Abstract
Direct and ketone (acetone or acetophenone)-sensitized photocycloaddition of 2,3-dimethyl-2-butene to 2′-deoxyuridine (1a), thymidine (1b), 5-fluoro-2′-deoxyuridine (1c), and their 3′,5′-di-O-acetyl derivatives 1d—1f by near-UV irradiation have been studied. The triplet excited state of the nucleosides was found to be the major intermeidate by the triplet quenching technique. From the respective reactants, a pair of diastereomeric products having a cyclobutane ring were isolated. The absolute configuration of the bridgehead carbon atoms was identified by X-ray crystallographic analyses with two of them as (1R, 6R)-isomer 3a and (1S, 6S)-isomer 2b and by 1H NMR. The conformations of the glycosyl bond, the pyrimidine ring, and the sugar portion in solution were identified by 1H NMR and compared with the ones in the crystalline state.
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