Nucleosides CIII. Anhydropyrimidine-C-nueleosides. Synthesis of 4,2′-anhydro-5-(β-D-arabinofuranosyl)- and 5-(β-D-arabinofuranosyl)pyrimidine C-nucleosides

Correlations of conformational parameters and equilibrium conformational states in a variety of beta-D-arabinonucleosides and their analogues

H nuclear magnetic resonance spectroscopy has been applied to a study of the conformations of a variety of purine and pyrimidine beta-D-arabinofuranosyl nucleosides. The experimental results, together with data collected from the literature, demonstrated the existence of reasonably good correlations between the coupling constants made it possible to define more accurately, than hitherto possible, the conformational states between which equilibria exist in solution. The equilibrium for the arabinonucleosides differs from that previously established for ribonucleosides; in particular, structural modifications and solvent effects may appreciably modify the conformational states between which equilibria exist. Preliminary measurements on some arabinosides in the syn conformation about the glycosidic bond indicated that these do not conform to the foregoing correlations, and will require separate study. A correlation has also been established between the conformation of the arabinose ring and that of the exocyclic 5'-CH2OH group. For both purine and pyrimidine arabinonucleosides, the conformational state 3E of the arabinose ring coexists to some extent with a gauche-gauche conformation of the exocyclic 5'-CH2OH, as in the case of pyrimidine (but not purine) ribonucleosides. Application of the foregoing to some biological problems is described.