PREFERRED CONFORMATION OF C-GLYCOSIDES .13. A COMPARISON OF THE CONFORMATIONAL BEHAVIOR OF SEVERAL C-FURANOSIDES, N-FURANOSIDES, AND O-FURANOSIDES

A qualitative comparison of C-furanosides, derived from arabinose and 2-deoxyribose, with the naturally occurring O- and N-furanosides was p erformed using solution H-1 NMR spectroscopy and X-ray crystallography . The ring conformations of beta- and alpha-C-arabino and beta- and al pha-C-2-deoxyribofuranosides tend to possess equatorial anomeric C-C b onds, in contrast to the O-furanosides, which have ring conformations stabilized by axial anomeric C-O bonds due to a stereoelectronic effec t. Installation of a quaternary anomeric center in C-furanosides intro duces unfavorable 1,3-diaxial-like steric interactions, which can shif t the ring conformational equilibrium in such a manner that the C-fura noside becomes more similar to the O-furanoside. The solid-state confo rmation of the C-nucleoside beta-arabinosyl pseudouridine was shown to be virtually identical to N-nucleoside beta-arabinosyluridine. In sol ution, however, the ring conformation of this N-furanoside, and its 2- deoxy analog, is intermediate to the corresponding oxygen- and carbon- derived entries. When compared to C-pyranosides, C-furanosides seem to have anomeric linkages prone to increased exo-anomeric conformational averaging. A quaternary furanosyl anomeric center such as in O-sucros e (1) and C-sucrose (2) appears to provide further flexibility at this anomeric linkage. Evidence is provided which suggests the flexible na ture of sucrose arises at the fructofuranosyl linkage, as the glucopyr anosyl linkage in this compound is conformationally well-defined in th e exo-anomeric sense. The analysis of the preferred syn/anti base conf ormation in C-nucleosides can be complicated by a stronger equatorial base preference.