Thymidine‐Psoralen Photoaddition: Stereoselective Formation of A trans‐syn Pyrone Adduct

Abstract

To study the photoreactions between thymidine and psoralen and to devise a synthetic route to prepare thymidine-5-methoxypsoralen adducts, we have synthesized a compound in which thymidine is linked to a 5-methoxypsoralen derivative by a short flexible chain located to allow only the formation of syn intramolecular [2+2] cycloadducts. Irradiation at 365 nm carried out at usual concentrations (10-2-10-3 M ) resulted in a regioselective dimerization involving the 3,4 double bonds. Extreme dilution (c I 5 x M ) was necessary to observe the intramolecular photoaddition, which produces exclusively the 3,4-trans-syn adduct. These results are compared to those obtained previously with similar heterodimers that lead exclusively to the 3,4-cis-anti thymine or thymidine-psoralen adduct on exposure to near UV light.