Conformation and structure of 2-amino-8-(beta-D-ribofuranosyl)imidazo [1,2-a]-s-triazin-4-one(5-aza-7-deaza guanosine), a potent antiviral nucleoside.

The crystal and molecular structure of one imidazo[1,2-a]-s-triazine nucleoside and its antiviral activity are described. The crystal structure of 2-amino-8-(beta-D-ribofuranosyl)imidazo[1,2-a]-s-triazin-4-one monohydroate (C10H13N5O5.H2O) was solved by X-ray counter data. The compound crystallizes in the monoclinic space group P21 with cell dimensions a = 7.353 (1), b = 6.465 (1), c = 13.701 (1) A, B = 104.64 (1) degrees. The structure was solved by direct methods and refined by full matrix least-squares technique to a final value of the conventional R-factor of 0.049 using 1998 observed intensities. The orientation of the base relative to the sugar ring defined in terms of rotating about the C(1')-N(8) glycosyl bond is anti (47.8 degrees). The ribose moiety exhibits C(2')-endo, E conformation. The conformation around C(4')-C(5') is gauche-. Molecular packing is dominated by hydrogen bonds. Base stacking occurs long the b axis. 5-Aza-7-deazaguanosine has shown a marked antiviral activity in vitro against herpes simplex virus despite the fact that N(3) is effective as the hydrogen acceptor only.