Conformational parameters of the carbohydrate moieties of α-arabinonucleosides

Abstract

The conformations of the sugar moieties of a number of α- d- and - l-arabino-furanosyl nucleosides in solution have been investigated, largely with the aid of a statistical procedure developed for this purpose. The overall results demonstrated the existence, for a broad range of analogs, of a two-state, conformational equilibrium, N⇌S (or 2 E⇌ 3 E). As between different analogs, there is a large variation in the relative populations of these two states that is related to the nature of the aglycon, the type of protecting groups on the sugar hydroxyl groups, and, to a much smaller extent, the solvent system. There is a striking correlation between the conformation of the sugar and the orientation of the exocyclic, hydroxymethyl group. Both for purine and pyrimidine nucleosides, a preponderance of the gauche-gauche rotamer of the exocyclic group is associated with the type-N state of the furanose ring, whereas, with the type-S state, the gauche-gauche rotamer population is virtually nonexistent. A comparison with X-ray diffraction data for 9-α- d-arabinofuranosyladenine demonstrated that, as for other classes of nucleosides, the solid-state conformation corresponds to one of the states participating in the equilibrium in solution.