Relative Photoreactivity of 5,7-Dimethoxycoumarin and cis-syn 8-MOP <TEX>$left<egin{array}{cc}4^{prime},5\5^{prime},6end{array} ight>$</TEX> Thd Monoadduct to Thymidine in a Dry Film State

The photocycloaddition reactivity of 5,7-dimethoxycoumarin (DMC) to thymidine is much higher than that of cis-syn 8-methoxypsoralen <TEX>$<_{5',6}^{4',5}>$</TEX>thymidine monoadducts, due to the steric effect. The different rate is also due to the relative reactivity of the singlet rather than the triplet excited state of the compounds. The biadducts of 8-methoxypsoralen and thymidine are first splitted into 4',5'-monoadducts and thymidine, not into 3,4'-monoadducts and thymidine by 254 nm light.

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