X-ray crystallographic study of several 2′-deoxy-β- d-ribonucleosides with 1-deazapurine-derived aglycones

Abstract

The 2′-deoxy-β- d-ribonucleosides of 1,3-deazapurine (benzimidazole ( 1)), 1-deazapurine (both 1 H-imidazo[4,5- b]pyridine ( 2) and 3 H-imidazo[4,5- b]pyridine ( 3)), and 6-benzoylamino-1-deazapurine (7-benzoylamino-3 H-imidazo[4,5- b]pyridine ( 4)) have been prepared and structurally characterized by X-ray crystallography. Especially compounds 1– 3 can serve as artificial nucleosides that may substitute 2′-deoxy adenosine because they lack the exocyclic amino group and one or two of the endocyclic nitrogen atoms and hence have a much smaller potential to engage in hydrogen bonds. In the latter respect, they are candidates for nucleosides in metal-ion mediated base pairs. The unit cell of compound 3 contains two crystallographically independent molecules. Compound 4 was crystallized from methanol and water, respectively, giving rise to two different solvates. Despite the closely related aglycones, the sugar conformations in 1– 4 are found to be highly variable ( 1: 2 T 1; 2: 3 T 2; 3: 3 E and E 4; 4: 2 E and 2 T 3). The structures reported here confirm that there is no simple correlation between the sugar conformation and the character of the nucleoside, and they will hopefully contribute to a better understanding of the complex interplay of different effects that are in control of the conformational equilibrium.