Conformational Analysis of 8-Azanucleosides. Crystal and Molecular Structure of 8-Azatubercidin Monohydrate, a Nucleoside Analogue Exhibiting the “High Anti” Conformation

The crystal structure of 8-azatubercidin monohydrate, C10N5O4H14·H2O, a cytotoxic analogue of tubercidin, has been determined. The unit cell constants are a=7.458 (2) Å, b=9.744 (3) Å, c=17.267 (5) Å, with Z=4 and space group P212121. The structure was solved by direct methods and refined by least squares to an R value of 0.040 using the intensities of 1112 reflections with I>1.50σ. The ribose exhibits a nearly symmetrical C(1')-exo-C(2')-endo pucker (21T), and the conformation about the exocyclic C(4')-C(5') bond is trans (ψ=179.5°). The glycosyl conformation is in the “high-anti” range (x=102.4°). There are close intramolecular contacts between the base ring nitrogen N(8) and the ribose C(2') and H(2') atoms of 2.81 and 2.49 A, respectively. All available sites on the ribose and pyrazolopyrimidine base rings, except N(8), are involved in intermolecular hydrogen bonds. These include an N(6)-H(6)⋯O(1') hydrogen bond of 2.99 Å involving the ribose ring oxygen atom. The screw-related bases exhibit a head-to-tail stacking with partial overlap of the rings and a minimum in-terplanar separation of 3.6 Å.