PHOTOCYCLOADDITION OF DEOXYURIDINES TO 2,3-DIMETHYL-2-BUTENE

Direct and ketone (acetone or acetophenone)-sensitized photocycloaddit ion 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 s tate 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 a bsolute configuration of the bridgehead carbon atoms was identified by X-ray crystallographic analyses with two of them as (1R, 6R)-isomer 3 a and (1S, 6S)-isomer 2b and by H-1 NMR. The conformations of the glyc osyl bond, the pyrimidine ring, and the sugar portion in solution were identified by H-1 NMR and compared with the ones in the crystalline s tate.