The phototransformation of 5-tert-butyl-2'-deoxynridine: An approach to the synthesis of new 1,2-dihydrocyclobuta[d]pyrimidin-2-ones

A new type of 2-oxo-1,2-dihydrocyclobuta[a]pyrimidin-1-yl-2'-deoxynucleoside 2a is obtained in 52% yield upon irradiation of aqueous solutions of 5-tert-butyl-2'-deoxyuridine 1a with short wavelength (254 nm) UV light. The identity and structure of the photoproduct is unequivocally established by H-1 and C-13 NMR (including a 2D INADEQUATE experiment), mass and UV spectroscopy. This new phototransformation represents an alternative route for the photolysis of 5-alkyl-substituted uracil derivatives where the side chain has more than one carbon atom. A mechanism for the phototransformation of compound 1a into 2a is proposed, and this requires the creation of a cyclobutanol-type intermediate (type-II photoprocess). A general pathway of UV-induced photolysis of 5-alkyl-substituted uracil derivatives (other than 5-methyl substituted) into 1,2-dihydrocyclobuta[d]pyrimidin-2-ones and photohydrated uracils is proposed.