4.2 Monosaccharides 255Table 4.5.Relative free enthalpies for hexopyranoses
Hexopyranose Conformation G◦
(kJ/mole)
α-D-Glucose^4 C 1 10. 03
(^1) C 4 27. 38
β-D-Glucose^4 C 1 8. 57
(^1) C
4 33.^44
α-D-Mannose^4 C 1 10. 45
β-D-Mannose^4 C 1 12. 33
α-D-Galactose^4 C 1 11. 91
β-D-Galactose^4 C 1 10. 45
α-D-Idose^4 C 1 18. 18
(^1) C 4 16. 09
β-D-Idose^4 C 1 16. 93
(^1) C
4 22.^36
α-D-Altrose^4 C 1 15. 26
(^1) C 4 16. 09
which destabilizes the anomeric HO-group in
the equatorial position, while it stabilizes this
group in the axial position. This is called the
anomeric effectand is attributed to the repul-
sion between the parallel dipoles. If the 1-OH
group (β-anomer) is in the equatorial position
(cf. Formula 4.17), repulsion results from the
polarized bonds carbon atom 5 – ring oxygen
and carbon atom 1 – oxygen of the anomeric
OH-group. The repulsion forces the anomeric
HO-group to take up the more stable axial or
α-position:
(4.17)
Table 4.6.Equilibrium compositionaof aldoses and ketoses in aqueous solution
Compound T (◦C)α-Pyranoseβ-Pyranoseα-Furanoseβ-Furanose
D-Glucose 20 36 64 −−
D-Mannose 20 67 33 −−
D-Galactose 20 32 64 1 3
D-Idose6031371616
D-Ribose 40 20 56 6 18
D-Xylose 20 35 65 −−
D-Fructose 20 − 76 4 20
aValues in %.
The other substituents also influence the
anomeric effect, particularly the HO-group
in C-2 position. Here, due to an antiparallel
dipole formation, the axial position enhances
stabilization better than the equatorial position.
Correspondingly, in an equilibrium state in
water,D-mannose is 67% in itsα-form, while
α-D-glucose orα-D-galactose are only 36% and
32%, respectively (Table 4.6). The anomeric
effect (dipole–dipole interaction) increases as
the dielectric constant of the solvent system
decreases e. g., when water is replaced by
methanol.
Alkylation of the lactol HO-group also enhances
the anomeric effect (Table 4.7). A reduction of the
dielectric constant of the solvent (e. g., transition
from water to methanol), resulting in an increase
in the dipole–dipole interaction, also enhances the
anomeric effect. Conformational isomers of fura-
nose also occur since its ring is not planar. There
are two basic conformations, the envelope (E) and
the twist (T), which are the most stable; in solu-
Table 4.7.Methylglucoside isomers in methanol (1%
HCl) at equilibrium statea
Compound α- β- α- β-
Pyrano- Pyrano- Furano- Furano-
side side side side
Methyl-D-
glucoside 66 32. 50. 60. 9
Methyl-D-
mannoside 94 5. 30. 70
Methyl-D-
galactoside 58 20 6 16
Methyl-D-
xyloside 65 30 2 3
aValues in %.