Journal of Organic Chemistry vol:60 issue:24 pages:7909-7919
Condensation of 3,4-bis-O-(p-nitrobenzoyl)-D-xylal with purines and pyrimidines (A, C, 6-chloropurine, G, T, U) without externally added acid catalyst leads to the 2',3'-unsaturated pentopyranosyl nucleosides in preparatively acceptable yields of both beta and alpha anomers and near complete suppression of formation of the 3'-substituted 1',2'-unsaturated regioisomers. Anomeric configurations of these analogues of nucleosides have been established for the 4'-O-deprotected derivatives by way of C-13 NMR. In all nine anomeric pairs the signals of the carbon atoms C-5' in alpha anomers are shifted upfield when compared with the corresponding signals of the beta anomers. Coupling constants J(4'5'ax) indicate pseudoaxial positions of purines in 33-40, 45, and 46. This is rationalized in terms of a pi-o*(c-1'-N-9) resonance and represents a case where aglycons occupy pseudoaxial positions via a mechanism'different than the anomeric effect. The same coupling constants of the alpha-pyrimidines 30, 32, 42, and 44 indicate H-4(0)<->H-0(4) equilibrium with a marginal preference toward the H-0(4) form, whereas the beta-pyrimidines 29, 31, 41, and 43 show a preference toward H-0(4) probably due to steric interactions.