Didehydromaleic anhydride

Belief in the existence of flve-membered hetarynes has waxed and waned since they were first postulated in the early part of this century. As described above, most of the development of the aryne concept took place with benzenoid compounds. Once this central idea became well established, however, it was natural to examine the scope of its application to other ring systems including flve-membered heterocycles. The first experiments in this direction in the early 1960s appeared to prove the existence of didehydromaleic anhydride (23), ° both 2,3-didehydrothiophene and 3,4-didehydrothiophene, 4 and 25 respectively, and even the patriarchal 2,3-didehydrobenzofuran (22). Older experimental results were reinterpreted in the light of these new claims to suggest that 2,3-didehydrobenzothiophene (26) and its dioxide (27) were probably known. The first reviews of these early results were generally optimistic about the existence of flve-membered hetarynes and doubtless... 

The formal Diels-Alder adduct (228) of anthracene and didehydromaleic anhydride (23) was subjected to flash vacuum thermolysis at 600°C in the hope that a retro-Diels-Alder reaction (Section II.1.E) might generate the aryne 23. Although the theoretical yield of anthracene was obtained, the aryne 23 could not be frozen out or indirectly detected by trapping experiments the only other identifiable products were CO and which could have arisen directly... 

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