Rotamers and isomers in the fulgide series. Part 1. Stereochemistry and conformational analysis of bis-(3,4-dimethoxybenzylidene)succinic anhydrides by X-ray crystallography and molecular mechanics

Abstract

Conformational analysis of bis-(3,4-dimethoxybenzylidene)succinic anhydride suggests that in addition to a Z,Z-isomer, the E,E-isomer of this diarylfulgide occurs as three distinct relatively freely interconvertible chiral rotamers and their enantiomers. The E,E-isomer undergoes facile dehydrogenation to form an arylnaphthalene derivative. Crystals were obtained of a symmetrical E,E-rotamer in two forms, the Z,Z-isomer and the arylnaphthalene derivative. X-Ray structures of these crystalline forms are reported. The conformations of the other rotamers were simulated by molecular mechanics, using an empirical force-field based on the observed structure of the symmetrical E,E-rotamer, described here. The aromatic rings of the E,E-isomers are eclipsed and under severe strain, which inhibits free rotation. Packing energy promotes the crystallization of one of the possible E,E-rotamers only and has a marked effect on the orientation of the methoxy substituents.