3-Deazauridine: crystal structure, conformational parameters, and molecular orbital calculations

The cytostatic compound 3-deazauridine crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules per unit cell of dimensions a = 13.394, b = 8.335, c = 9.639 Å. On a four-circle diffractometer 998 reflexions were measured. A trial structure was obtained by direct methods and refined by least-squares procedures to R = ∑||F_o| − |F_c||/∑|F_o| = 4.0%. The tautomeric form present in the crystal is the enol, 4-hydroxy-l-(β-D-ribo-pentafuranosyl)-2-pyridone, which also predominates in solution. Conformational features of the nucleoside include a glycosidic bond conformation in the anti range, a ribose moiety in the C(2′)-endo envelope form and a gauche–gauche arrangement about the C(4′)—C(5′) bond. Pyridone units form endless chains parallel to c firmly linked by O(4)—H...O(2) hydrogen bonds only 2.549 Å long. Base planes make a 21° angle with the ac plane; stacking interactions appear insignificant. Instead, the chains are tethered at the ribose oxygens by hydrogen bonds involving H(O3′) in a bifurcated system as well as H(O2′) and H(O5′). The bifurcated hydrogen bond involves a very unusual intramolecular O(3′)—H...O(2′) and an O(3′)—H...O(4) hydrogen bond. Extended Hϋckel molecular orbital calculations of the electronic properties are reported.