Through a sequence that can be performed in a single vessel, involving regio- and diastereoselective copper-catalyzed carbomagnesiation of cyclopropenes, reaction with acylsilanes, and addition of THE as cosolvent, Brook rearrangement can be triggered to furnish a wide range of cyclobutenes with exceptional diastereoselectivity. Accordingly, stereodefined and highly substituted cyclobutenes with contiguous quaternary carbon centers can be synthesized easily and in high yield. The new strategy constitutes an unprecedented application of Brook rearrangement, one which involves the intermediary of carbene species.