A Stereoselective Rule for the Cross-bicyclization in Linear Conjugated Polyenes

A concept of the cross-bicyclization complementary to that of the cycloaddition is defined for interpreting the mechanism of the simultaneous bicyclizations in linear conjugated polyenes based on the perturbation treatment in the frame of Hückel MO’s. Stereoselective modes in thermal and photo-induced reactions are shown to depend on the sign of the bond-bond-polarizability, Πrs,tu, of the concerned polyene, where r, s, t, and u refer to the atomic positions at which the bicyclization occurs. From the properties of Πrs,tu of linear conjugated polyenes, these modes are simply determined by the relationship among r, s, t, and u in thermal cases, but not in photo-induced ones. The predicted stereoselective modes are discussed in line with experimental observations. It is demonstrated that the “symmetry-disfavored” reactions inferred from the usual cycloaddition mechanism can be interpreted as the concerted cross-bicyclization reactions.