Photochromic heterocyclic fulgides. Part 1. Rearrangement reactions of (E)-α-3-furylethylidene(isopropylidene)succinic anhydride

The title compound (6) undergoes photochemical conrotatory and thermal disrotatory ring-closure to give red 7,7a-dihydro-4,7,7-trimethylbenzo[b]furan-5,6-dicarboxylic anhydride (7,7a-DHBF)(8) in near quantitative yield. The latter ring-opens in a conrotatory mode to the title compound on exposure to white light, and shows a slow suprafacial 1,5-H shift on heating above 100 °C. The 7a-deuterio-derivative of (8) does not undergo a 1,5-D shift even at 140 °C. The 2,5-dideuterio-derivative of fulgide (6) shows a reversible intramolecular hydrogendeuterium exchange at room temperature to give (E,E)-α-5-deuterio-3-furylethylidene(monodeuterioisopropylidene)succinic anhydride. The acid-catalysed reactions of fulgide (6) and 7,7a-DHBF (8) are described.