σ-Skeletal-Rearrangement of Heptalenes: Thermal Transformation of Heptalene-1,2-dicarboxylates into Heptalene-1,3-dicarboxylates

It is shown that dimethyl heptalene-1,2-dicarboxylates undergo rearrangements at temperatures > 200° to yield the corresponding 1,3-dicarboxylates, which are isolated as the more stable 3,5-dicarboxylates. ²H- and ¹³C-labelling experiments with dimethyl 7-isopropyl-5,10-dimethylheptalene-1,2-dicarboxylate ( 1 ) which is rearranged into dimethyl 9-isopropyl-1,6-dimethylheptalene-3,5-dicarboxylates. ( 2 ) indicate that the reaction occurs by interchange of C(2) and C(3) in the heptalene skeleton of 1 . Thus, the transformation of 1 into 2 represents the first thermal σ-skeletal rearrangement of heptalenes. The structures of 1 and 2 are discussed in terms of an X-ray analysis and the spectral data.