Nucleosides and Nucleotides. 108. Synthesis and Optical Properties of Syn-Fixed Carbon-Bridged Pyrimidine Cyclonucleosides

6, 1'-Propanouridine (10), a carbon-bridged cyclouridine fixed in the syn-conformation, was synthesized from D-fructose. Two additional carbon-units were introduced at the 1'-position of 1'-hydroxymethyl-O2, 2'-anhydrouridine 13 and inversion of the 2' hydroxyl group was achieved by sequential oxidation-reduction reactions. Finally, the spiro-carbon bridge was constructed by radical cyclization of the 1'-iodopropyl derivative of 5-chlorouridine. Dehydrochlorination followed by deprotection gave the desired 10. The circular dichroism (CD) spectrum of 10 showed a negative Cotton effect ([θ]=-6100) at the main absorption region, whereas 5'-O-tert-butyldimethylsilyl-2', 3'-O-isopropylidene-6, 1'-propanouridine (30) showed almost no Cotton band at the same absorption region. These results suggest that the critical region in which the CD Cotton effect changes from negative to positive is present in the syn region where 10 is located. Correlation of the magnitude and the direction of the sign of the CD Cotton effect and the torsion angle (χ) is also discussed.