MONOFUNCTIONAL3,4-and4',5'-PHOTOCYCLOADDUCTS BETWEEN 4,5'-DIMETHYLANGELICIN and THYMINE

By irradiating (365 nm) an aqueous liquid solution of 4,5'-dimethylangelicin. a monofunctional photosensitizing furocoumarin, in the presence of an excess of thymine, two new compounds, I and II, have been obtained; they do not show fluorescence when observed with Wood's light. The nuclear magnetic resonance data, the marked similarity of UV absorption and fluorescence spectra of these compounds with those of synthetic 3.4-dihydro-4,5'-dimethylangelicin and their capacity to undergo photodissociation (254 nm) yielding the starting thymine and 4,5'-dimethylangelicin in equimolecular amounts, are consistent with C₄-cycloadducts between the 3,4-double bond of the furocoumarin and 5,6-double bond of thymine. Nuclear magnetic resonance data indicate for I and II a head-to-head and a head-to-tail structure, respectively. When irradiation is carried out in the frozen state, two adducts. III and IV, fluorescent at Wood's light, have been obtained other than the two above-mentioned compounds I and II. Compounds III and IV have been identified as 4'.5'-fluorescent adducts between the 4',5'-double bond of the furocoumarin and the 5.6-double bond of thymine; one of them (III) is identical to that formed in the photoreaction between DNA and 4,5'-dimethylangelicin; for this last compound a cis head-to-head structure has been suggested.