Rotamers and isomers in the fulgide series. Part 2. Stereochemistry and conformational analysis of bis-(p-methoxybenzylidene)succinic anhydrides by X-ray crystallography and molecular mechanics

Abstract

Crystalline bis-(p-methoxybenzylidene)succinic anhydrides occur as the methoxy rotamers of both E,E and Z,Z geometrical isomers. Three of these forms have been synthesized and characterized crystallographically while a fourth was simulated by force-field methods. Only one methoxy rotamer, the unsymmetrical variety of the Z,Z series, was observed, but two rotamers of the E,E form were obtained. A third possible E,E rotamer, not obtained experimentally, but predicted by molecular mechanics conformational analysis, is also described. The number of possible rotamers is restricted by the tendency of methoxy groups to be coplanar with the associated phenyl rings. Additional forms of the E,E series appear because of chirality introduced by steric repulsion between the superimposed phenyl rings.