Stereoselective single-step synthesis and X-ray crystallographic investigation of acetylated aryl 1,2-trans glycopyranosides and aryl 1,2-cis C2-hydroxy-glycopyranosides

Article abstract of DOI:10.1016/j.carres.2005.10.010

Reported is an attractive and environmentally friendly method for the synthesis of the title compounds in moderate yield using inexpensive 1,2,3,4,6-penta-O-acetyl-β-d-gluco- and galactopyranoses as sugar donors, five different phenols as acceptors and H-β zeolite as the catalyst. The yield (23-28%) of aryl 3,4,6-tri-O-acetyl-α-d-glycopyranosides obtained in this single-step procedure is considerably higher than that obtained using previously reported methods. Treatment of an orthoacetate, 3,4,6-tri-O-acetyl- [1,2-O-(1-p-fluorophenoxyethylidene)]-α-d-glucopyranose, with p-fluorophenol under the same solvent-free reaction conditions also led to the formation of the title compounds in similar yield and composition. X-ray crystallographic analysis of phenyl 3,4,6-tri-O-acetyl-α-d-glucopyranoside and p-fluorophenyl 3,4,6-tri-O-acetyl-α-d-glucopyranoside showed that the molecular packing is stabilized by C-H...O, C-H...π and C-H...F interactions, in addition to regular hydrogen bonding patterns.

Full text of DOI:10.1016/j.carres.2005.10.010

Carbohydrate
RESEARCH
Carbohydrate Research 341 (2006) 19–28
Stereoselective single-step synthesis and X-ray
crystallographic investigation of acetylated aryl 1,2-trans
glycopyranosides and aryl 1,2-cis C2-hydroxy-glycopyranosides
Udayanath Aich and Duraikkannu Loganathan*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
Received 3 August 2005; received in revised form 9 October 2005; accepted 14 October 2005
Available online 22 November 2005
Abstract—Reported is an attractive and environmentally friendly method for the synthesis of the title compounds in moderate yield
using inexpensive 1,2,3,4,6-penta-O-acetyl-b-D-gluco- and galactopyranoses as sugar donors, five different phenols as acceptors and
H-b zeolite as the catalyst. The yield (23–28%) of aryl 3,4,6-tri-O-acetyl-a-D-glycopyranosides obtained in this single-step procedure
is considerably higher than that obtained using previously reported methods. Treatment of an orthoacetate, 3,4,6-tri-O-acetyl-[1,2-
O-(1-p-fluorophenoxyethylidene)]-a-D-glucopyranose, with p-fluorophenol under the same solvent-free reaction conditions also led
to the formation of the title compounds in similar yield and composition. X-ray crystallographic analysis of phenyl 3,4,6-tri-O-acetyl-
a-D-glucopyranoside and p-fluorophenyl 3,4,6-tri-O-acetyl-a-D-glucopyranoside showed that the molecular packing is stabilized by
C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀF interactions, in addition to regular hydrogen bonding patterns.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Carbohydrates; Helferich glycosidation; Aryl C-2-hydroxy-glycoside; H-b Zeolite; X-ray crystallography; C–Hꢀ ꢀ ꢀO; C–Hꢀ ꢀ ꢀF and
C–Hꢀ ꢀ ꢀp interactions
1. Introduction
ease of preparation. The various catalysts4,5 employed
for Helferich glycosidation include ZnCl2, p-TsOH,
POCl3, H2SO4, BF3–Et2O, SnCl4, FeCl3 and AlCl3.
Depending on the nature of catalyst and reaction
conditions, either a- or b-glycopyranosides have been
obtained as the major product in varying yield.
Although a few speculations have been made previously,
the mechanism of the Helferich glycosidation still
remains to be elucidated.
Acetylated aryl glycopyranosides carrying a free C2–
OH group have been isolated from several medicinal
plants belonging to the genera Bidens6a,c–e and Sene-
cio,6b the fern Phegopteris connectilis6f and also from
the marine organism, Euplexaura anastomosans.7 One
such natural product viz., moritoside, inhibits cell divi-
sion of fertilized starfish eggs,7a while others have been
shown to exhibit diverse bioactivities including moder-
ate cytotoxicity7b,c and inhibition of viral replication.6c
Many reports have appeared in the recent literature on
the stereoselective formation of protected aryl 1,2-trans
Aryl O-glycosides occur widely in nature and their com-
plex structures and diverse biological activities make
them attractive targets for synthesis. Applications of
aryl O-glycosides include their use as chromogenic
substrates for glycosidases1 and as inhibitors of carbo-
hydrate–lectin interactions.2 Preparation of fully
acetylated aryl glycosides using sugar peracetates as
donors and Lewis/Brønsted acids as catalysts was first
reported by Helferich and Schmitz-Hillebrecht in 1933.3
Although several effective donors, such as glycosyl tri-
chloroacetimidates and thioglycosides have subse-
quently been introduced, sugar peracetates are still
commonly used for the chemical synthesis4 of O-glyco-
sides in view of their ready availability, low cost and
*
Corresponding author. Tel.: +91 44 22574206; fax: +91 44
22570509; e-mail: loganath@iitm.ac.in
0008-6215/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.10.010

20
U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
C2-hydroxyglycopyranosides8 and all these are based
on multi-step procedures. The stereoselective formation
of 1,2-cis linkages is, in general, considerably more chal-
lenging.9 Extension of the Stork silicon tether approach
by Bols10 represents the only report on the preparation
of aryl 1,2-cis C2-hydroxy-glycopyranosides. This five-
step synthesis starting from b-D-glucose pentaacetate
(1) afforded phenyl 3,4,6-tri-O-acetyl-a-D-glucopyran-
oside (6a) in less than 12% overall yield.
As part of our major research program aimed at
developing environmentally friendly synthetic method-
ologies for carbohydrate transformations, we have
found that H-b zeolite is an efficient catalyst for the per-
acetylation of sugars.11 Exploration of the Helferich
reaction of several phenols with 1,2,3,4,6-penta-O-ace-
tyl-b-D-gluco- and galactopyranoses has now led to the
stereoselective synthesis of aryl 2,3,4,6-tetra-O-acetyl-
b-D-glycopyranosides, 4a–e and 5a–e and aryl 3,4,6-tri-
O-acetyl-a-D-glycopyranosides 6a–e and 7a–e (Scheme
1) in moderate yields. We also demonstrate, for the first
time, that the H-b zeolite-catalyzed transformation of an
orthoacetate, 3,4,6-tri-O-acetyl-[1,2-O-(1-p-fluorophen-
oxyethylidene)]-a-D-glucopyranose, to 4e and 6e in
similar yield. Single crystal X-ray analysis of 6a and 6e
reveals similarities in molecular recognition patterns
involving C–Hꢀ ꢀ ꢀO as well as C–Hꢀ ꢀ ꢀp interactions in
the crystal packing of both compounds as well as differ-
ences of which the most notable is a C–Hꢀ ꢀ ꢀF interac-
tion in the latter compound.
lites examined as catalysts for peracetylation of
sugars,11 H-b zeolite proved to be highly efficient due
probably to its greater acid strength and larger pore
openings and channel intersections. Hence, the same
catalyst was chosen for the present study. The feasibility
of H-b zeolite catalyzed Helferich glycosidation was ini-
tially examined using b-D-glucose pentaacetate as the
donor (1) and phenol (3a, 5 equiv) as the acceptor at
140 ꢁC under solvent free conditions (Scheme 1). After
12 h, when the conversion reached a maximum as
shown by TLC analysis, the reaction mixture was
cooled to room temperature and worked-up. Column
chromatography of the crude product over silica gel
afforded predominantly phenyl 2,3,4,6-tetra-O-acetyl-
b-D-glucopyranoside (4a; 46%) as well as phenyl 3,4,6-
tri-O-acetyl-a-D-glucopyranoside (6a, 23%) both of
which were characterized based on comparison of their
physical and spectral data with those reported in the lit-
erature.13,10 Careful 1H NMR analysis of two minor
fractions obtained from column chromatography did re-
veal the formation of the corresponding a-anomer of 4a
and b-anomer of 6a each in less than 5% yield. The ste-
reoselectivity of the H-b zeolite catalyzed reaction was
found to be general with respect to different substituted
phenols (3a–e) and also with b-D-galactose pentaacetate
(2) (Table 1).
All the other known peracetylated aryl b-D-glyco-
pyranosides (4b–e) were characterized based on compar-
ison of their physical and spectral data with those
reported in the literature.13 The hitherto unknown aryl
3,4,6-tri-O-acetyl-a-D-glycopyranosides 6b–e and 7a–e
have been fully characterized using two-dimensional
1H NMR spectroscopy and ESI-MS analysis. All com-
pounds 6a–e and 7a–e exhibited only three methyl
2. Results and discussion
2.1. Synthesis and mechanistic study
1
signals of the acetyl groups in their H NMR spectra.
Zeolites have been shown to be efficient catalysts for
several liquid-phase organic transformations.12 The
inherent shape selectivity and the potential to fine tune
the Brønsted/Lewis acidity of zeolites makes them ideal
catalysts for Helferich glycosidation yet they are hith-
erto unexplored in this regard. Among the various zeo-
The anomeric proton signal in the NMR spectra of
6a–e and 7a–e appears at around 5.50 ppm as a doublet
with a coupling constant value of about 3.7 Hz revealing
the presence of 1,2-cis glycosidic linkage. The H-2 signal
is observed in the region 3.80–4.20 ppm, up-field shifted
from the region of 5.10–5.30 ppm noted for the corre-
OAc
R1
O
OH
OAc
R1
R2
O
AcO
O
R2
H-β Zeolite
OAc
OAc
R
OAc
+
AcO
+
140 o C
R1
OAc
4a-e & 5a-e
36 - 46 %
O
R
R2
3a - 3e
1. Glc : R1 = H, R2 = OAc
2. Gal : R1 = OAc, R2 = H
AcO
H
O
3a: R = H, 3b: R = CH3,
3c: R = OCH3, 3d: R = Cl,
& 3e: R = F
O
6a-e & 7a-e
23 - 28 %
R
Scheme 1. H-b Zeolite catalyzed synthesis of acetylated aryl 1,2-trans-glycopyranosides and aryl 1,2-cis C2-hydroxy-glycopyranosides.

U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
Table 1. H-b Zeolite catalyzed reaction of sugar peracetates with phenols
21
Entry no.
Phenols
R
Sugar
Product 4/5
% Yielda
Product 6/7
% Yielda
Total 4+6/5+7 % Yielda
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3a
3b
3c
3d
3e
H
1
1
1
1
1
2
2
2
2
2
4a
4b
4c
4d
4e
5a
5b
5c
5d
5e
46
40
40
40
38
43
43
42
36
37
6a
6b
6c
6d
6e
7a
7b
7c
7d
7e
23
25
23
25
26
26
28
27
24
28
69
65
63
65
64
69
71
69
60
65
CH3
OCH3
Cl
F
H
CH3
OCH3
Cl
10
F
a Based on isolated pure product.
sponding fully protected aryl glycosides, confirming the
free nature of the C2 hydroxyl group in 6a–e and 7a–e.
The 13C–19F coupling constants observed for the p-flu-
orophenyl glycosides 6e and 7e are in reasonable agree-
ment with those reported for p-fluorophenyl ethers.14
Furthermore, the identity of 6a and 6e as phenyl 3,4,6-
tri-O-acetyl-a-D-glucopyranoside and p-fluorophenyl
3,4,6-tri-O-acetyl-a-D-glucopyranoside, respectively, has
been unambiguously established by single crystal
X-ray diffractometry (see below). The total yield (60–
71%) of the aryl 2,3,4,6-tetra-O-acetyl-b-D-glycopyran-
oside and aryl 3,4,6-tri-O-acetyl-a-D-glycopyranoside
obtained in each of the reactions is quite good consider-
ing that the reaction conversion is about 85–90%. As
phenols in general are poor nucleophiles as compared
to alcohols, earlier methods of Lewis acid catalyzed
Helferich glycosidation often employed activated forms
of phenols such as higher-toxicity tributyltin deriva-
tives,15 to achieve higher yields. The excellent activation
of the sugar peracetate by the solid acid H-b zeolite
obviates the need for such modification of phenols.
In spite of the fact that the Helferich method has been
widely employed for aryl glycoside synthesis for the past
70 years, the reaction mechanism remains to be estab-
lished. The role of neighbouring group participation
by the carbonyl oxygen of 2-O-acyl group on the stereo-
selectivity of various chemical glycosidation methods
has been extensively studied.16 Brønsted acid-catalyzed
Helferich glycosidations have invariably been performed
using 1,2-trans peracetylated sugars. An early key step in
the mechanism of the H-b zeolite-catalyzed Helferich
glucosidation is expected to be the C2 acetate assisted
cleavage of the glycosidic bond of the activated (proton-
ated) peracetate leading to the formation of the 1,3-
dioxolenium ion (I, Scheme 2). Nucleophilic attack by
the phenol at the anomeric carbon of I results in the ste-
reoselective formation of aryl 2,3,4,6-tetra-O-acetyl-b-D-
glucopyranoside. Alternatively, conversion of I to the
protonated form of the orthoacetate, III, followed by
a nucleophilic attack at the anomeric carbon could also
lead to the same product, particularly in view of the fact
that phenol is used in excess. Formation of a minor
amount of the corresponding a-glucopyranoside could
be rationalized through the intermediacy of the glycosyl
oxocarbenium ion, II. A control reaction involving pure
p-fluorophenyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyran-
oside as a substrate and 4 equiv of p-fluorophenol under
same the conditions employed in Scheme 1 led to only
5% anomerization revealing the predominance of the
1,3-dioxolenium ion I over II as otherwise the thermo-
dynamically more stable a-anomer would have been ob-
tained as the major product. The use of ZnCl2 has been
known to bring about 34% anomerization of phenyl
2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside under Helfe-
rich reaction conditions.13a
The formation of acetylated alkyl 2-hydroxy-glyco-
pyranosides during Helferich-type glycosidations has
been reported earlier.17 Acyl transfer accompanied by
the formation of glycopyranosides with a free C2 OH
group has been studied in detail.18 Whitfield and
co-workers18f have recently shown from theoretical cal-
culations that orthoesters are not intermediates in acyl
transfer but can be converted to an intermediate or
intermediates that lead to either acyl transfer or glyco-
side formation. To gain some insight into the formation
of C2–OH derivatives 6a–e and 7a–e, we have examined
the reaction of an orthoacetate, 3,4,6-tri-O-acetyl-[1,2-
O-(1-p-fluorophenoxyethylidene)]-a-D-glucopyranose,19
with p-fluorophenol (4 equiv) in the presence of H-b zeo-
lite under the same conditions employed in Scheme 1.
This reaction afforded 4e and 6e in 31% and 26% yields,
respectively. The a-anomer of 4e (7%) and b-anomer
of 6e (<2%) were also formed. The yields of these two
minor products obtained from the reaction of 1 with
3e are 4% and <2%, respectively, while the major prod-
ucts 4e and 6e are obtained in 38% and 26% yields,
respectively (see Table 1, entry 5), pointing out reason-
ably good agreement between these two reactions in
terms of product yield, composition and stereoselectiv-
ity. Based on the results described above and consider-
ing that the activation provided by H-b zeolite does
not appear to be strong enough for the facile depar-
ture of the anomeric acetate to lead to the direct forma-
tion of the intermediate II, it is likely that the aryl

22
U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
OAc
OAc
O
OAc
O
AcO
AcO
a
O
AcO
AcO
AcO
AcO
+
CH3
O
O
O
O
O
+
H
+
I
O
II
H3C
O
CH3
O
b
H3C
- ArOH
+ ArOH
a
b
OAc
O
OAc
c
AcO
AcO
OAc
O
H
AcO
AcO
+
AcO
AcO
O
ArOH
O
+
Ar
OAc
O
c
III
O
H
V
CH3
HO
+
H+
+
ArOCOCH3
O
+ H+
- H+
~
Ar
ArOH
OAc
OAc
AcO
AcO
O
OAc
O
O
AcO
AcO
AcO
AcO
O
Ar
O
IV
OAc
+
OH
O
CH3
O
H
Ar
O
Ar
Scheme 2. Plausible reaction mechanism.
2-hydroxy-glucopyranosides result from the intermedi-
ate V, which in turn is formed via IV from the proto-
nated orthoacetate, III, by proton transfer.
actions between the hydrophobic C–H groups of carbo-
hydrate residues and the p-electron systems of aromatic
amino-acid residues have been suggested to play an
important role in the ligand-recognition function of car-
bohydrate-binding proteins.22
2.2. X-ray crystallographic study
The details of the crystal data, intensity data collec-
tion and structure refinement for 6a and 6e are provided
in Table 2. The ORTEP depictions for 6a and 6e with
atom numbering are shown in Figure 1. Selected bond
lengths and bond angles involving non-hydrogen atoms
are provided in Table 3. The C–C bond lengths, which
A systematic crystallographic study of the structural and
conformational aspects of glycosides provided the key
understanding of the differences in the electronic proper-
ties of a- and b-pyranosides and the preferred gauche
conformation of the glycosidic bond.20 Crystal structure
data of a relatively large number of alkyl glycosides are
available but, in contrast, those of aryl glycosides are
rather limited. A single-crystal X-ray investigation of
6a and 6e was undertaken not only to unambiguously
establish their structures but also to unravel their
conformation and the features of non-covalent weak
interactions such as the regular hydrogen bond and
the C–Hꢀ ꢀ ꢀX (where X = O/F/p) interactions that could
govern their molecular assembly. This knowledge would
be valuable in correlating the structure with the biolo-
gical activity7 of the naturally occurring acetylated aryl
C2-hydroxy-glycopyranosides. The occurrence of C–
Hꢀ ꢀ ꢀO interactions in carbohydrate crystal structures
have been well documented in the literature.21 The inter-
˚
vary between 1.488(9) and 1.530(8) A, are comparable
to those observed in most sugar derivatives. Between
the endocyclic C–O bond lengths, namely C1–O5 and
C5–O5, the former is found to be shorter than the latter.
This trend is generally observed for several glycopyran-
osides and the shortening of the C1–O5 bond has been
attributed to a stabilizing effect arising from the delocal-
ization of the glycosidic oxygen lone pair of electrons
into the anti-bonding C1–O1 orbital.23 Both experimen-
tal and theoretical studies on aldopyranoses have shown
earlier that the C1–O1 bond as well as O5–C1–O1 angle
4
in C1(D)-a configuration are greater than those in the
4C1(D)-b configuration.24 Comparative analysis of these
parameters for compounds 6a, 6e, p-nitrophenyl 2,3,4,6-

U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
Table 2. Data collection and refinement parameters for compounds 6a and 6e
23
Parameter
Compound 6a
Compound 6e
Empirical formula
Formula weight
C
366.36
18H22O8
C18H21FdO9
400.35
Temperature
Wavelength
293(2) K
293(2) K
0.71073 A
˚
˚
0.71073 A
Crystal system space group
Unit cell dimensions
Volume
P212121, orthorhombic
a = 8.5875(15) A, b = 9.7868(16) A, c = 22.431(4) A
1885.2(5) A3
P212121, orthorhombic
a = 8.653(5) A, b = 9.787(4) A, c = 22.437(3) A
1900.0(13) A3
˚
˚
˚
˚
˚
˚
Z
4
4
Calculated density
Absorption coefficient
F(000)
1.291 Mg/m3
1.400 Mg/m3
0.102 mmꢁ1
776
0.112 mmꢁ1
840
Crystal size
0.3 · 0.2 · 0.2 mm
0.3 · 0.2 · 0.2 mm
Theta range for data collection
Index ranges
2–25ꢁ
2–25ꢁ
0 <= h <= 10, 0 <= k <= 11, 0 <= l <= 26
1916/1916 [R(int) = 0.0000]
99.4%
Full-matrix least-squares on F2
1916/1/249
0 <= h <= 10, 0 <= k <= 11, 0 <= l <= 26
1937/1937 [R(int) = 0.0000]
99.9%
Full-matrix least-squares on F2
1937/0/258
Reflections collected/unique
Completeness to 2 theta
Refinement method
Data restraint parameters
Goodness-of-fit on F2
Final R indices [2<sigma>(I)]
R indices (all data)
1.018
1.032
R1 = 0.0530, wR2 = 0.1242
R1 = 0.1085, wR2 = 0.1490
R1 = 0.0588, wR2 = 0.1251
R1 = 0.1192, wR2 = 0.1554
tetra-O-acetyl-b-D-glucopyranoside (8)25a and the free
p-nitrophenyl a-D-glucopyranoside (9)25b (Table 3)
supports these earlier findings.
The torsion angles, C4–C5–C6–O6 and O5–C5–C6–
O6, observed for 6a and 6e are both close to ꢁ60ꢁ and
180ꢁ, respectively (Table 3), pointing out that the
hydroxymethyl group in these compounds adopts a gt
conformation. The O5–C1–O1–C10 torsion angle values
of compounds 6a and 6e are ꢁ73.9(5)ꢁ and ꢁ77.4(6)ꢁ,
respectively, falling within the range 60–85ꢁ observed
for other aryl pyranosides.25 Similarly, the C60–C10–
O1–C1 torsion angle of compounds 6a and 6e are
ꢁ0.3(6)ꢁ and ꢁ0.7(9)ꢁ, which are close to ꢁ7.3ꢁ reported
for 8.25b Thus, the phenyl ring in both 6a and 6e is
coplanar with the anomeric carbon atom C1 and this
orientation facilitates maximum delocalization of elec-
trons from the lone-pair orbitals of O1 with the p orbi-
tals of the phenyl ring. All three acetate groups in each
of the compounds 6a and 6e adopt the Z conformation.
The molecular assembly of 6a and 6e in the solid state
is driven by an infinite chain of hydrogen bonds involv-
ing C2–OH as the donor and O9 as the acceptor and
extending along the crystallographic a-axis (Table 4).
The packing in compound 6e is also stabilized by a
unique and strong finite chain of C5–Hꢀ ꢀ ꢀF hydrogen
˚
bonds [d(Dꢀ ꢀ ꢀA) = 3.526 A, <DHA = 168ꢁ] (Fig. 2).
Other features of molecular packing common to both
the compounds include several C–Hꢀ ꢀ ꢀO interactions
(Table 4). Besides accepting the hydrogen from C2–
OH, O9 is also hydrogen bonded to C2–H thus serving
as a bifurcated acceptor (Fig. 3). The oxygen atom
(O2) of C2–OH is, in turn, tri-coordinated as it
is bonded to C4-H as well as C6-H. Furthermore,
Figure 1. ORTEP with atom numbering of compounds 6a and 6e.

24
U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
Table 3. Selected bond lengths, bond angles and torsion angles of compounds 6a and 6e
OAc
OH
OAc
Parameter
OAc
O
O
O
O
AcO
AcO
HO
HO
AcO
AcO
AcO
AcO
O
OH
OH
OAc
OH
O
O
NO
O
2
F
NO
825a
2
6a
6e
925b
Bond lengths
C1–O1
C1–O5
1.408(3)
1.426(5)
1.416(7)
1.424(7)
1.408(7)
1.452(7)
1.374(6)
—
1.417(3)
1.432(3)
1.383(3)
1.393(5)
1.422(5)
1.393(5)
1.387(8)
1.437(7)
1.384(7)
1.384(8)
C5–O5
C10–O1
C40–F
Bond angle
O5–C1–O1
107.9(2)
63.5(2)
ꢁ177.9(2)
ꢁ73.6(2)
18.4(6)
111.8(3)
112.8(5)
111.0(3)
Torsion angles
O5–C5–C6–C6
C4–C5–C6–C6
O5–C1–O1–C10
C60–C10–O1–C1
ꢁ73.9(5)
165.9(3)
ꢁ66.8(4)
ꢁ0.3(6)
ꢁ77.4(6)
165.0(5)
ꢁ67.0(7)
ꢁ0.7(9)
—
—
71.8
ꢁ7.3
Table 4. Hydrogen bonds in compound 6a and 6e
˚
˚
Hydrogen bonds
Hꢀ ꢀ ꢀA in A
Dꢀ ꢀ ꢀA in A
<DHA in ꢁ
Phenyl 3,4,6-tri-O-acetyl-a-D-glucopyranoside (6a)
C8–Hꢀ ꢀ ꢀO11
2.700
2.645
2.713
2.673
2.561
2.163
2.724
2.893
3.406
3.454
3.488
3.529
3.361
2.904
3.644
3.506
131
140
128
149
139
140
151
125
C4–Hꢀ ꢀ ꢀO2
C6–Hꢀ ꢀ ꢀO2
C12–Hꢀ ꢀ ꢀO7
C2–Hꢀ ꢀ ꢀO9
O2–Hꢀ ꢀ ꢀO9
C6–Hꢀ ꢀ ꢀp(C20)
C20–Hꢀ ꢀ ꢀp(C40)
p-Fluorophenyl 3,4,6-tri-O-acetyl-a-D-glucopyranoside (6e)
C5–Hꢀ ꢀ ꢀF
2.559
2.686
2.671
2.600
2.566
2.232
2.858
3.526
3.490
3.466
3.529
3.356
2.957
3.717
168
139
139
150
138
140
148
C4–Hꢀ ꢀ ꢀO2
C6–Hꢀ ꢀ ꢀO2
C12–Hꢀ ꢀ ꢀO7
C2–Hꢀ ꢀ ꢀO9
O2–Hꢀ ꢀ ꢀO9
C6–Hꢀ ꢀ ꢀp (C20)
Figure 2. Molecular packing of p-fluorophenyl 3,4,6-tri-O-acetyl-a-D-
glucopyranoside (6e) showing an infinite chain (C2–OHꢀ ꢀ ꢀO9) and a
finite chain (C5–Hꢀ ꢀ ꢀF) of hydrogen bonds. Most of the hydrogen
atoms are omitted for clarity.
C–Hꢀ ꢀ ꢀp interactions lend additional stability to the
packing in both these compounds (Table 4 and Fig. 3).
In conclusion, we have developed an attractive
and environmentally friendly method for the synthesis
of acetylated aryl 1,2-trans glycopyranosides and aryl
1,2-cis C2-hydroxy-glycopyranosides from readily avail-
able and inexpensive hexopyranose pentaacetates. The
present single-step procedure affording aryl 3,4,6-tri-O-
acetyl-a-D-glycopyranosides in a moderate yield of
23–28% is preferable to the reported multi-step method-
ology that gives an overall yield of less than 12%. Except
the phenyl derivative, all the nine aryl 3,4,6-tri-O-acetyl-
a-D-gluco- and galactopyranosides synthesized are
hitherto unknown. Besides the biological importance
mentioned earlier, C2-hydroxy-glycosides are also valu-
able intermediates for the synthesis of 2-deoxy, 2-azido
and 2-halo glycosides. Furthermore, acetylated glyco-
sides with free C2–OH group serve as key intermediates
for the synthesis of oligosaccharides and glycoconju-
gates containing 1,2-linkages that occur in bacteria,
plants as well as higher animals.26 Given the significance
of C2-hydroxy-glycosides, the present methodology
should prove to be useful for diverse applications in gly-
cobiology research. X-ray crystallographic analysis of
two aryl 3,4,6-tri-O-acetyl-a-D-glucopyranosides has
shown that the molecular packing is stabilized by
C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀF interactions in addi-
tion to regular hydrogen bonding. These findings will be

U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
25
Freshly activated H-b-zeolite (200 mg) was added to
the melt obtained. The resulting mixture was magneti-
cally stirred and the temperature raised to 140 ꢁC. The
progress of the reaction was monitored by TLC
(EtOAc/hexane, 1:2). Following the completion of reac-
tion (10–16 h), the reaction mixture was cooled to room
temperature, diluted with chloroform (50 mL) and
heated at reflux for 1.5 h to extract the product from
the zeolite pores. The catalyst was then filtered. This
extraction procedure was repeated twice. The combined
filtrate was concentrated to give a residue that was sub-
jected to flash column chromatography over silica gel
using a mixture of EtOAc and hexane as the eluent to
furnish the title compounds.
3.3. Phenyl 3,4,6-tri-O-acetyl-a-D-glucopyranoside (6a)
Figure 3. The molecular packing of phenyl 3,4,6-tri-O-acetyl-a-D-
glucopyranoside (6a) displaying several C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀp inter-
actions. Most of the hydrogen atoms are omitted for clarity.
[a]D +173 (c 0.5, CH2Cl2) [lit.10 [a]D +170 (c 0.5,
CH2Cl2)]; 1H NMR (400 MHz, CDCl3): d 7.30–6.94
(m, 5H, Ph), 5.54 (d, 1H, J = 3.6 Hz, H-1), 5.39 (t,
1H, J = 9.8 Hz, H-3), 5.05 (t, 1H, J = 9.8 Hz, H-4),
4.20 (dd, 1H, J = 4.8, 12.4 Hz, H-6a), 4.04 (m, 1H, H-
5), 3.98 (dd, 1H, J = 2.0, 12.4 Hz, H-6b), 3.80 (dd, 1H,
J = 3.6, 9.9 Hz, H-2), 2.14–1.90 (3s, 9H, COCH3).
useful in studies directed towards obtaining a better
understanding of carbohydrate–protein interactions.
3. Experimental
3.4. p-Cresyl 3,4,6-tri-O-acetyl-a-D-glucopyranoside (6b)
3.1. General information
1
[a]D +52.3 (c 1, CH2Cl2); H NMR (400 MHz, CDCl3):
Thin layer chromatograms were performed on 25 mm
E. Merck silica gel plates (60F-254). Detection was done
by spraying the plates with 10% sulfuric acid in ethanol
and heating on a hot plate. Column chromatography
was performed using silica gel (230–400 mesh) under
flash conditions using a mixture of ethyl acetate and
hexane. Optical rotations were measured at 30 ꢁC on a
JASCO-DIP 200 digital polarimeter using a cell of
10 mm length. NMR spectra were recorded on a Bruker
AV400 spectrometer. ESI-MS spectra were measured on
a Micromass Q-Tof mass spectrometer. All free sugars
used were purchased from Sigma–Aldrich USA or from
Pfanstiehl Laboratories Inc. USA. Sugar peracetates
were prepared by using NaOAc and Ac2O at 100 ꢁC.
d 7.13 (d, 2H, J = 8.5 Hz, H-20, H-60), 7.02 (d, 2H,
J = 8.5 Hz, H-30, H-50), 5.56 (d, 1H, J = 3.7 Hz, H-1),
5.46 (t, 1H, J = 9.8 Hz, H-3), 5.13 (t, 1H, J = 9.8 Hz,
H-4), 4.28 (dd, 1H, J = 4.6, 12.3 Hz, H-6a), 4.16–4.09
(m, 1H, H-5), 4.05 (dd, 1H, J = 2.1, 12.3 Hz, H-6b),
3.86 (dd, 1H, J = 3.8, 9.9 Hz, H-2), 2.32 (s, 3H,
PhCH3-p), 2.14 (s, 3H, COCH3), 2.06 (s, 6H, COCH3);
13C NMR (100 MHz, CDCl3): d 171.3, 170.6, 169.6
(3 · COCH3), 153.9 (C-10), 132.7 (C-40), 130.1 (C-20,
C-60), 116.6 (C-30, C-50), 97.2 (C-1), 73.4, 70.9, 68.3,
67.8, 61.7 (C-6), 29.6 (PhCH3), 20.7, 20.6, 20.5
(COCH3);
ESI-MS:
Calcd
for
C19H24O9Na
([M+Na]+): 419.1318, found: 419.1297.
Beta-zeolite (Na form) was obtained from Sud-Chemie
¨
3.5. p-Methoxyphenyl 3,4,6-tri-O-acetyl-a-D-gluco-
pyranoside (6c)
India Ltd., New Delhi and activated by heating to
540 ꢁC prior to converting into the H+-form. Ethyl
acetate, hexane, chloroform and Ac2O were of labora-
tory grade and all were distilled once before use.
1
[a]D +25.8 (c 1, CH2Cl2); H NMR (400 MHz, CDCl3):
d 7.07 (d, 2H, J = 9.1 Hz, H-20, H-60), 6.86 (d, 2H,
J = 9.1 Hz, H-30, H-50), 5.50 (d, 1H, J = 3.7 Hz, H-1),
5.42 (t, 1H, J = 9.8 Hz, H-3), 5.12 (t, 1H, J = 9.8 Hz,
H-4), 4.28 (dd, 1H, J = 4.8, 12.2 Hz, H-6a), 4.18–4.12
(m, 1H, H-5), 4.08 (dd, 1H, J = 2.1, 12.2 Hz, H-6b),
3.86 (dd, 1H, J = 3.8, 9.9 Hz, H-2), 3.80 (s, 3H,
PhOCH3-p), 2.14, 2.07, 2.06 (3s, 3H each, COCH3);
13C NMR (100 MHz, CDCl3): d 171.3, 170.6, 169.6
(3 · COCH3), 155.6, 150.0, 118.0, 114.7, 97.8 (C-1),
73.3, 70.9, 68.2, 67.8, 61.8 (C-6), 55.7 (OCH3), 20.9,
3.2. General procedure for the synthesis of aryl 2,3,4,6-
tetra-O-acetyl-b-D-glycopyranosides (4a–e and 5a–e) and
aryl 3,4,6-tri-O-acetyl-a-D-glycopyranosides (6a–e and
7a–e)
A mixture of sugar peracetate (1 or 2, 1.0 mmol) and
phenol (3, 5 mmol) taken into a 100 mL round-bottom
flask and was heated to ꢂ100 ꢁC in a silicone oil bath.

26
U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
20.7, 20.6 (3 · COCH3); ESI-MS: Calcd for
3), 4.34 (t, 1H, J = 6.6 Hz, H-5), 4.16–4.07 (m, 3H, H-
2, H-6a, H-6b), 2.32 (s, 3H, PhCH3-p), 2.18, 2.11, 2.08
(3s, 3H each, COCH3); 13C NMR (100 MHz, CDCl3):
d 170.8, 170.3, 170.1 (3 · COCH3), 154.1 (C-10), 133.0
(C-40), 130.1 (C-20, C-60), 116.9 (C-30, C-50), 98.0 (C-
1), 70.6, 68.1, 67.5, 67.2, 61.6 (C-6), 29.6 (PhCH3-p),
20.8, 20.6, 20.5 (COCH3); ESI-MS: Calcd for
C19H24O9Na ([M+Na]+): 419.1318, found: 419.1339.
C19H24O10Na ([M+Na]+): 435.1267, found: 435.1280.
3.6. p-Chlorophenyl 3,4,6-tri-O-acetyl-a-D-glucopyran-
oside (6d)
1
[a]D +60.2 (c 1, CH2Cl2); H NMR (400 MHz, CDCl3):
d 7.30 (d, 2H, J = 8.9 Hz, H-30, H-50), 7.09 (d, 2H,
J = 8.9 Hz, H-20, H-60), 5.57 (d, 1H, J = 3.7 Hz, H-1),
5.45 (t, 1H, J = 9.8 Hz, H-3), 5.13 (t, 1H, J = 9.8 Hz,
H-4), 4.27 (dd, 1H, J = 4.6, 12.1 Hz, H-6a), 4.10 - 4.02
(m, 2H, H-5, H-6b), 3.89 (dd, 1H, J = 3.7, 9.9 Hz, H-
2), 2.14, 2.07, 2.06 (3s, 3H each, COCH3); 13C NMR
(100 MHz, CDCl3): d 171.3, 170.5, 169.5 (3 · COCH3),
154.6 (C-10), 129.7 (C-40), 128.4 (C-20, C-60), 118.0 (C-
30, C-50), 97.2 (C-1), 73.2, 70.8, 68.5, 67.6, 61.6 (C-6),
20.8, 20.6, 20.5 (COCH3); ESI-MS: Calcd for
C18H21O9NaCl ([M+Na]+): 439.0772, found: 439.0772.
3.10. p-Methoxyphenyl 3,4,6-tri-O-acetyl-a-D-galacto-
pyranoside (7c)
[a]D +117.1 (c 1, CH2Cl2); 1H NMR (400 MHz, CDCl3):
d 7.05 (d, 2H, J = 9.1 Hz, H-20, H-60), 6.86 (d, 2H,
J = 9.1 Hz, H-30, H-50), 5.54 (d, 1H, J = 3.6 Hz, H-1),
5.50 (d, 1H, H-4), 5.36 (dd, 1H, J = 3.2, 10.4 Hz, H-
3), 4.40 (t, 1H, J = 6.6 Hz, H-5), 4.14–4.10 (m, 3H, H-
2, H-6a, H-6b), 3.80 (s, 3H, PhOCH3-p), 2.18, 2.11,
2.20 (3s, 3H each, COCH3); 13C NMR (100 MHz,
CDCl3): d 170.9, 170.4, 170.2 (3 · COCH3), 155.6 (C-
10), 150.2 (C-40), 118.4 (C-20, C-60), 114.7 (C-30, C-50),
98.6 (C-1), 70.6, 68.2, 67.5, 67.2, 61.8 (C-6), 55.6
(PhOCH3-p), 20.8, 20.7, 20.6 (COCH3); ESI-MS: Calcd
for C19H24O10Na ([M+Na]+): 435.1267, found:
435.1262.
3.7. p-Fluorophenyl 3,4,6-tri-O-acetyl-a-D-glucopyran-
oside (6e)
Mp 136 ꢁC; [a]D +30.2 (c 1, CH2Cl2); 1H NMR
(400 MHz, CDCl3): d 7.14–6.99 (m, 4H, C6H4F-p),
5.52 (d, 1H, J = 3.7 Hz, H-1), 5.44 (t, 1H, J = 9.8 Hz,
H-3), 5.12 (t, 1H, J = 9.8 Hz, H-4), 4.25 (dd, 1H,
J = 4.0, 12.1 Hz, H-6a), 4.13–4.06 (m, 2H, H-5, H-6b),
3.87 (dd, 1H, J = 3.8, 9.9 Hz, H-2), 2.13, 2.07, 2.06
(3s, 3H each, COCH3); 13C NMR (100 MHz, CDCl3):
0
3.11. p-Chlorophenyl 3,4,6-tri-O-acetyl-a-D-galacto-
pyranoside (7d)
d 171.3, 170.5, 169.6 (3 · COCH3), 158.7 (JC4 ,F
=
[a]D +147.3 (c 1, CH2Cl2); 1H NMR (400 MHz, CDCl3):
d 7.30 (d, 2H, J = 8.8 Hz, H-30, H-50), 7.07 (d, 2H,
J = 8.8 Hz, H-20, H-60), 5.62 (d, 1H, J = 3.7 Hz, H-1),
5.49 (d, 1H, H-4), 5.36 (dd, 1H, J = 3.2, 10.4 Hz, H-
3), 4.30 (t, 1H, J = 6.4 Hz, H-5), 4.19–4.05 (m, 3H, H-
2, H-6a, H-6b), 2.18, 2.11, 1.98 ppm (3s, 3H each,
COCH3); 13C NMR (100 MHz, CDCl3): d 170.8,
170.3, 170.1 (3 · COCH3), 154.7 (C-10), 129.6 (C-40),
128.3, 118.2, 97.9 (C-1), 70.4, 68.0, 67.8, 67.1, 61.6 (C-
6), 20.8–20.6, (3 · COCH3); ESI-MS: Calcd for
C18H21O9NaCl ([M+Na]+): 439.0772, found: 439.0781.
240.1 Hz, C-40), 152.2 (C-10), 118.1 (JC ,F = 8.2 Hz,
0
C-20, C-60), 116.2 (JC ,F = 23.1 Hz, C-30, C-50), 97.2
0
(C-1), 73.2, 70.8, 68.4, 67.7, 61.8 (C-6), 20.8, 20.6, 20.5
(COCH3); ESI-MS: Calcd for C18H21O9FNa
([M+Na]+): 423.1067, found: 423.1058.
3.8. Phenyl 3,4,6-tri-O-acetyl-a-D-galactopyranoside (7a)
Mp 168 ꢁC; [a]D +117.1 (c 1, CH2Cl2); 1H NMR
(400 MHz, CDCl3): d 7.38–7.08 (m, 5H, Ph), 5.68 (d,
1H, J = 3.8 Hz, H-1), 5.50 (d, 1H, H-4), 5.39 (dd, 1H,
J = 3.5, 10.8 Hz, H-3), 4.35 (t, 1H, J = 6.9 Hz, H-5),
4.20–4.06 (m, 3H, H-2, H-6a, H-6b), 2.19, 2.07, 1.98
(3s, 3H each, COCH3); 13C NMR (100 MHz, CDCl3):
d 170.8, 170.4, 170.1 (3 · COCH3), 156.1 (C-10), 129.7,
128.1, 116.8, 97.6 (C-1), 70.6, 68.2, 67.6, 67.1, 61.6 (C-
6), 20.8, 20.6, 20.5 (COCH3); ESI-MS: Calcd for
C18H22O9Na ([M+Na]+): 405.1162, found: 405.1161.
3.12. p-Fluorophenyl 3,4,6-tri-O-acetyl-a-D-galacto-
pyranoside (7e)
Mp 127 ꢁC; [a]D +177.9 (c 1, CH2Cl2); 1H NMR
(400 MHz, CDCl3): d 7.09–6.99 (m, 4H, C6H4F-p),
5.56 (d, 1H, J = 3.9 Hz, H-1), 5.47 (d, 1H, H-4), 5.35
(dd, 1H, J = 3.4, 10.8 Hz, H-3), 4.32 (t, 1H,
J = 6.8 Hz, H-5), 4.15–4.08 (m, 3H, H-2, H-6a, H-6b),
2.20, 2.07, 2.00 (3s, 3H each, COCH3); 13C NMR
(100 MHz, CDCl3): d 170.8, 170.3, 170.1 (3 · COCH3),
3.9. p-Cresyl 3,4,6-tri-O-acetyl-a-D-galactopyranoside
(7b)
158.7 (JC4 ,F = 240.1 Hz, C-40), 152.3 (C-10), 118.4
0
[a]D +152.8 (c 1, CH2Cl2); 1H NMR (400 MHz, CDCl3):
d 7.13 (d, 2H, J = 8.4 Hz, H-20, H-60), 7.01 (d, 2H,
J = 8.4 Hz, H-30, H-50), 5.60 (d, 1H, J = 3.8 Hz, H-1),
5.49 (d, 1H, H-4), 5.37 (dd, 1H, J = 3.3, 10.4 Hz, H-
(JC ,F = 8.1 Hz, C-20, C-60), 116.1 (JC ,F = 23.1 Hz,
C30, C-50), 98.4 (C-1), 70.5, 68.1, 67.7, 67.2, 61.7 (C-6),
20.8, 20.6, 20.5 (3 · COCH3); ESI-MS: Calcd for
C18H21O9FNa ([M+Na]+): 423.1067, found: 423.1071.
0
0

U. Aich, D. Loganathan / Carbohydrate Research 341 (2006) 19–28
27
5. Conchie, J.; Levvy, G. A.. In Method Carbohydr. Chem.;
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graphic analysis of phenyl 3,4,6-tri-O-acetyl-a-D-gluco-
pyranoside (6a) and p-fluorophenyl 3,4,6-tri-O-acetyl-a-
D-glucopyranoside (6e)
6. (a) Hoffmann, B.; Hoelzl, J. Planta Med. 1988, 54, 450–
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Compound 6a and 6e were crystallized from a mixture
of EtOAc and hexane at room temperature. X-ray dif-
fraction data were collected at room temperature in
the x–2h scan mode on an Enraf-Nonius CAD4 diffrac-
tometer and the relevant details of data collection and
refinement are given in Table 2. The structure was
solved by direct methods using SHELXS-97 and the refine-
ment was done by full matrix using SHELXL-97.27a OR-
TEP of the compound was drawn using Wingx27b and
the packing diagrams using Mercury 1.2.1., provided
by CCDC, Cambridge.
Supplementary data
Complete structural data (CCDC # 230387 and 230388)
have been deposited at the Cambridge Crystallographic
Data Centre which can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; fax: (+44) 1123-336-
033; or email: deposit@ccdc.cam.ac.uk).
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Acknowledgements
10. Bols, M. J. Chem. Soc., Chem. Commun. 1992, 913–
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11. (a) Bhaskar, P. M.; Loganathan, D. Tetrahedron Lett.
1998, 39, 2215–2218; (b) Bhaskar, P. M.; Loganathan, D.
Synlett 1999, 129–131.
Funding provided by the Department of Science and
Technology, New Delhi, for the purchase of the
400 MHz NMR under IRPHA Scheme and ESI-MS
under the FIST program is gratefully acknowledged. A
research grant awarded by Indo-French Centre for
Promotion of Advanced Research (IFCPAR), New
Delhi is also gratefully acknowledged. The authors
thank the Sophisticated Analytical Instrumentation
Facility (SAIF), IIT Madras for the X-ray data collec-
tion. We would like to thank Dr. Babu Varghese for
his valuable technical help. One of us (UA) is thankful
to the Council of Scientific and Industrial Research
(CSIR), New Delhi, for award of a Junior Research
Fellowship.
12. (a) Clark, J. H.; Macquarrie, D. J. Chem. Soc. Rev. 1996,
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