2'-Deoxy-5-propynyluridine: a nucleoside with two conformations in the asymmetric unit.

The title compound, 1-(2-deoxy-beta-D-erythro-pentofuranosyl)-5-(prop-1-ynyl)pyrimidin-2,4(1H,3H)-dione, C(12)H(14)N(2)O(5), shows two conformations in the crystalline state: conformer 1 adopts a C2'-endo (close to (2)E; S-type) sugar pucker and an anti nucleobase orientation [chi = -134.04 (19) degrees], while conformer 2 shows an S sugar pucker (twisted C2'-endo-C3'-exo), which is accompanied by a different anti base orientation [chi = -162.79 (17) degrees]. Both molecules show a +sc (gauche, gauche) conformation at the exocyclic C4'-C5' bond and a coplanar orientation of the propynyl group with respect to the pyrimidine ring. The extended structure is a three-dimensional hydrogen-bond network involving intermolecular N-H...O and O-H...O hydrogen bonds. Only O atoms function as H-atom acceptor sites.