From solid state photodimers of ethyl coumarin-3-carboxylate to their alcoholysis derivatives. A supramolecular study

The solid state photodimerization of ethyl coumarin-3-carboxylate and its 6-Cl and 6-Br (1a-c) derivatives as well as the methyl and ethyl alcoholysis derivatives of ethyl coumarin-3-carboxylate photodimer are reported in the context of crystal engineering. Ethyl coumarin-3-carboxylates photodimerize topochemically to form anti head-to-tail stereoisomers (2a-c). The extent of lactone methanolysis of 2a depends on the boiling temperature of the solvent to produce 2-(2-hydroxyphenyl)-3-oxo-8bH-4-oxa-cyclobuta[a]naphthalene-1,1,2a- tricarboxylic acid 1,2a-diethyl ester 1-methyl ester (3a) and 2,4-bis-[(2-hydroxyphenyl]cyclobutane-1,1,3,3-tetracarboxylic acid diethyl ester dimethyl ester (4a) in 1:2 proportion, whereas the ethanolysis of 2a only yields 2-(2-hydroxyphenyl)-3-oxo-8bH-4-oxa-cyclobuta[a]naphthalene-1,1,2a- tricarboxylic acid triethyl ester (5a). The molecular structure of 2a-c and 3-5a were elucidated by 1H and 13C NMR spectroscopy. Also the molecular and supramolecular structures of 2a,b and 3,4a were studied by X-ray diffraction. Most of the C-H⋯X (X = O, Ph), π⋯π and dipolar interactions in the photodimers 2a,b and derivative 3a are preserved from the corresponding original coumarin monomers. Thus the supramolecular structure of ethyl coumarin-3-carboxylate is conserved through this group of compounds as if they would have a "supramolecular memory". [image omitted].