Unexpected conversion of a polycyclic thiophene to a macrocyclic anhydride

Oxygenation of 2,5,9,12-tetra(tert-butyl)diacenaphtho[1,2-b:1′,2′-d]-thiophene (, C40H44S) by peracids gave the cyclic sulfonic ester (2,7,10,13-tetra(tert-butyl)diacenaphtho[1,2-c:1′,2′-e]oxathiin 5,5-dioxide, C40H44O3S) which, when heated in nitrobenzene, is converted into a complex, macrocyclic anhydride (C80H88O3), which is derived from two molecules of . Further investigation found a likely intermediate in this reaction, 4,4′,7,7′-tetra(tert-butyl)-1,1′-biacenaphthylenylidene-2,2′-dione (, C40H44O2), apparently formed from by additional oxidation. Anhydride plausibly arises by Diels–Alder reaction of and followed by several ring fragmentations. The structures of , , and were unambiguously established by X-ray crystallography.