2,2'-Anhydro-1-(3',5'-di-O-acetyl-β-D-arabinofuranosyl)uracil, a cyclouridine nucleoside with a C4'-endo furanosyl conformation

2,2′-Anhydro-1-(3′,5′-di-O-acetyl-β-D-arabinofuranosyl)uracil, C13H14N2O7, was obtained by refluxing 2′,3′-O-(methoxymethylene)uridine in acetic anhydride. The structure exhibits a nearly perfect C4′-endo(4E) conformation. The best four-atom plane of the five-membered furanose ring is O—C—C—C, involving the C atoms of the fused five-membered oxazolidine ring, and the torsion angle is only −0.4 (2)°. The oxazolidine ring is essentially coplanar with the six-membered uracil ring [r.m.s. deviation = 0.012 (5) Å and dihedral angle = −3.2 (3)°]. The conformation at the exocyclic C—C bond isgauche–transwhich is stabilized by various C—H...π and C—O...π interactions.