Brook Rearrangement as Trigger for Carbene Generation: Synthesis of Stereodefined and Fully Substituted Cyclobutenes

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 THF 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 intermediacy of carbene
species.