1,3- and 1,2-Zwitterionic Complexes of Donor-Acceptor Cyclopropanes in Reactions with Arylallenes.

The reaction of 2-arylcyclopropane-1,1-dicarboxylates (ACDC) with GaCl3 and other Lewis acids in the presence of 3-arylbuta-1,2-dienes, which acts as a trapping agent for the 1,2- or 1,3-zwitterionic intermediates being generated, has been studied. The reaction of 1,2-zwitterionic gallium complexes at low temperature has been found to give products of annulation at the phenyl substituent in the allene, 2-aryl-1-(inden-2-yl)ethylmalonates, as the main products. Only in the case of naphthylcyclopropanedicarboxylate, the formation of a polycyclic frame by annulation at both aromatic moieties occurred upon an increase in temperature to 0°C. Trapping of 1,3-zwitterions, the primary intermediates of cyclopropane ring opening, by the indicated allene was most successful if catalytic amounts of GaCl3 were added directly to the reaction mixture. In this case, the reaction followed the pathway of the usual formal (3+2)-cycloaddition of 1,3-zwitterions at each of the double bonds of the allene to give the corresponding exo-methylenecyclopentane-1,1-dicarboxylates in up to 93% yields. This process occurred with the highest efficiency at the unsubstituted double bond of 3-arylbuta-1,2-diene.