Carbon-13 Chemical Shifts of Furanosides and Cyclopentanols. Configurational and Conformational Influences

Abstract

Assignments for the 13C chemical shifts of methyl aldopento- and hexofuranosides and of cyclopentane polyols are reported, and these chemical shifts are evaluated with respect to all combinations of configurational arrangements of the ring substituents. A cis configuration of vicinal substituents is generally associated with a substantial increase in shielding, as compared with the trans analog. Among the furanosides, however, changes in configuration involving O-3 and C-5 are accompanied by a complex shielding pattern: e.g., effects on C-3 are minor, C-4 is more shielded in cis than in trans isomers, and C-4 and C-5 of hexosides are more shielded than those of pentosides. Variations in shielding attributable to cis or trans arrangements of substituents in a 1,3-relationship are encountered in only a few instances; however, it is noteworthy that C-4 is less shielded when O-l and O-3 are cis than when trans. These diverse effects are considered also in relation to steric interactions and to conformational influences on 13C chemical shifts in the five-membered ring series.