38
1
General Principles
2.3.1 Chemical Shifts
1 H and 13 C NMR chemical shifts for unsubstituted and simple substituted carbohydrate derivatives and the effects of substituents on these shifts have been summarized [130,350,351,378,
379,380,381]. The bulk of carbohydrate protons are observed between 3.2 and 4.2 ppm
with equatorial protons being less shielded than axial and primary less shielded than secondary [130,375,382,383]. Anomeric protons are more deshielded (∼4.3 to 5.5 ppm) and
similarly anomeric carbons (∼ 90 to 110 ppm) are deshielded with respect to the bulk of
the secondary (∼ 67 to 82 ppm) and primary carbons (∼60–65 ppm). The factors that influence hydroxyl proton chemical shifts have been summarized and evaluated computationally [239,251].
2.3.2 Scalar Coupling
One-Bond Coupling The magnitudes of 1 J C,H are about 125 Hz in saturated alkanes but
increase in the presence of electronegative substituents [384] and are typically 140–150 Hz
for non-anomeric carbons. The sizes of anomeric 1 J C,H are related to the relative orientations of the bonds at the anomeric center of pyranoses, which are in turn related to CH
bond lengths [76]; values are ∼170–175 Hz if H-1 is equatorial (typically α-D anomers
in 4 C 1 conformations) but ∼160–165 Hz if H-1 is axial (typically β-D anomers in 4 C 1
conformations) [385,386,387]. Values from both furanoside anomers are normally similar,
> 170 Hz [388,389] although conformational restriction makes the 1 J C,H values for 2,3-anhydrofuranosides diagnostic of configuration [390]. For the same reasons, 1 J C,H values for
the anomeric CH units of septanoses are unreliable indicators of configuration [307]. Additional electronegative substituents increase the magnitude of 1 J C,H again to 176–185 Hz in
orthoesters [391]. In oligosaccharides, the sizes of anomeric 1 J C,H are also influenced by the
angles in predictable ways [387].
Two-Bond Coupling A number of different two-bond couplings can provide useful structural
and conformational information. The magnitudes of H–C–H coupling constants in saturated systems range from about 0 to −15 Hz and adjacent electronegative atoms cause them to
increase algebraically in an orientationally dependent fashion [392], for instance, 2 J H,H is
∼−6 Hz in 1,3-dioxanes but > −2 Hz in 1,3-dioxolanes. 2 J H,H has been used for conformational studies [196,393] and its magnitude at C-6 of pyranose sugars is related to both the C-5
C-6 torsional angle as well as the C-6 O-6 torsional angle [196]. It could be utilized more,
particularly in the study of 1,6-linked oligosaccharides.
The magnitude of 2 J C,H depends on substitution, electronegativity, and bond angle and can be
positive or negative, increasing with increased numbers of electronegative substituents [394].
Values range from about −8 to +10 Hz in carbohydrates with larger values if one of the carbons
is the anomeric carbon [395,396,397]. Rules that relate the size of 2 J C,H to the orientation
of the oxygen atoms have been formulated [395,397] and used to assign the configuration
in N-acetylneuraminic acid and derivatives [398,399] and in oligosaccharides [400]. More
recently, conformational information has been obtained by calculating these values for all
possible conformational minima for comparison with experimental values for hydroxymethyl
rotamers [197].
1
General Principles
2.3.1 Chemical Shifts
1 H and 13 C NMR chemical shifts for unsubstituted and simple substituted carbohydrate derivatives and the effects of substituents on these shifts have been summarized [130,350,351,378,
379,380,381]. The bulk of carbohydrate protons are observed between 3.2 and 4.2 ppm
with equatorial protons being less shielded than axial and primary less shielded than secondary [130,375,382,383]. Anomeric protons are more deshielded (∼4.3 to 5.5 ppm) and
similarly anomeric carbons (∼ 90 to 110 ppm) are deshielded with respect to the bulk of
the secondary (∼ 67 to 82 ppm) and primary carbons (∼60–65 ppm). The factors that influence hydroxyl proton chemical shifts have been summarized and evaluated computationally [239,251].
2.3.2 Scalar Coupling
One-Bond Coupling The magnitudes of 1 J C,H are about 125 Hz in saturated alkanes but
increase in the presence of electronegative substituents [384] and are typically 140–150 Hz
for non-anomeric carbons. The sizes of anomeric 1 J C,H are related to the relative orientations of the bonds at the anomeric center of pyranoses, which are in turn related to CH
bond lengths [76]; values are ∼170–175 Hz if H-1 is equatorial (typically α-D anomers
in 4 C 1 conformations) but ∼160–165 Hz if H-1 is axial (typically β-D anomers in 4 C 1
conformations) [385,386,387]. Values from both furanoside anomers are normally similar,
> 170 Hz [388,389] although conformational restriction makes the 1 J C,H values for 2,3-anhydrofuranosides diagnostic of configuration [390]. For the same reasons, 1 J C,H values for
the anomeric CH units of septanoses are unreliable indicators of configuration [307]. Additional electronegative substituents increase the magnitude of 1 J C,H again to 176–185 Hz in
orthoesters [391]. In oligosaccharides, the sizes of anomeric 1 J C,H are also influenced by the
angles in predictable ways [387].
Two-Bond Coupling A number of different two-bond couplings can provide useful structural
and conformational information. The magnitudes of H–C–H coupling constants in saturated systems range from about 0 to −15 Hz and adjacent electronegative atoms cause them to
increase algebraically in an orientationally dependent fashion [392], for instance, 2 J H,H is
∼−6 Hz in 1,3-dioxanes but > −2 Hz in 1,3-dioxolanes. 2 J H,H has been used for conformational studies [196,393] and its magnitude at C-6 of pyranose sugars is related to both the C-5
C-6 torsional angle as well as the C-6 O-6 torsional angle [196]. It could be utilized more,
particularly in the study of 1,6-linked oligosaccharides.
The magnitude of 2 J C,H depends on substitution, electronegativity, and bond angle and can be
positive or negative, increasing with increased numbers of electronegative substituents [394].
Values range from about −8 to +10 Hz in carbohydrates with larger values if one of the carbons
is the anomeric carbon [395,396,397]. Rules that relate the size of 2 J C,H to the orientation
of the oxygen atoms have been formulated [395,397] and used to assign the configuration
in N-acetylneuraminic acid and derivatives [398,399] and in oligosaccharides [400]. More
recently, conformational information has been obtained by calculating these values for all
possible conformational minima for comparison with experimental values for hydroxymethyl
rotamers [197].