Cu-promoted [2 + 2] cycloaddition of 1,4-bisallenes.

The thermal reaction of 1,4-bisallenes with the aid of Cu salt/amine significantly suppressed the formal [3,3] sigmatropic rearrangement resulting in the highly selective formation of the bicyclo[4.2.0]octadiene framework. This reaction could be applied to the one-pot synthesis of bicyclo[4.2.0]octadienes from 1,5-hexadiynes via the Crabbé homologation.