Light-Induced and Thermal π-Skeletal Rearrangement of Heptalenes with Retention of Configuration

It is shown that dimethyl 5, 6, 8, 10-tetramethyl- ( 3 ) and 8- (tert-butyl)-5,6,10-trimethylheptalene-1, 2-dicarboxylate ( 5 ), and their derivatives rearrange reversibly on irradiation or on heating to yield the corresponding 1,6,8,10-tetramethyl- ( 4 ) and 8-(tert-butyl)-1,6,10-trimethylheptalene-1,2-dicarboxylate ( 6 ), and their derivatives by double-bond shift (π-skeletal rearrangement) via a transition state with D₂ symmetry as the highest possible one. This follows from the fact that (?)-(P)- 3 is photochemically as well as thermally rearranged to give (?)-(P)- 4 i.e. the π-skeletal rearrangement occurs with retention of configuration of the heptalene skeleton and without loss of optical purity.