Conformational behaviour of some d-arabino-, d-lyxo-and d-xylofuranosyl pyridine C-nucleosides

Abstract

A conformational study is presented of 3-substituted pyridine C-nucleosides with a D-arabino-, D-lyxo- or D-xylofuranosyl moiety at the 5 position of the pyridine ring. From vicinal proton-proton coupling constants, the ring conformation was deduced with the use of an appropriate Karplus equation in conjunction with the concept of pseudorotation. The solution conformation of the α-D-arabinofuranosides is a dynamic equilibrium between an 2T 1 N-type and an 3 2T S-type conformation with a strong preference for the N-type. The β-D-arabinofuranosides are engaged in a 2T 1 S-type and a 2E N-type equilibrium shifted toward the 2T 1 S-conformer. The conformation of the α-D-lyxo- and α-D-xylofuranosides is exclusively N-type characterized as P N = −13° ± 2°, Γ N = 41.4° ± 0.6° and P N = −7.0 ± 3.8°, Ψ N = 39.6° ± 0.6° respectively. The β-D-lyxofuranosides have a slight preference for the 2 1T N-conformer over the 1 2T S-conformer while the β-D-xylofuranoside favours the 2 3T N-type over the 1 0T S-type. X-ray crystallographic information of (2β), (3α) and (9α) indicates a close agreement between solid state geometry and the preferred conformation in solution.