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K. Paradowska et al. / Carbohydrate Research 343 (2008) 2299–2307
2301
Table 2
gen bond structure of
a
-anomer (3) consists of O-2–HÁ Á ÁO-3 and O-
Selected bond lengths, bond angles and major torsion angles for methyl
lyxopyranosides (1) and methyl b-D-lyxopyranosides (2)
a-D-
3–HÁ Á ÁO-2 infinite chains as well as the isolated bonds to the ring
oxygen O-4–HÁ Á ÁO-5. In 4, there are only O-2–HÁ Á ÁO-3, O-3–HÁ Á ÁO-
4 and O-4–HÁ Á ÁO-2 bonds, that is, both the ring and glycosidic
oxygen atoms are excluded from the hydrogen bond formation.
Atoms
Compound
1
2
The crystal structures of methyl a- and b-D-xylopyranosides (5
Bond lengths (Å)
C-5–O-5
O-5–C-1
C-1–O-1
C-6–O-1
and 6) are quite different. In particular, 5 crystallizes with two
molecules in the asymmetric unit, while 6 has one molecule in
the asymmetric unit. The neutron diffraction study of 510 showed
that it has four molecules per unit cell. The X-ray diffraction data11
indicate that the only significant conformational difference
between two independent molecules of 5 is in the dihedral angle
O-5–C-1–O-1–CH3 (the difference of ca. 6°). In the crystals of 6,
the bond to the ring oxygen atom O-2–HÁ Á ÁO-5 is the longest, as
it would be expected, and the donor-only bond O-4–HÁ Á ÁO-3 is
shorter than the donor–acceptor bond O-3–HÁ Á ÁO-2.
1.438 (2)
1.426 (2)
1.401 (2)
1.440 (2)
1.4340 (18)
1.4351 (19)
1.385 (2)
1.435 (2)
Bond angles (°)
C-1–O-5–C-5
O-1–C-1–O-6
O-1–C-1–O-5
111.83 (13)
112.29 (15)
112.91 (15)
111.50 (12)
108.36 (13)
114.26 (13)
Torsion angles (°)
O-5–C-1–O-1–C-6
C-2–C-1–O-1–C-6
O-1–C-1–O-5–C-5
66.46 (19)
À75.79 (17)
À172.35 (15)
164.40 (14)
58.58 (18)
À179.17 (13)
The structure of orthorhombic crystal of methyl b-D-ribopyran-
oside (7), the only known crystallographic form of 7, was refined
using both X-ray and neutron-diffraction methods.12 Compound
7 crystallized with P212121 space group with one molecule in the
asymmetric unit (1C4 conformation). The crystal structure of 7
revealed four intermolecular hydrogen bonds and one intramole-
cular hydrogen bond per molecule. Each hydroxyl group of this
compound was both donor and acceptor in the hydrogen bonding
scheme. The intramolecular hydrogen bond in 7 was placed
between the syndiaxially oriented O-2–H and O-4, with distance
between OÁ Á ÁO atoms of 2.768 Å. Intermolecular and intramolecular
interactions in 7 excluded the ring and glycosidic oxygen atoms.
Table 3
Hydrogen bonds in methyl a-D-lyxopyranoside (1) and methyl b-D-lyxopyranoside (2)
Donor D-H
Acceptor A
d (DÁ Á ÁH)
d (HÁ Á ÁA)
\DHA
Compound 1
O-2–H
O-2–H
O-3–H
O-4–H
O-5a
O-4
O-2
O-3
0.7792
0.7792
0.7684
0.8005
2.5109
2.0386
1.9680
1.9176
111.95
157.54
177.53
177.14
Compound 2
O-2–H
O-3–H
2.2. 13C CP MAS NMR spectroscopy
O-5
O-4
O-3
0.8711
0.8367
0.8565
1.8829
1.9402
1.8331
173.45
170.26
166.13
O-4–H
The series of seven methyl glycopyranosides (1–7), was studied
by means of solid-state 13C CP MAS NMR spectroscopy, thus com-
plementing the crystal structures of these compounds. The cross-
polarization was effective, as frequently occurs in carbohydrates,
since the maximum intensity of signals was achieved with contact
time of 1.5 ms and the spectra of reasonably good quality were
obtained when accumulating ca. 200–300 scans. The 13C CP MAS
NMR spectra of 3, 4 and 7 are illustrated in Figure 3. Close correla-
tions between XRD and MAS NMR data are expected for both lyxo-
pyranosides 1 and 2 because the X-ray and NMR studies were
performed using the same samples in both techniques (crystals
were powdered for MAS NMR measurements).
a
Intramolecular hydrogen bond.
and 0.590° for 2, while the other puckering parameters, Q and /,
are equal to 0.5°, 14.49° and 0.6°, 251.84° for 1 and 2, respectively.
Both methyl a- and b-D-lyxopyranosides (1, 2) crystallize in the
P212121 space group. An independent part of the unit cell is formed
by four molecules of sugar in 1 and 2. Due to the anomeric effect in
1 some differences in selected bond lengths between both anomers
are observed. In 1, the C-5–O-5 bond is slightly longer than that of
C-1–O-5, but in 2, the opposite situation is observed, that is, the C-
1–O-5 bond is longer than that of C-5–O-5. Moreover, the glyco-
sidic bond O-1–C-1 is shorter in 2 than in 1 (the difference in
lengths is ca. 0.02 Å). The molecules in crystals of 1 and 2 are linked
by hydrogen bonds; the common feature is that all three OH
groups are hydrogen bond donors. The pattern of interactions is
different, however, for 1 and 2 (Fig. 2) as donors and acceptors of
hydrogen bonds are not the same for these compounds. In the crys-
tals of 1 an intramolecular hydrogen bond is observed between hy-
droxyl group at C-2 atom and the ring oxygen atom of the same
molecule, whilst in crystals of 2 this kind of hydrogen bonds is
missing. An intramolecular hydrogen bonding, typical for crystals
of 1, is rarely observed in monosaccharides. The O-2–HÁ Á ÁO-5 intra-
molecular hydrogen bond in 1 is slightly longer than others and
shows large deviation (of ca. 68°) from linearity (Table 3).
The chemical shifts for solids and solutions (in DMSO-d6)
and the differences between solution and solid state
D =
dsolution À dsolid > 1 ppm for 1–7 are given in Table 4. Since solid-
state techniques, such as dipolar diphase or short contact pulse
sequences, are less helpful in distinguishing C–H resonances of car-
bohydrate than solution techniques. Therefore, the chemical shifts
were assigned mainly by comparison with solution data and the
calculated shielding constants. The dependence of solid-state
chemical shifts on intermolecular interactions result from the close
proximity of neighboring molecules in the crystals, although it
should also be kept in mind that the intramolecular interactions
associated with conformational effects can also produce different
chemical shifts.
To assess the conformational effects, for example, freezing the
rotation around glycosidic bond, the rotation of OH groups or the
differences in the dihedral angle O-5–C-1–O-1–CH3, the compari-
son of chemical shifts in solid state and solution has been made
which allows detection of the rigid and flexible structural frag-
ments of the molecules. The flexible fragments should exhibit
larger shielding changes than the rigid ones. It is expected that
for a rigid system such as pyranosidic ring of the chair-like confor-
mation in 1–7, only the OCH3 group at anomeric carbon C-1 can
undergo reorientation. As shown in Table 4, the differences in
The crystal structures of methyl glycopyranosides 3–7 were
determined by X-ray diffraction and neutron diffraction or by these
two methods (5 and 7). The crystal structures of methyl a-L-arabi-
nopyranoside (3) and methyl b-L-arabinopyranoside (4) refined by
X-ray diffraction measurements were reported elsewhere.9 3 and 4
adopt 4C1 conformations with very small differences between their
geometrical parameters. In both anomers of
(3 and 4), the O-1–C-1 bond is the shortest, and the O–C-1–O
valence angle is smaller in as compared to b anomer. The hydro-
L-arabinopyranosides
a