stepwise process via a key intermediate containing a seven-membered ring rather than a concerted pathway.