Monomeric Aldoses and Ketoses

The conformations and 13C chemical shifts of the most common aldo- and ketopyranoses [89, 132, 689-694] and aldo- and ketofuranoses [132, 682, 690, 693, 695] are surveyed in Tables 5.15 and 5.16 the chemical shifts of selected monosaccharide derivatives are given in Table 5.17 [88, 89, 677, 680-682, 694, 696-725], The signals were assigned according to rules (1) to (11). 

Upon 3-O-methylation of D-galactose the resonance of C-4 shifts about 4.5 ppm upheld. This effect is a general rule Methylation of pyranose hydroxy groups causes upheld shifts of the jS-carbons with axial hydroxy groups of about 4.5 ppm [705], 

A mutarotational equilibration is demonstrated in Fig. 5.5 by the time dependent 13C NMR spectrum of a solution of D-glucose. Initially, the solution contains almost pure a-D-glucose, but the signals at lower field corresponding to the /i-anomer soon appear. At equilibirium, the relative signal intensities indicate that the /i-anomer is favored (60% P, 40% a Fig. 5.5) [685], 

Hydroxylated furanose ring carbons resonate at lower field than the corresponding pyranose carbons, as is demonstrated by the PFT 13C NMR spectrum of ribose in Fig. 5.6 [677], 

An anomeric carbon with an axial hydroxy group usually resonates at higher field than the corresponding C-1 with an equatorial hydroxy group (rule (5)). This rule allows one to prove, experimentally, that arabinose exists in the C4 and not in the 4C, conformation, in which most sugars occur in solution [88, 726]. However, rule (5) is not valid for the mannose, rhamnose and talose anomers. This was attributed to the presence of three closely spaced dipoles (Reeves effect) present in the /J-anomeric configuration of these three sugars [683, 684] (Fig. 5.7). 

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