Photochromic heterocyclic fulgides. Part 8. The condensation of 2,6-dichlorobenzaldehyde with diethyl [1-(2,5-dimethyl-3-furyl)ethylidene]succinate to give 6-chloro-2-(2,5-dimethyl-3-furyl)-1H-benzocycloheptene-3,4-dicarboxylic anhydride and photochromic (7S,7aS)-7-(2,6-dichlorophenyl)-2,4,7a-trimethyl-7,7a-dihydrobenzofuran-5,6-dicarboxylic anhydride

Abstract

The dark red photochromic (7S,7aS)-7-(2,6-dichlorophenyl)-2,4,7a-trimethyl-7,7a-dihydrobenzofuran-5,6-dicarboxylic anhydride has been synthesised by the Stobbe condensation of 2,6-dichlorobenzaldehyde with diethyl [1-(2,5-dimethyl-3-furyl)ethylidene]succinate in the presence of potassium tert-butoxide in tert-butyl alcohol. When tert-butyl alcohol is replaced by toluene, 6-chloro-2-(2,5-dimethyl-3-furyl)-1H-benzocycloheptene-3,4-dicarboxylic anhydride is produced. Use of an aprotic solvent provides a new general method of preparing 2-substituted 6-chloro-1H-benzocycloheptene-3,4-dicarboxylic anhydrides via a mechanism involving nucleophilic substitution of chlorine in 2,6-dichlorobenzaldehyde, followed by intramolecular condensation.