Chemistry Reference
In-Depth Information
2.3
How to Describe the Shape of Monosaccharides
Intramolecular hemiacetal formation of monosaccharides yields two different ring
structures. In comparison, the fl exibility of the furanose ring is higher than that
of the pyranose ring. The level of fl exibility of the pyranose ring is schematically
depicted in Figure 2.1 (please see Chapter 1.2, where the chair/boat forms were
introduced), showing the pseudorotational pathway in the ring interconversion.
Polar coordinates
q
,
describe the ring conformation in a rather simple
manner in comparison to the traditional ring-torsion angles. The polar
q
,
θ
and
φ
θ
and
φ
coordinates correspond to the ring-puckering parameters introduced by Cremer
and Pople [3] and their visualizations [4]. The more stable chair conformations are
localized on the poles of the sphere (
4
C
1
with
180 ° ; the
4
C
1
/
1
C
4
terminology is explained in Chapter 1.2); the less-stable boat (starting from
3,0
B at
θ
∼
0 ° and
1
C
4
with
θ
∼
φ
∼
0 ° ) and skew (beginning with
3
S
1
at
φ
∼
30 ° ) conformations reside around
the equator (
90 ° ). The half - chair and half -boat or half-skew forms lie some-
where between the pole and the equator on the meridian of the sphere. The
4
C
1
form is typical for
D
- sugars, whereas
L
- sugars prefer the
1
C
4
conformation. The
energy difference, based on
ab initio
quantum chemical calculations, is only
1.2 kcal/mol in favor for the
4
C
1
form over the
1
C
4
conformation of methoxytetra-
hydropyran shown in Figure 2.1. Actual structural parameters also depend on the
presence of ring substitutions. In addition to steric factors, the conformational
θ
∼
Figure 2.1
Conformational fl exibility of the
pyranose ring presented in the spherical visual-
ization according to the Cremer- Pople ring -
puckering parameters. The chair conformations
occupy the poles of the sphere, the twist (skew)
and boat forms are located around the equator.
The
S
chirality of the anomeric C1 carbon is
maintained along the pseudo-rotational path-
way. The half-boat/half-chair forms (not shown)
are located around the meridians. The intra-
ring torsion angles of all 14 forms are presented
in the on-line version of Supplementary Material.
