Further evidence for the gelation ability-structure correlation in sugar-based gelators

Article abstract of DOI:10.1016/S0008-6215(01)00035-0

Eight methyl glycosides of 4,6-O-benzylidene derivatives of the monosaccharides D-glucose, D-mannose, D-allose and D-altrose were synthesized to systematically study the effect of small configurational changes on the ability to gelate organic solvents. Among the β anomers, only the D-mannose glycoside exhibits a strong gelation ability, whereas in the α-series the D-glucose and D-mannose derivatives act as versatile gelators. Also, as a general rule we found that the β anomers possess a higher ability to gelate solvents than the α anomers. The gelation properties are discussed on the basis of SAXS, FTIR, differential scanning calorimetric (DSC) measurements and scanning electron microscopy (SEM) observations. The temperature-dependent SAXS measurements were carried out to elucidate the sol-gel transition temperature. The present study emphasizes that the saccharide family provides, not only valuable information of the structural requirements for the design of new gelators, but also for molecular assembly systems in general.

Full text of DOI:10.1016/S0008-6215(01)00035-0

www.elsevier.nl/locate/carres
Carbohydrate Research 331 (2001) 307–318
Further evidence for the gelation ability–structure
correlation in sugar-based gelators
Oliver Gronwald,a Kazuo Sakurai,a Roman Luboradzki,b,1 Taro Kimura,a
Seiji Shinkaia,*
aChemotransfiguration Project, Japan Science and Technology Corporation, Kurume Research Center Building,
2432 Aikawa, Kurume, Fukuoka 839-0861, Japan
bDepartment of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu Uni6ersity, Hakozaki,
Hisgashi-ku, Fukuoka 812-8581, Japan
Received 14 November 2000; accepted 23 January 2001
Abstract
Eight methyl glycosides of 4,6-O-benzylidene derivatives of the monosaccharides
and -altrose were synthesized to systematically study the effect of small configurational changes on the ability to
gelate organic solvents. Among the b anomers, only the -mannose glycoside exhibits a strong gelation ability,
whereas in the a-series the -glucose and -mannose derivatives act as versatile gelators. Also, as a general rule we
D-glucose, D-mannose, D-allose
D
D
D
D
found that the b anomers possess a higher ability to gelate solvents than the a anomers. The gelation properties are
discussed on the basis of SAXS, FTIR, differential scanning calorimetric (DSC) measurements and scanning electron
microscopy (SEM) observations. The temperature-dependent SAXS measurements were carried out to elucidate the
sol–gel transition temperature. The present study emphasizes that the saccharide family provides, not only valuable
information of the structural requirements for the design of new gelators, but also for molecular assembly systems
in general. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Carbohydrates; Gels; Hydrogen bonds; Sol–gel process; Supramolecular chemistry
aggregation, gelators can be classified into two
categories: hydrogen-bond-based gelators and
nonhydrogen-bond-based gelators. Aliphatic
amide derivatives1–4 are typical examples of
the former group, whereas cholesterol
derivatives6–9 are the main representatives of
the latter group. Saccharide-containing gela-
tors have also been reported to form a hydro-
gen-bond-based gel network as well, but their
examples in literature are very limited.8,13–16 In
recent studies some methyl glycosides of 4,6-
O-benzylidene monosaccharides have been in-
vestigated with regard to their ability to gelate
organic solvents. However, the exact relation-
ship between the structure and the gelation
potential has remained unknown. In this pa-
1. Introduction
The development of new gelators for or-
ganic solvents has recently received much at-
tention. They not only gelate various organic
solvents, but also create novel networks with
fibrous superstructures that can be character-
ized by SEM pictures of xerogels.1–12 Accord-
ing to their driving forces for molecular
* Corresponding author. Tel.: +81-942-399011; fax: +81-
92-642-3611.
E-mail address: seijitcm@mbox.nc.kyushu-u.ac.jp (S.
Shinkai).
1 Present address: Institute of Physical Chemistry, Polish
Academy of Sciences, Kasprzaka 44/52, Warsaw, Poland.
0008-6215/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(01)00035-0

308
O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
per, we report the synthesis and detailed ex-
amination of the a and b isomers of gluco,
manno, allo and altropyranoside, from which
further evidence for the correlation between
the absolute configuration and the gelation
ability has been obtained.
(1) and b-
out with ZnCl2 and benzaldehyde.16 Methyl
4,6-O-benzylidene-a and b- -glucopyranoside
D
-glucopyranoside (2) was carried
D
(7 and 8) served not only as compounds for
gelation experiments but also as important
key intermediates for synthesis of the allo and
altro monosaccharides. Compound 8 was con-
verted according to literature18 into the 2,3-di-
O-(methylsulfonyl) derivative 9 and then
treated with sodium benzoate in N,N-
dimethylformamide to replace the sulfoxy-
group at C-3, with inversion of the
configuration, to give methyl 3-O-benzoyl-4,6-
2. Results and discussion
Synthesis.—The syntheses of the eight
sugar compounds tested are outlined in
Scheme 1. Treatment of methyl a,b- -
D
O - benzylidene - 2 - O - (methylsulfonyl) - b - -
D
mannopyranoside (3, 4) with a,a-dimethoxy-
toluene under slightly acidic conditions17
yielded the 4,6-O-benzylidenated derivatives 5
and 6 as the main products. The acetalation of
the C-4 and C-6 hydroxyl groups in methyl a-
allopyranoside (12) in 61% yield. Treatment of
12 with lithium aluminum hydride in 1,4-diox-
ane simultaneously removed the methylsul-
fonyloxy group, with splitting of the OꢀS
Scheme 1. (i) PhCH(OCH3)2, PPTS, DMF, 5–6.5 h at 80 °C. (ii) PhCHO, ZnCl2, stirring overnight at rt. (iii) TsCl, pyridine, 4
days at rt (7, 8), MsCl, pyridine, 20 h at rt (8). (iv) CH3ONa, ClCH2CH2Cl, 3 days refrigerator and then 2 days at rt. (v) KOH,
H2O, 2 days reflux. (vi) PhCOCl, imidazole, CHCl3, 2 h reflux. (vii) DMSO, Ac2O, 9 h at rt. (viii) NaBH4, DMF, MeOH, 45 min
at rt and then reflux for 10 min. (ix) PhCO2Na, DMF, reflux. (x) LiAlH4, 1,4-dioxane, 80 °C for 18 h.

O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
309
Table 1. Some solutions gelled at a gelator
concentration below 1 wt.%. These results are
marked with an ‘*’. As with previous stud-
ies,16a the solvents tested were classified into
two groups: Group I solvents are gelated by
some compounds, whereas Group II solvents
mainly result in solutions. In contrast, this
time only carefully dried solvents were used to
exclude the possible influence of water on the
gelation results. Furthermore, we introduced
the self-supporting precipitate (PS) as a new
classification additional to gel (G) and precipi-
tation (P). In this stage the monosaccharide
establishes a turbid self-supporting structure
in the solvent consistent with fibrous aggre-
gates immobilizing, similar to the gel, the
complete volume of the solvent. However, this
aggregation type does not completely fulfill
the requirements of a true physical gel. The
transition from ‘gel phase’ to a solution is not
reversible since slow cooling causes crystalliza-
tion, and mechanical disruption (shaking)
causes a visible separation into a solid and a
liquid phase. When the compound aggregated
only parts of the solvent in a similar manner
to those of PS, we denoted this as a partial
self-supporting precipitation (PPS). Due to
these changes we reinvestigated the previously
tested16a compounds 5, 7 and 8 and found that
some of the reported gels (G) have to be
rejudged as PS or PPS (5: entry 4; 7: entry 1, 3
and 5; 8: entries 11 and 13), whereas in some
cases P changes into PS (5: entry 6; 7: entries
2, 4, 18 and 20; 8: entries 1–10, 12, 15, 16, 18,
21–25, 27 and 30–34). On the other hand, we
observed the gel-formation instead of precipi-
tation for 7 in tetraethoxysilane (entry 19).
From comparison of all compounds listed
in Table 1, several conclusions can be drawn:
Firstly, among a-monosaccharides 5, 7, 16
and 20 only the gluco (7) and the manno (5)
diastereomers act as gelators. Both gelate aro-
matic solvents like toluene, p-xylene and
diphenyl ether. Additionally, the a-manno
diastereomer (5) can also gelate apolar hydro-
carbons (n-hexane, n-heptane, n-octane,
methylcyclohexane), carbon disulfide, triethyl-
silane and water, whereas the a-gluco com-
pound (7) gelates benzene and tetraethoxy-
silane. In apolar hydrocarbon solvents (n-hex-
ane to methylcyclohexane) triethylsilane and
tetraethoxysilane 7 forms a gel-like pre-
bond and the benzoyl group, to afford
methyl 4,6-O-benzylidene-b- -allopyranoside
D
(13)19 in 69% yield. The corresponding a-allo
anomer (20) was prepared by selective benzoy-
lation of the C-2 hydroxyl group in 7, fol-
lowed by oxidation of the C-3 position with
dimethyl sulfoxide and acetic anhydride to
afford methyl 2-O-benzoyl-4,6-O-benzylidene-
a-
D
-ribo-hexopyranosid-3-ulose (19)20 in 60%
yield. Finally, methyl 4,6-O-benzylidene-a-
D-
allopyranoside (20) was obtained by reduction
of 19 with sodium borohydride in MeOH–
N,N-dimethylformamide. The yield of 72%
was found to be better than that reported in
literature (55%).21
Tosylation of methyl 4,6-O-benzylidene-a
and b- -glucopyranoside (7 and 8) to the 2,3-
D
di-O-tosyl derivatives 10 and 11, and their
treatment with sodium methylate in 1,2-
dichloroethane gave the corresponding methyl
2,3-anhydro-4,6-O-benzylidene-a- and b- -al-
D
lopyranosides (1422 and 1523) in quantitative
yields. Hydrolysis with alkali22,24 induced the
ring opening with inversion at C-2 and the
formation of a mixture of methyl 4,6-O-ben-
zylidene-a and b-
D
-glucopyranoside (7 and 8)
and methyl 4,6-O-benzylidene-a and b-
D
-al-
tropyranoside (16 and 17), with the latter as
the principal product. The altro:gluco ratio
1
was estimated via H NMR spectroscopy for
the a anomer to be 85:15 and 70:30 for the b
anomer, respectively. All compounds were
identified by spectroscopic methods and com-
pared with published data. HPLC analysis
(u=210 nm) confirmed that the purity of the
target compounds used for the gelation test
was higher than 98%.
Gelation test for 6arious sol6ents.—The
gelation test was carried out as follows: the
gelator (2.4–3.5 mg) was mixed in a closed-
capped test tube with the appropriate amount
of solvent (80–110 mL) to result in a concen-
tration of 3 wt.%, and the mixture was heated
until the solid was dissolved. By this proce-
dure the solvent bp became higher than under
standard atmospheric pressure. The sample
vial was cooled in air to 25 °C, left for 1 h at
this temperature, and then turned upside
down. When the gelator formed a clear or
slightly opaque gel by immobilizing the sol-
vent at this stage, it was denoted by a ‘G’ in

310
O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
Table 1
Organic solvents tested for gelation by 5, 6, 7, 8, 13, 16, 17 and 20 a
Organic solvent
5
7
16
20
6
8
13
17
Group I
1
2
n-hexane b
G*
G*
G*
PS*
G*
PS*
G*
G*
S
PS*
PS*
PS*
PS*
PS*
G
G*
G*
S
G*
P*
S*
G
S
S
S
S*
PS*
G*
PS
I*
I*
P*
P*
P
P
P
PPS
PPS
PPS
S
P
P*
P
PPS
S
S
S
S*
P
G*
G*
G*
G*
G*
G*
G*
G*
S
G*
G*
S*
G
S
S
P
S
G*
P
S
PS*
PS*
PS*
PS*
PS*
PS*
PS*
PS*
PS
PS*
PP*S
PS*
PS*
P*
PS
PS*
PS*
PS*
PS*
PS*
PS
PS*
PS*
S
PS*
PS*
P*
PS
S
S
PS
S
PS*
P
PS
P*
P*
P*
PS*
P*
PPS
PS
PS
S
n-heptane b
3
n-octane b
P*
P*
PP*S
PPS
PPS
PPS
S
P*
P*
P*
PPS
S
S
S
S*
S*
PP*S
S
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
cyclohexane b
methylcyclohexane b
benzene b
toluene b
p-xylene b
nitrobenzene b
carbon tetrachloride b
carbon disulfide c
diethyl ether b
diphenyl ether b
ethyl formate b
methyl acetate b
n-octanol b
P
P
G*
S*
G
S
S
S
S
G*
S
G
PS
P*
PPS
S
P
P
P
P*
P
PS
triethylamine
triethylsilane
tetraethoxysilane
water
P*
PS*
P*
P*
P
S
P
Group II
21
22
23
24
25
26
27
28
29
30
31
32
33
34
1,2-dichloroethane
dichloromethane b
chloroform
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
P
P
S
S
S
S
S
S
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
P
P
PS
P
S
S
PS*
PS
PS
PS
PS
P
PS
P
P
PS
PS
PS
PS
PS
S
S
S
P
S
P
P
S
P
S
S
P
P
S
S
S
P
S
S
ethyl acetate b
ethyl malonate b
acetone b
S
methyl ethyl ketone b
acetonitrile b
ethanol b
P
P
P
P
PS
PS
S
n-propanol b
n-butanol b
hexanoic acid
acetic anhydride
glycerol
S
a 3.0 wt./vol% unless specified otherwise: *, 1.0 wt./vol%; G, gel; PS, self-supporting precipitate; PPS, partial self-supporting
precipitate; P, precipitation; S, solution; I, insoluble.
b
,
Dried over 4 A molecular sieves.
c Dried over anhydr MgSO4.
a gelator. It gelates a wide range of solvents
such as apolar hydrocarbons and aromatic
solvents (entries 1–8 and 13), carbon tetra-
chloride, carbon disulfide and triethylsilane
mainly at 1 wt.% concentration. In contrast, the
b-gluco and allopyranosides (8 and 13) tend to
form mainly self-supporting precipitates as de-
noted by the numerous ‘PS’ marks. Similarly,
the b-altropyranoside (17) exhibits a low solu-
bility in the major part of the solvents tested
reflected by the ‘P’ and ‘PS’ marks.
cipitation that is denoted by ‘PS’. In contrast,
the a-allo (13) and a-altropyranoside (16) tend
to be insoluble or precipitate in the Group I
solvents. In some cases theses saccharides par-
tially immobilize the solvents by the formation
of the self-supporting precipitates denoted as
‘PPS’. In accordance with previous studies, all
a compounds form mainly solutions in Group
II solvents.
On the other hand, among the b isomers only
the manno diastereomer 6 can be considered as

O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
311
plotted against the gelator concentration in
order to compare their gelation properties.
The use of dried solvents caused for com-
pound 6 no change in Tgel compared to the
previously obtained data.16a For the same con-
centration the Tgel values always appear in the
order of 6\7\5. However, the Tgel for 7
were only evaluated in between 0.5 and 1.0
wt.%. At higher concentrations a self-support-
ing precipitation (PS) is formed, and below 0.5
wt.% no gelation occurs. Furthermore, the gel
of 7 formed in p-xylene is less stable than that
of the manno compounds. Within a few days
crystals are formed, whereas the manno gels
are stable for weeks. Together with the results
from Table 1, these findings suggest the order
for the gelation ability to be b-manno\a-
manno\a-gluco. Although the Tgel for 7 in
p-xylene between 0.5 and 1.0 wt.% and in
diphenyl ether is higher than that for 5, the
smaller variety of other solvents gelated by
a-gluco (7) implies that compound is inferior
to a-manno (5).
At first a general conclusion that one can
draw from Table 1 is that the absolute confor-
mation of the monosaccharides strongly influ-
ences the gelation ability. Among the
a-monosaccharides, the gluco and the manno
diastereomer can be considered as gelators,
whereas among the b anomers only methyl
4,6-O-benzylidene-b- -mannopyranoside (6)
D
possesses the ability to gelate solvents. A more
detailed picture is given by the investigation of
the concentration dependence of the sol–gel
transition temperatures (Tgel).
Concentration dependence.—In Fig. 1 the
sol–gel phase-transition temperatures (Tgel) of
5, 6 and 7 in p-xylene and diphenyl ether are
Eq. (1), which is derived from Schrader’s
relation, allows the calculation of DH for the
dissolution of solid compounds in organic sol-
vents.7 DH can be determined from the slope
of a plot of log[gelator] (with the gelator
concentration given in mol/dm3) against T
.
−1
gel
Because these DH values are comparable or
slightly larger than the latent heat of melting
(DHm) determined from DSC measurements at
the melting point of the solid, it has been
assumed that this DHgel value reflects the heat
released at the sol–gel phase-transition tem-
perature.7 Recent studies7,16,25 proposed, there-
fore, a low degree of solvation for the fiber
structures formed by the gelator molecules in
Fig. 1. Plots of Tgel against gelator concentration in p-xylene
and diphenyl ether.
the organic solvent. Fig. 2 shows the straight
−1
lines of the plots of log[gelator] against T
gel
with k (correlation coefficients) \0.98. The
DHgel values are summarized, together with
the DHm values as obtained by DSC measure-
ments at the melting point in Table 2.
DH
2.303R Tgel
1
log[gelator]= −
×
+constant
(1)
In general, the DHgel values in p-xylene are
higher than those in diphenyl ether. This cor-
responds with the higher concentrations (\2
Fig. 2. Plots of log[gelator] (gelator concentration in mol/
dm3) against T
in p-xylene and diphenyl ether.
−1
gel

312
O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
Table 2
tion by FTIR spectroscopy is given. Due to
intermolecular and intramolecular hydrogen-
bonding interactions, no wOH peak for a free
OH group (around 3600 cm−1) could be de-
tected for the solid samples (KBr) of all the
monosaccharides. In the gel-state, all signals
are more broadened, and the wOH for the OH
groups appear in the range of 3047–3483
cm−1 [5: 3047 (vs), 6: 3262 (s), 3467 (s), 3566
(s), 7: 3047 (vs); (all in carbon tetrachloride, 1
wt.%)] and can therefore be assigned to the
intermolecular hydrogen bonds in the gel net-
work. For only the b-manno derivative (6) a
sharp peak appears at 3566 cm−1, which indi-
cates the presence of nonhydrogen-bonded
OH groups.
DHgel obtained from [gelator] vs. Tgel plots in p-xylene and
diphenyl ether and DHm at the melting point obtained from
DSC measurements a
DHgel (kJ/mol)
DHm (DSC)
(kJ/mol)
p-xylene Diphenyl
ether
a-Series
b-Series
7 (gluco)
56.8
36.2
37.7
16.0
10.1
30.9
23.1
5 (manno) 52.3
20 (allo)
16 (altro)
8 (gluco)
27.2
18.2
28.0
25.4
6 (manno) 46.5
13 (allo)
34.8
17 (altro)
Synchrotron small angle X-ray scattering.—
Synchrotron small-angle X-ray scattering
(SAXS) is a powerful method to explore
supramolecular structure. Since the synchro-
tron X-ray is almost 106 times stronger than
conventional X-rays, it has great advantage
for diluted systems such as organogels. Terech
et al.26–28 analyzed SAXS from different non-
sugar-based organogelators. We measured
SAXS for the first time from sugar-based
organogelators and investigated from the b-se-
ries the gluco, manno and allo diastereomers
and a-mannopyranoside (5) at 1 wt.% in p-xy-
lene. The scattering patters obtained are
clearly different (Fig. 3). The manno
diastereomer of the b-series (6) shows at room
temperature a broad peak at q=0.018 that is
a All correlation coefficients are \0.98.
wt.%) required for gel-formation in this sol-
vent if one attributes a small DHgel value to
the affinity (or partial dissolution) of the gela-
tor for the solvents molecules. Furthermore,
diphenyl ether can act as a hydrogen-bond
acceptor and weakens the network formation
based on the intermolecular hydrogen-bond-
ing interaction. For p-xylene DHgel decreases
in the order 7\5\6, whereas in diphenyl
ether the order was estimated to be 5\7\6.
In general, the b-manno derivative (6) has the
highest affinity to the solvent molecules.
As expected for all monosaccharides, the
DHgel values are larger than the corresponding
DHm values, since DHgel values are associated
with solvation, whereas DHm values are not.
Interestingly, a comparison of DHm values for
all compounds reveals that the monosaccha-
rides acting as gelators possess, in general,
smaller values (16.0, 10.1, 18.7 kJ/mol) than
the non-gelators (20–30 kJ/mol). Therefore,
the less cohesive nature of the monosaccha-
rides seems to be beneficial for their ability to
gelate solvents.
probably due to the supramolecular structure
−1
,
of the gel and a second peak at q=0.014 A
.
Also the a-mannopyranoside (5) exhibits simi-
−1
,
lar broad peaks at q=0.04 and 0.09 A
from which the first can be assigned to the gel
structure as well. SAXS measurements of the
self-supporting precipitation (PS) from b-
gluco- and allopyranoside (7, 13) gave typical
scattering curves that are obtained from poly-
disperse systems by the superposition of the
component curves. This supports the assump-
tion that PS contains particles of identical
shape but too different in size to form a gel.
An attempt to obtain scattering data from the
a-glucosaccharide (7) 1 wt.% in p-xylene
failed, probably due to the small contrast to
give scattering.
FTIR spectroscopy.—The evidence that 4,6-
O-benzylidene monosaccharides form a hy-
drogen-bond-based gel-network has been
investigated in detail by FTIR and tempera-
ture-dependent 1H NMR spectroscopy.16a
Therefore, in this paper only a brief confirma-

O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
313
the peaks remains unchanged; however, the
In order to investigate which peaks are re-
lated to the gel structure, a temperature-de-
pendent measurement was carried out with 6
in p-xylene (1.5 wt.%). Fig. 4 shows the scat-
tering profiles obtained at 50, 60, 65, 70 and
80 °C. At 50 °C the curve contains two broad
−1
,
intensity of the peak at q=0.018 A
de-
creases about 50% and it disappears com-
pletely at 70 °C. Because the sol–gel transition
temperature (Tgel) of this system was estimated
by the oil-bath method to be 70 °C, this result
supports the assumption that the first peak
can be assigned to the supramolecular struc-
ture of the gel. A similar experiment was
peaks at q=0.018 and 0.11 A−1. With in-
,
creasing temperature (60 °C) the position of
carried out with 5 in p-xylene (1.5 wt.%), and
−1
,
the peak at q=0.04 A
disappeared as ex-
pected between 60 and 65 °C (Tgel=60 °C).
Furthermore, the clearer scattering pattern of
6 compared to 5 is probably due to a more
homogenous particle size in the gel, which
corresponds with the higher Tgel. Therefore,
the SAXS measurements support the conclu-
sions that 5 is inferior to 6 derived by the Tgel
measurements. The problem of improving the
scattering quality is currently under investiga-
tion to obtain more intense peaks that allow a
fit to a model for the molecular packing in the
gel state.
SEM obser6ation of xerogels.—To obtain
visual insights into the aggregation mode, dry
samples of organic gel fibers for SEM studies
have been prepared.29 It is clear that gelators
form a three-dimensional fiber network. Fig. 5
shows typical pictures obtained from xerogel
of 6 in carbon disulfide and the self-support-
ing precipitates (PS) of 8, 13, and 17 in p-xy-
lene and carbon tetrachloride. As expected,
the gelator 6 forms a three-dimensional net-
work with puckered fibrils (200–500 nm di-
ameter) that are connected with junction
nodes. Probably a disorder in aggregation is
the origin of these knots. We expected that the
hydrogen-bonding interactions among chiral
saccharide moieties might create a helical fiber
structure, but such a periodic, regular struc-
ture was not found in these SEM pictures.
Therefore, the absolute configuration is not
reflected in the fiber shape. In contrast, the
self-supporting precipitates show linear fibers
with lengths over 30 mm and diameters around
5 mm. In none of these cases could junction
points be observed. These results show that
compounds possessing the ability to form a
fiber structure do not necessarily act as good
gelators.
Fig. 3. Scattering profile for 5, 6, 7 and 13 (1 wt.% in
p-xylene) at room temperature.
Fig. 4. Temperature dependence of the SAXS profiles for 6
(1.5 wt.% in p-xylene).

314
O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
Fig. 5. SEM pictures of 6, 8, 13 and 17. (a) 6 prepared from carbon disulfide [1% (wt./vol)]; (b) 8 prepared from carbon
tetrachloride [1% (wt./vol)]; (c) 17 prepared from p-xylene [3% (wt./vol)]; (d) 13 prepared from p-xylene [1% (wt./vol)].
3. Conclusions
gelators. Partly these differences can be ex-
plained by the mode of action as to these
molecules gelate organic solvents. Since they
form a one-dimensional hydrogen-bonding ar-
ray under participation of both hydroxyl
The present study has demonstrated that
among the methyl glycosides of 4,6-O-benzyli-
dene monosaccharides only gluco and manno
in the a-series and methyl 4,6-O-benzylidene-
groups,30 methyl 4,6-O-benzylidene-a-
D
-altro-
pyranoside (16) and methyl 4,6-O-benzyli-
dene-a- -allopyranoside (20) cannot serve as
b- -mannopyranoside (6) act as efficient hy-
D
drogen-bond-based gelators. Judging from the
Tgel and the variety of solvents gelated, the
gelation capability for organic solvents de-
creases in the order of 6\5\7. The compari-
son of both mannopyranosides confirms the
conclusion derived from a previous study of
D
efficient gelators because of the possible
strong intramolecular 3-OH–1-OMe interac-
tion between both axial orientated groups.31
For the efficient formation of a gel, the hydro-
gen-bonding network to both hydroxyl groups
should be orientated so that they can form
one-dimensional intermolecular hydrogen
bonds.
D
-galactose derivatives16a that the b anomer is
more efficient than the a anomer. In sum-
mary, among ten investigated 4,6-O-benzyli-
dene monosaccharides, only
D
-mannose,
In addition, the gelation process depends on
the gelator’s ability to assemble into fiber-like
aggregates. Although the b-series shows a
D-galactose derivatives and methyl 4,6-O-ben-
zylidene-a- -glucopyranoside (7) serve as
D

O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
315
strong ability for fiber structure formation
indicated by the SEM pictures, 8, 13, and 17
do not meet the requirements to gelate sol-
vents. Due to the lack of junction nodes and
the enhanced fiber thickness, these compounds
are not able to form a three-dimensional fiber
network. Based on the DHm values obtained
from DSC, these compounds must be consid-
ered as too cohesive to act as gelators. Conse-
quently, they form instead of gels self-
supporting precipitates. From this point of
view a-gluco occupies an intermediate posi-
tion because it forms Ps as well as gels. There-
fore, the optimal requirements to be a gelator
are fulfilled only by the 4,6-O-benzylidene
erating voltage of SEM was 5 kV and the
emission current was 10 mA.
Apparatus for spectral measurements.—1H
and 13C NMR spectra were measured with a
Bruker ARX 300 instrument. Chemical shifts
are reported downfield from the internal stan-
dard, Me4Si. Spin–spin coupling is reported in
Hz. For 13C NMR data, DEPT assignments
are indicated as follows: +, positive; −,
negative; *, no signal. UV spectra were ob-
tained using a JASCO V-570 spectrometer. IR
spectra were recorded in KBr pills and NaCl
cells (0.5 mm) with a Shimazu FTIR 8100
spectrometer. A Hitachi M-2500 spectrometer
was used recording the mass spectra. The
SIMS measurements were carried out in a
glycerol or 3-nitrobenzylalcohol (NBA) matrix
with xenon as a primary ion.
derivatives of
D-mannose and
D-galactose.
The foregoing findings must be useful for
the discovery and the design of new gelators.
We believe that the saccharide library pro-
vided by nature can be applied further, in
particular to the design of molecular assem-
blies, such as macrocyles, DNA mimics,
monolayers, bilayer membranes and liquid
crystals.
HPLC analysis.—HPLC analysis was car-
ried out with a Waters 600 apparatus with a
Waters 490E detector. A Millipore puresil
,
column (5 mm, 120 A C18 0.6×150 mm) was
used with a gradient of 30:70–70:30 MeCN–
water in 20 min. Flow 1.5 mL/min. Detector:
u=210 nm, aufs 1.0.
Synchrotron SAXS measurement.—The syn-
chrotron SAXS measurement was carried out
at the BL-15A SAXS station at the Photon
Factory High Energy Research Organization
in Japan. The SAXS intensity was collected as
an accumulation of the scattered intensity dur-
ing 100 s by use of a one-dimensional position
sensitive detector with 20-cm length and 512
channels. Details of the optics and the instru-
mentation are described elsewhere.32
4. Experimental
General.—Compounds 9,18 10,22 11,23 12,19
14,22 15,23 18,20 and 1920 were prepared ac-
cording to known literature methods. All
1
compounds gave H NMR spectra and mp’s
that correlated with those reported. New com-
pounds were characterized by NMR, UV, IR
and MS spectroscopy and elemental analysis.
The purity of all target compounds 5,17 6, 7,16
8,16 13,19 16,22 17,24 and 2021 was confirmed by
HPLC analysis.
Preparation of methyl 4,6-O-benzylidene-h-
-mannopyranoside (5).—Compound 5 was
D
prepared according to the published method.17
Rf (1:1 EtOAc–hexane): 0.23. HPLC: TR
10.87 min. mp 149.3–150.0 °C (lit. 141–
143 °C17). 1H NMR (300 MHz, CDCl3): l 2.73
(d, 1 H, J3-H, 3-OH 2.1 Hz, 3-OH), 2.76 (d, 1 H,
J2-H, 2-OH 3.2 Hz, 2-OH), 3.39 (s, 3 H, OCH3),
3.75–3.93 (m, 3 H, 2-H, 4-H, 6-HA), 4.01–
4.07 (m, 2 H, 3-H, 5-H), 4.28 (dd, 1 H, J6-H-B,
6-H-A 8.2, J6-H-B, 5-H 2.8 Hz, 6-HB), 4.74 (s, 1
H, 1-H), 5.56 (s, 1 H, 7-H), 7.36–7.39 (m, 3
H, Ar H), 7.47–7.50 (m, 2 H, Ar H). SIMS (6
keV, glycerol): m/z (%): 283 (79) [M+H]+,
282 (4) [M+].
Gel–sol transition temperatures.—The test
tube containing the gel was immersed in-
versely in a thermostated oil bath. The tem-
perature was raised at a rate of 2 °C/min.
Here, the Tgel was defined as the temperature
at which the gel turned into the sol-phase.
SEM obser6ations.—A Hitachi S-900S
scanning electron microscope was used for
taking the SEM pictures. The gel was pre-
pared in a sample tube and frozen in liquid
N2. The frozen specimen was evaporated by a
vacuum pump for 10–15 h. The dry sample
obtained was shielded with platin. The accel-

316
O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
3.42 (s, 3 H, OCH3), 3.46 (dd, 1 H, J4-H, 3-H
9.2, J4-H, 5-H 9.2 Hz, 4-H), 3.59 (dd, 1 H, J2-H,
1-H 3.9, J2-H, 3-H 9.2 Hz, 2-H), 3.71 (dd, 1 H,
J6-H-A, 6-H-B 9.5, J6-H-A, 5-H 9.5 Hz, 6-HA), 3.71–
3.79 (m, 1 H, 5-H), 3.90 (dd, 1 H, J3-H, 2-H 9.2,
J3-H, 4-H 9.2 Hz, 3-H), 4.26 (dd, 1 H, J6-H-B, 6-H-A
9.5, J6-H-B, 5-H 3.7 Hz, 6-HB), 4.75 (d, 1 H, J1-H,
2-H 3.9 Hz, 1-H), 5.50 (s, 1 H, 7-H), 7.33–
7.37 (m, 3 H, Ar H), 7.46–7.49 (m, 2 H, Ar
H). SIMS (6 keV, glycerol): m/z (%): 283 (21)
[M+H]+.
Methyl 4,6-O-benzylidene-i-
noside (6).—Methyl b- -mannopyranoside
D
-mannopyra-
D
isopropylate (4, 0.30 g, 1.18 mmol) and pyrid-
inum p-toluenesulfonate (PPTS, 3.10 mg, 0.01
mmol) were placed in a flask and dissolved in
3 mL of dry DMF. The mixture was heated to
80 °C, and a,a-dimethoxytoluene (578 mL,
3.75 mmol) was added dropwise under a
steady steam of dry N2 to remove the liberated
MeOH (6.5 h, 80 °C). The solvent was evapo-
rated, and the residue was subjected to
column chromatography (Wakogel C-300, 1:1
EtOAc–hexane) yielding 152.4 mg of a solid
as a mixture of 4,6- and (R,S)-2,3-O-benzyli-
dation products. This mixture was treated
with tert-butyldiphenylchlorosilane (207 mL,
0.80 mmol) and N,N-dimethylaminopyridine
(DMAP, 10.00 mg, 0.09 mmol) in 2.4 mL of
dry pyridine for 4 h. The solvent was removed
and the residue was subjected to column chro-
matography (Wakogel C-300, 2:1 EtOAc–
hexane). Yield: 63 mg (0.22 mmol, 19%) of 6
as a colorless solid. Rf (2:1 EtOAc–hexane):
0.10. HPLC: TR 7.99 min. mp 169.5–173.7 °C.
1H NMR (300 MHz, CDCl3): l 2.57 (d, 1 H,
J3-H, 3-OH 1.9 Hz, 3-OH), 2.66 (d, 1 H, J2-H, 2-OH
6.4 Hz, 2-OH), 3.33–3.41 (m, 1 H, 5-H), 3.58
(s, 3 H, OCH3), 3.80–3.93 (m, 3 H, 2-H, 4-H,
6-HA), 4.11–4.12 (m, 1 H, 3-H), 4.35 (dd, 1 H,
J6-H-B, 6-H-A 10.5, J6-H-B, 5-H 5.0 Hz, 6-HB), 4.50
(d, 1 H, J1-H, 2-H 1.1 Hz, 1-H), 5.56 (s, 1 H,
7-H), 7.36–7.38 (m, 3 H, Ar H), 7.48–7.51
Preparation of methyl 4,6-O-benzylidene-i-
D-glucopyranoside (8).—Compound 8 was
prepared according to the published method.16
Rf (3:1 EtOAc–hexane): 0.31 HPLC: TR 9.05
min. mp 207.3–208.6 °C (lit. 174–175 °C16).
1H NMR (300 MHz, CDCl3): l 2.67, 2.83 (d,
1 H, J 2.5, J 2.4 Hz, 2-OH, 3-OH), 3.34–3.53
(m, 2 H, 2-H, 5-H), 3.52 (dd, 1 H, J4-H, 3-H 9.0,
J4-H, 5-H 9.0 Hz, 4-H), 3.58 (s, 3 H, OCH3),
3.79 (dd, 1 H, J6-H-A, 6-H-B 10.5, J6-H-A, 5-H 10.5
Hz, 6-HA), 3.82 (ddd, 1 H, J3-H, 3-OH 2.3, J3-H,
2-H 9.0, J3-H, 4-H 9.0 Hz, 3-H), 4.33 (d, 1 H,
J1-H, 2-H 7.7 Hz, 1-H), 4.35 (dd, 1 H, J6-H-B, 6-H-A
10.5, J6-H-B, 5-H 9.2 Hz, 6-HB), 5.54 (s, 1 H,
7-H), 7.36–7.38 (m, 3 H, Ar H), 7.48–7.51
(m, 2 H, Ar H). SIMS (6 keV, glycerol): m/z
(%): 283 (13) [M+H]+.
Preparation of methyl 4,6-O-benzylidene-i-
-allopyranoside (13).—Compound 13 was
D
prepared according to the published method.19
Rf (3:1 EtOAc–hexane): 0.23. HPLC: TR 8.93
min. mp 171.5–172.5 °C (lit. 173–174 °C19).
1H NMR (300 MHz, CDCl3): l 2.56 (bs, 1 H,
3-OH), 2.67 (d, 1 H, J2-OH, 2-H 6.3 Hz, 2-OH),
3.47–3.53 (m, 1 H, 2-H), 3.57 (s, 3 H, 1-
OCH3), 3.58 (dd, 1 H, J4-H, 3-H 2.5, J 4-H, 5-H 9.3
Hz, 4-H), 3.76 (dd, 1 H, J6-H-A, 6-H-B 10.3,
J6-H-A, 5-H 10.3 Hz, 6-HA), 3.95–4.04 (m, 1 H,
5-H), 4.37 (bs, 1 H, 3-H), 4.40 (dd, 1 H, J6-H-B,
6-H-A 10.4, J6-H-B, 5-H 5.0 Hz, 6-HB), 4.62 (d,
J1-H, 2-H 7.9 Hz, 1 H, 1-H), 5.57 (s, 1 H, 7-H),
7.35–7.39 (m, 3 H, Ar H), 7.46–7.51 (m, 2 H,
Ar H). SIMS (6 keV, NBA): m/z (%): 283 (36)
[M+H]+, 282 (4) [M+].
13
(m, 2 H, Ar H). C NMR (75 MHz, CDCl3):
l 66.5, 70.7, 70.8, 78.5 (+, C-2, C-3, C-4,
C-5, OCH3), 68.4 (−, C-6), 101.2, 102.0 (+,
C-1, C-7), 126.2, 128.2, 129.1 (+, Ar C),
137.0 (*, Ar C). IR (KBr) wmax (cm−1): 696
(m), 723 (w), 760 (w), 787 (w), 980 (m), 991
(m), 1049 (s), 1103 (vs), 1172 (m), 1238 (m),
1385 (m), 1452 (m), 2883 (s), 3454 (vs), 3534
(vs). UV(MeOH) umax (log m): 207.5 nm (3.87).
SIMS (6 keV, glycerol): m/z (%): 283 (100)
[M+H]+, 282 (4) [M+]. Anal. Calcd for
C14H18O6 (282.3): C, 59.56; H, 6.42. Found C,
59.59, H, 6.48.
Preparation of methyl 4,6-O-benzylidene-h-
Preparation of methyl 4,6-O-benzylidene-h-
D
-glucopyranoside (7).—Compound 7 was
D
-altropyranoside (16).—Compound 16 was
prepared according to the published method.16
Rf (3:1 EtOAc–hexane): 0.17. HPLC: TR 9.71
min. mp 164.6–166.4 °C (lit. 165.4–
prepared according to the published method.22
Rf (2:1 EtOAc–hexane): 0.40. HPLC: TR 9.31
min. mp 170.4–172.8 °C (lit. 169–170 °C22).
1H NMR (300 MHz, CDCl3): l 2.05, 2.88 (d,
1
166.8 °C16). H NMR (300 MHz, CDCl3): l

O. Gronwald et al. / Carbohydrate Research 331 (2001) 307–318
317
References
1 H, J 5.9, J 6.7 Hz, 2-OH, 3-OH), 3.45 (s, 3
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J6-H-A, 5-H 10.1 Hz, 6-HA), 3.98 (dd, 1 H, J4-H,
3-H 3.0, J4-OH, 5-H 9.1 Hz, 4-H), 4.03–4.06,
4.10–4.13 (m, 1 H, 2-H, 3-H), 4.17–4.26 (m,
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