Carbon-13 chemical shifts for sugars and methyl glycosides are reported. The following general effects have been observed: (a) a comparison of chemical shifts with expected electron densities suggests that 13C-2 and -4 experience relatively stronger shielding than do 13C-1, -3, and -5, possibly because they oppose the ring 0 ; (b) inversion of an equatorial 0 to axial 0 is associated with increased shielding of the 13C nucleus to which it is bonded, of adjacent I3C nuclei, and of those in the p-position (i.e., having a n opposing axial H or 0 ) ; (c) removal of a gauche 0 , O interaction is associated with 13C deshielding. It appears that since these same steric relationships affect stability, the overall shielding states of I3C nuclei of the different isomers reflect variations in the magnitude of repulsive interactions in the molecules; on this basis, the anomeric effect does not arise through instability of the equatorial anomeric C-0 bond. A destabilizing interaction appears to cause polarization of most C-H bonds in a molecule, 13C becoming more shielded, and ' H less shielded, suggestive of a concerted means for delocalizing the instability. In comparing anomers or epimers, this polarization is evident as a reordering of the shielding state of different nuclei, e.g., for several sugars and their glycosides increased shielding o f 13C-l is accompanied by deshielding of the bonded H and increased shielding of the appended 0-H or methoxyl 13C nucleus.
. 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, a s compared with the trans analog. Among the furanosides, however, changes in configuration involving 0-3 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 0-1 and 0-3 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.
R. GEORGE S. RITCHIE, NATSUKO CYR, BRIAN KORSCH, HANS J. KOCH et ARTHUR S, PERLIN.Can. J. Chem. 53,1424Chem. 53, (1975.On rapporte les attributions de d6placements chimiques du I3C pour les aldopento-et hexofurannosides de m6thyle et pour une sCrie de cyclopentanepolyols; on &value ces d6place-ments chimiques par rapport a toutes les combinaisons d9&angements configurationnels des substituants du novau. Par o~~o s i t i o n aux analogues trans. on associe dn6ralement une augmentation substantielle du biindage avec une configuration cis des substituants vicinaux. Toutefois parmi les furannosides, les changements de configuration impliquant 0 -3 et C-5 sont accompagnes par un patron de blindage complexe, e.g. les effets sur C-3 sont mineurs, C-4 est plus blind6 dans les isomkres cis que dans les isomeres trans et C-4 et C-5 des hexosides sont plus blind& que ceux des pentosides. On ne rencontre que quelques cas oh des variations dans le blindage sont attribuables aux arrangements cis et trans des substituants en position 1,3; toutefois il est a noter que C-4 est moins blind6 quand 0-1 et 0-3 sont cis que lorsqu'ils sont trans. On considere aussi ces divers effets en relation avec les interactions stkriques et les influences conformationnelles sur les d6placements chimiques du 13C dans une skrie de cycles a cinq membres.
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IntroductionEarlier studies (1-6) have dealt with configurational and conformational effects on 13C n.m.r. chemical shifts in the pyranose series of carbohydrates and selected substituted cyclohexanols. The recognition of such steric effects in the spectra of these compounds proved to be relatively straightforward because of the unique geometry of the six-membered ring: namely, the existence of two chair conformations, in which the substituents are either axial or equatorial. By contrast, the furanose and cyclopentane rings are more
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