Photochromic heterocyclic fulgides. Part 6. Base-catalysed cyclizations of 1-(2,5-dimethyl-3-furyl and 2,5-dimethyl-3-thienyl)ethylidene(isopropylidene)succinic anhydrides and related compounds

Abstract

The title fulgides 1a and 3a(X = O or S) rearrange in boiling pyridine to yield deep red cis-8,8a-dihydro-2,4,7,8a-tetramethyl-7H-cyclohepta[b]-furan and -thiophene-5,6-dicarboxylic anhydrides 6a(X = O or S) respectively. The corresponding 2-thienyl fulgide 8 undergoes a similar rearrangement but with a subsequent 1,5-shift of its 3-hydrogen to give the colourless cis-5,6-dihydro-5,8-dimethyl-4H-cydohepta[b]thiophene-6,7-dicarboxylic anhydride 11. The 3-thienyl-fulgide 1d(X = S) undergoes a similar rearrangement to yield the colourless trans-7,8-dihydrc-4,7-dimethyl-6H-cyclohepta[b]thiophene-5,6-dicarboxylic anhydride 7. 3-Methyl-5,5-diphenyl-cyclohepta-1(7),2-diene-1,2-dicarboxylic anhydride 13 is obtained in an analogous reaction from (E)-3,3-diphenylallylidene(isopropylidene)succinic anhydride 12. Structural assignments are based on NMR studies. The mechanistic implications of the reactions are discussed.