Abstract
The structure and conformation of 2′,3′-dideoxy-2′,3′-didehydrocytidine (2′,3′-dideoxycytidin-2′-ene, d4C), a potent inhibitor of the human immunodeficiency virus, was determined by X-ray crystallography. The nucleoside crystallizes in the orthorhombic space group P212121 with cell dimensions a = 8.603(1), b = 9.038(1), c = 25.831(2) A and with two independent molecules in the asymmetric unit (Z = 8). Atomic parameters were refined by full-matrix least squares to a final value of R = 0.033 for 2258 observed reflections. The molecules are quite flexible: in molecule A the glycosyl torsion angle (XCN) is 61.3° and the -CH2OH side chain is in the gauche + orientation while in molecule B XCN = 19.8° and the side chain is trans. The five-membered rings are slightly puckered (∼0.1 Å), 04′ being endo in molecule A and exo in molecule B. A mechanism is proposed for the known instability of 2′,3′-unsaturated nucleosides.