CRYSTAL-STRUCTURE OF A NUCLEOSIDE ANALOG, 2',3'-DIDEOXY-3'-FLUORO-5-CHLOROCYTIDINE

The title compound, 1-(2,3-dideoxy-3-fluoro-beta-D-erythro pentofurano s-1-yl)-5-chlorocytosine, crystallises in the orthorhombic space group P2(1)2(1)2(1) with a = 5.142(1), b = 14.177(2), c = 15.721(2) angstro m, Z = 4. The crystal structure was solved by the heavy atom method an d refined by full-matrix least-squares method to a final R value of 0. 031 for 1629 unique observed reflections. The N-glycosidic torsion ang le is -156.1(2)-degrees and the sugar moiety is anti to the cytosine b ase. The sugar pucker is 3(2)T with P = 178.2(1)-degrees and psi = 31( 1)-degrees. The atom 05' is in a +sc conformation with respect to the furanose ring. The molecular packing in the crystal is stabilized by N -H . . . N, N-H . . . O, O-H . . . O hydrogen bonds and C-H . . . O cl ose contacts.