Stereochemistry of photoadducts between 5,7-dimethoxycoumarin and thymidine in solid state as a dry film

Abstract

The photocycloaddition reaction of 5,7-dimethoxycoumarin to thymidine was studied as a model for photosensitizing reaction of psoralen with DNA bases at the long wavelength UV light. To obtain the major photoadducts, a mixture of DMC and thymidine as a model nucleoside was irradiated in solid state as a dry film at 350 nm. The major photoadducts were isolated by silica gel column chromatography and gel permeation chromatography. Each stereoisomer of the photoadducts was separated by high performance liquid chromatography using reverse bonded phase column ($\mu$-Bondapak $C_{18}$). The stereochemistry of each isomer was studied by Fourier transform NMR, UV, IR and mass spectra. The photoadducts between DMC and thymidine were proved to be $C_4$-cycloadducts through the cycloaddition of 3,4 pyrone double bond of DMC to 5,6 double bond of thymidine. Theoretically four possible stereoisomers may be considered ; $\mbox{\underline{syn}}$ head-to-head, $\mbox{\underline{syn}}$ head-to-tail, $\mbox{\underline{anti}}$ head-to-head, and $\mbox{\underline{anti}}$ head-to-tail. The stereochemistry of the major photoadducts isolated by the HPLC is consistent with the $\mbox{\underline{anti}}$ configuration. A small amounts of $\mbox{\underline{syn}}$ isomers are also isolated.