Dicarbomethoxycarbene and dichlorocarbene add to 1, 2, 2-trimethylbicyclo [1.1. 0] butanc in such a fashion as to suggest a concerted addition in which the central and side bonds are cleaved simultaneously. MNDO calculations support such a pathway and suggest that endo attack on the bicyclobutane is preferred to exo. Reaction of the same two carbenes with quadricyclane gives substituted derivatives of the exo-tricyclo [3.2. 1.02, 4] oct-6-ene ...