Intermediates in the von Pechmann reaction

Condensation of meta-cresol with ethyl acetoacetate in dehydrated phosphoric acid is slow enough to observe formation and disappearance of the intermediates viz 1,2-dihydro-4,7-dimethyl-4-(4-hydroxy-2-methylphenyl)-coumarin (3) and 1,2-dihydro-4,7-dimethyl-4-(2-hydroxy-4-methylphenyl)-coumarin (4). GCMS analyses of the reaction mixtures indicated that 4,7-dimethylcoumarin (5) was formed from 3 and 4 by elimination of meta-cresol molecule. The compounds 3 and 4 were isolated by the column chromatography. Their structures were elucidated by the spectroscopic methods but unequivocal assignment of the structure to the particular isomer was not possible. The structures were resolved by X-ray diffraction studies.