Photochromic heterocyclic fulgides. Part 5. Rearrangement reactions of (E)-α-1,2,5-trimethyl-3-pyrrylethylidene(isopropylidene)succinic anhydride end related compounds

Abstract

The pale yellow (E)-isopropylidene-[α-(1,2,5-timethyl-3-pyrryl)ethylidene]succinic anhydride 3b undergoes a reversible photochemical reaction to give the deep blue 7,7a-dihydro-1,2,4,7,7,7ahexamethylindole- 5,6-dicarboxylic anhydride 4b which is thermally stable at ambient temperatures. The pale yellow anhydride 3b, on heating at 140° C in the presence of an acid catalyst, rearranges quantitatively to the colourless 4,5-dihydro-1,2,3,4, 4,7-hexamethylisoindole-5,6-dicarboxylic anhydride 6b. The corresponding ispropylidene(2,5-dimethyl-1-phenyl-3-pyrrylethylidene)succinic anhydride 3c undergoes an analogous sequence of reactions but the related isopropylidene(2,5-dimethyl-1-phenyl-3-pyrrylmethylene)succinic anhydride 3g undergoes neither photochemical nor thermal ring closure to indole nor isoindole derivatives but only E–Z isomerisation under similar reaction conditions.