名称:
Preferred Conformation of C-Glycosides. 13. A Comparison of the Conformational Behavior of Several C-, N-, and O-Furanosides
摘要:
A qualitative comparison of C-furanosides, derived from arabinose and 2-deoxyribose, with the naturally occurring O- and N-furanosides was performed using solution H-1 NMR spectroscopy and X-ray crystallography. The ring conformations of beta- and alpha-C-arabino and beta- and alpha-C-2-deoxyribofuranosides tend to possess equatorial anomeric C-C bonds, in contrast to the O-furanosides, which have ring conformations stabilized by axial anomeric C-O bonds due to a stereoelectronic effect. Installation of a quaternary anomeric center in C-furanosides introduces unfavorable 1,3-diaxial-like steric interactions, which can shift the ring conformational equilibrium in such a manner that the C-furanoside becomes more similar to the O-furanoside. The solid-state conformation of the C-nucleoside beta-arabinosyl pseudouridine was shown to be virtually identical to N-nucleoside beta-arabinosyluridine. In solution, 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-sucrose (1) and C-sucrose (2) appears to provide further flexibility at this anomeric linkage. Evidence is provided which suggests the flexible nature of sucrose arises at the fructofuranosyl linkage, as the glucopyranosyl linkage in this compound is conformationally well-defined in the exo-anomeric sense. The analysis of the preferred syn/anti base conformation in C-nucleosides can be complicated by a stronger equatorial base preference.