Light‐Induced, Thermal, and Acid‐Catalyzed π‐Skeletal Rearrangements of Five‐Ring‐Annelated Heptalenes

Abstract

AbstractIt is shown that, upon irradiation in CDCl3 solution, 5,6,8,10‐tetramethylheptalene‐1,2‐dicarboxylic anhydride (6) rearranges to its double‐bond‐shift (DBS) isomer 7 in an equilibrium reaction (Scheme 2). The isomer 7 is DBS stable at −50°. At ca. 30°, a thermal equilibrium with 97.8% of 6 and 2.2% of 7 is rapidly established. Similarly, the ‘ortho’‐anhydrides 9 and 11 (Schemes 4 and 5) can be rearranged to their corresponding DBS isomers 12 and 13, respectively. Whereas 12 is DBS stable at 30° (at 100° in tetralin, 94.0% of 9 are in equilibrium with 6.0% of 12), the i‐Pr‐substituted isomer 13 is already at 30° in thermal equilibrium with 11 leading to 98.7% of 11 and 1.3% of 13. It is shown by rearrangement of diasteroisomeric ‘ortho’‐anhydrides of known relative and absolute configuration (Scheme 6) that the DBS in such five‐ring‐annelated heptalenes occurs with retention of the configuration of the heptalene skeleton as already established for other heptalene compounds. It is found that the DBS process may also take place under acid catalysis (e.g. HCl/CH3OH), thus yielding 9 from 12 (Scheme 9). The ‘ortho’‐anhydrides 21 and 23 (Scheme 10) which are isomeric with 9 and 11 (Scheme 3) undergo rapid DBS' already at room temperature. The thermal equilibrium 21⇄22 consists of 18% of 21 and 82% of 22 at 30° and that of 23⇄24 of 17% of 23 and 83% of 24 at −30°. From these equilibrium mixtures, the pure DBS isomer 22 can be obtained by crystallization. Again, these rapid DBS' occur with retention of configuration of the heptalene skeleton (Fig. 4).