Topochemical characterization of photostable 4-hydroxyaryl-1,4-dihydropyridines by X-ray crystal structure analysis

X-ray crystal structures of photostable 4-(hydroxyphenyl)-1,4-dihydropyridines have been determined to investigate their topochemistry. Different reasons have been found for the observed photostability. The 3-hydroxyphenyl compound showed an unfavorable crystal packing of neighboring molecules with resulting distances of potentially reacting double bonds which lie far beyond published limits permitting the reaction. The 4-hydroxyphenyl compound proved to be photostable although the distances are within these limits, because the steric influence of the aromatic and carbonyl ester substituents in the adopted crystal packing prevents the movements that are necessary for the dimerization reaction.