Modified one-pot protocol for the preparation of thioglycosides from unprotected aldoses via S-glycosyl isothiouronium salts

Article abstract of DOI:10.1080/07328300500256775

An efficient one-pot protocol for the direct preparation of thioglycosides starting from unprotected reducing sugars via S-glycosyl isothiouronium salts is reported. In this one-pot methodology, BF₃·OEt₂ has been used as a general catalyst for both per-O-acetylation of sugars and conversion of sugar per-O-acetates into S-glycosyl isothiouronium salts, which was allowed to react with alkylating agents in the presence of a base to furnish thioglycosides in excellent yield. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/07328300500256775

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Modified One‐Pot Protocol for the
Preparation of Thioglycosides from
Unprotected Aldoses via S‐Glycosyl
Isothiouronium Salts
Pallavi Tiwari a , Geetanjali Agnihotri a & Anup Kumar Misra a
a Medicinal and Process Chemistry Division, Central Drug Research
Institute, Lucknow, UP, India
Published online: 20 Aug 2006.
To cite this article: Pallavi Tiwari , Geetanjali Agnihotri & Anup Kumar Misra (2005): Modified One‐Pot
Protocol for the Preparation of Thioglycosides from Unprotected Aldoses via S‐Glycosyl Isothiouronium
Salts , Journal of Carbohydrate Chemistry, 24:7, 723-732
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Journal of Carbohydrate Chemistry, 24:723–732, 2005
Copyright # Taylor & Francis, Inc.
ISSN: 0732-8303 print 1532-2327 online
DOI: 10.1080/07328300500256775
Modified One-Pot Protocol
for the Preparation of
Thioglycosides from
Unprotected Aldoses via
S-Glycosyl Isothiouronium
Salts
Pallavi Tiwari, Geetanjali Agnihotri, and Anup Kumar Misra
Medicinal and Process Chemistry Division, Central Drug Research Institute,
Lucknow, UP, India
An efficient one-pot protocol for the direct preparation of thioglycosides starting from
unprotected reducing sugars via S-glycosyl isothiouronium salts is reported. In this
.
one-pot methodology, BF3 OEt2 has been used as a general catalyst for both per-O-
acetylation of sugars and conversion of sugar per-O-acetates into S-glycosyl isothiouro-
nium salts, which was allowed to react with alkylating agents in the presence of a base
to furnish thioglycosides in excellent yield.
Keywords Acetylation, Thioglycosides, One-pot, Stoichiometric, Aldoses, Thiourea,
.
BF3 OEt2
INTRODUCTION
There has been explosive growth in the field of glycobiology in the last decade.
Particular interest has been drawn to the complex carbohydrates, which play
important roles in many biologic recognition events, including cell–cell
adhesion, bacterial attachment, and viral infections.[1–4] In this context,
Received March 14, 2005; accepted May 12, 2005.
Address correspondence to Anup Kumar Misra, Medicinal and Process Chemistry
Division, Central Drug Research Institute, Chattar Manzil Palace, Lucknow 226001
UP, India. E-mail: akmisra69@rediffmail.com
C.D.R.I. communication no. 6767.
Authors contributed equally to this work.
723

P. Tiwari et al.
724
1-thiosugars have attracted considerable attention because of their close
structural similarity to the natural O-glycosides. Due to the stability of thiogly-
cosidic bond to enzymatic cleavage, thioglycosides have been considered as
very promising candidates for the preparation of carbohydrate-based thera-
peutics.[5–9] Per-O-acetylated sugars and thioglycosides have found
enormous applications in the field of synthetic carbohydrate chemistry,
especially in the synthesis of oligosaccharides.[10–13] Among glycosyl donors,
thioglycosides are widely used because of their high degree of stability in
many organic reactions.
The most often employed approaches toward the synthesis of thioglycosides
are the treatment of per-O-acetylated sugars with malodorous and toxic alkyl/
aryl thiols or expensive alkyl/aryl thiotrimethylsilanes in the presence of a
Lewis acid,[14–19] which often leads to the formation of anomerized products.
Recently, we have emphasized the one-pot reaction protocol[20] for the pre-
paration of acetylated thioglycosides directly from unprotected reducing
sugars. However, the above-mentioned process had few limitations in the use
of malodorous thiols and limited availability for carrying out a variety of reac-
tions. As a result, we needed to develop a generalized reaction protocol for the
preparation of a wide variety of thioglycosides without using malodorous and
toxic thiols. Earlier, few reports appeared in the literature regarding the use
of nonmalodorous thioglycoside donors in the glycosylation reactions, which
involves the formation of O-methoxycarbonylphenyl thioglycoside from the
corresponding glycosyl bromides and methylthiosalicylate.[21,22] However, a
general method for the preparation of thioglycosides avoiding the use of
the above-mentioned toxic thiols employs reaction of alkyl halides with
S-glycosyl isothiouronium salts.[23–28] Conventionally, S-glycosyl isothiouro-
nium salts are prepared from the reaction of thiourea with glycosyl halides,
which are generally prepared from per-O-acetylated sugars. In practice,
this environmentally safer preparation of per-O-acetylated thioglycosides
from unprotected reducing sugars involves at least four steps consisting of
(1) acetylation using excess acetic anhydride and pyridine or pyridine deriva-
tives as solvent and activator despite their known toxicity and unpleasant
odor, (2) bromination using HBr-AcOH, and (3) treatment of acetobromo
sugar with thiourea followed by (4) the reaction of alkyl halide with the
S-glycosyl isothiouronium salts, which require intermediate isolation and
purification through conventional workup, causing the synthetic sequence to
be tedious. Prompted by a recent report describing the reaction of glycosyl
acetates with thiourea toward the formation of 1-thiosugars,[29] we have
envisioned that the above-mentioned intermediate steps could be minimized
by using a general catalyst for the acetylation and formation of S-glycosyl
isothiouronium salts and reacting S-glycosyl isothiouronium salts produced
in situ with alkylating agents in one-pot directly from free sugars. In the
course of our ongoing thioglycoside syntheses using nonmalodorous reagents

Modified One-Pot Protocol
725
in a minimum number of steps, we describe herein a modified one-pot protocol
for the preparation of thioglycosides directly from unprotected reducing
sugars.
RESULTS AND DISCUSSION
.
To standardize the reaction protocol, BF3 OEt2 (1.5 mmol) was added to a well-
stirred suspension of free sugar (1.0 mmol) in acetic anhydride (5.1 mmol) at rt.
An exothermic reaction started immediately and a clear reaction mixture was
obtained within a few minutes with a clean formation of per-O-acetylated sugar
.
(TLC; hexane:EtOAc; 1:1). Reducing the quantity of BF3 OEt2 from 1.5 eq. to
1.0 eq. or 0.5 eq. led to a slow reaction, and the reaction was not complete even
after 24 hr. After a series of experiments, it was optimized that use of 1.5 eq. of
.
BF3 OEt2 and 1.02 eq. of acetic anhydride per hydroxy group of the free sugar
produced excellent yield of per-O-acetylated products in a very fast and effi-
cient manner. Although all acetylation reactions in milligram scale have
been performed at rt, a cooling arrangement is required for the multigram
scale to avoid the loss of reagents and decomposition of products due to over-
heating resulting from the exothermic reaction. After formation of the per-O-
acetylated sugars using stoichiometric acetic anhydride, a small amount of
CH3CN was added to mobilize the thick syrupy mass of the sugar per-O-
acetates. To the reaction mixture was added thiourea (2.0 eq.) and the
reaction mixture was heated to 808C for 15 min. TLC (EtOAc) showed
complete conversion of sugar per-O-acetates to slower-moving S-glycosyl iso-
thiouronium salts. The reaction mixture was cooled to rt and alkyl halides
(1.5 eq.) were added to the reaction mixture followed by excess Et3N. The
above reaction mixture was allowed to stir at ambient temperature until
S-glycosyl isothiouronium salts were consumed completely to a faster-moving
component. After completion of the reaction as monitored by TLC (hexane:
EtOAc 1:1), the solvent was evaporated and the resulting syrup was diluted
with CH2Cl2 and washed with water, which on evaporation furnished pure
alkyl thioglycoside in one-pot starting from free sugars. Following the similar
reaction sequence, a series of alkyl and aralkyl 1,2-trans-1-thioglycosides
were successfully synthesized starting from free sugars (Sch. 1, Table 1).
Per-O-acetylated alkyl and aralkyl thioglycosides prepared from commonly
Scheme 1

P. Tiwari et al.
726
Table 1: One-pot preparation of 1,2-trans-thioglycosides from unprotected
reducing sugars via acetylation and formation of S-glycosyl isothiouronium salts.
Ac2O
Alkyl
Yield
Entry Sugars (1) (equiv)
halides
Products (2)
(%) Ref.
a
D-glucose
5.1
Allyl
bromide
92
[30]
b
c
d
D-glucose
D-glucose
D-glucose
5.1
5.1
5.1
Benzyl
bromide
95
92
85
[30]
—
4-Nitro
benzyl
bromide
Trityl
chloride
[27]
e
D-glucose
5.1
Acetic
95
[27]
anhydride
f
D-glucose
5.1
5.1
Ethyl bromo
acetate
92
90
[31]
[32]
g
D-mannose
Allyl
bromide
h
i
D-mannose
D-mannose
5.1
5.1
Octyl
bromide
90
92
[33]
[34]
Benzyl
bromide
j
D-galactose
D-galactose
5.1
5.1
Ethyl
bromide
95
90
[27]
[35]
k
Methyl
iodide
(continued)

Modified One-Pot Protocol
727
Table 1: Continued.
Ac2O
Entry Sugars (1) (equiv)
Alkyl
halides
Yield
(%) Ref.
Products (2)
l
D-galactose
5.1
4-Nitro
benzyl
bromide
95
92
[27]
m
D-galactose
5.1
Benzyl
bromide
[30]
n
o
D-lactose
D-lactose
8.2
8.2
Octyl
bromide
85
90
[36]
[37]
Ethyl
bromide
p
q
D-cellobiose
D-maltose
8.2
8.2
Methyl
iodide
85
92
[38]
[38]
Methyl
iodide
r
D-galactose
5.1
Aceto
80
[39]
bromo
galactose
available sugars gave acceptable 1H NMR and 13C NMR spectra that matched
data reported in the cited references. It is important to note that only single
isomers of 1,2-trans-1-thioglycosides were obtained, which were confirmed
from their NMR spectral data. This methodology has been further extended
toward the synthesis of 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-(1 ! 1)-
2,3,4,6-tetra-O-acetyl-1-thio-b-D-galactopyranoside (Table 1, entry r) by
reacting acetobromogalactose with D-galactosyl isothiouronium salt in the
presence of Et3N. Only a single b-linked disaccharide was isolated with no
traces of formation of a-disaccharide, which was further confirmed from its
NMR spectra that matched with the literature report.[39]
The exclusive formation of 1,2-trans-1-thioglycosides can be explained by
considering the formation of 1,2-acyloxonium ions as the reaction intermediate
as a result of neighboring group participation, which allows the approach of
thiourea from one possible site to form 1,2-trans-S-glycosyl isothiouronium
salts only. As S-glycosyl isothiouronium salts do not undergo anomerization

P. Tiwari et al.
728
Scheme 2
as evident from the earlier report,[29] they always produce only a single isomer
of 1,2-trans-1-thioglycosides on reacting with alkyl halides in the presence of a
base (Sch. 2).
In summary, the present methodology offers a convenient one-pot protocol
to prepare per-O-acetylated 1,2-trans-1-thioglycosides from the free sugars
using a stoichiometric acetic anhydride and sequential thioglycosidation via
S-glycosyl isothiouronium salt formation. This mild, operationally simple,
one-pot reaction protocol for the preparation of thioglycosides directly from
free sugars will certainly find application in oligosaccharide synthesis and
carbohydrate-derived drug discovery programs. A series of thioglycosides
prepared following the present protocol are under use in the oligosaccharide
synthesis, which will be published in due course.
EXPERIMENTAL
General Methods
All the reactions were monitored by thin layer chromatography over silica
gel coated TLC plates. The spots on TLC were visualized by warming ceric
sulphate (2% Ce(SO4)2 in 2 N H2SO4) sprayed plates in hot plate. Silica gel
230–400 mesh was used for column chromatography. FAB mass spectra were
recorded on JEOL SX 102/DA-6000 mass using Argon/Xenon (6 KV, 10 MA)
1
as the FAB gas. H and 13C NMR was recorded on Brucker Advance DPX
200 MHz using TMS as internal reference. Chemical shift value is expressed
in dppm. Elementary analysis was carried out on Carlo ERBA-1108 analyzer.
Optical rotations were measured at 258C on a Rudolf Autopol III polarimeter.
Commercially available grades of organic solvents of adequate purity are
used in many reactions.
Typical procedure for the preparation of 1,2-trans-1-thioglycosides
4-Nitrobenzyl 2,3,4,6-tetra-O-acetyl-1-thio-b-D-glucopyranoside (2c): A sus-
pension of D-glucose (1.8 g, 10.0 mmol) in acetic anhydride (4.82 mL, 51.0 mmol)

Modified One-Pot Protocol
729
was placed in an ice bath with continuous stirring. To the cold suspension of the
.
reaction mixture was added BF3 OEt2 (1.9 mL, 15.0 mmol) at a time. An
exothermic reaction started immediately and the reaction mixture was
allowed to stir for 5.0 min. After completion of the per-O-acetylation (monitored
by TLC; hexane:EtOAc 1:1), anhydrous CH3CN (10.0 mL) was added to the
reaction mixture followed by thiourea (1.52 g, 20.0 mmol) and the reaction
mixture was placed on a preheated oil bath at 808C for 15 min with constant
stirring. After full consumption of the sugar per-O-acetates (as judged by
TLC, EtOAc), the reaction mixture was cooled to rt. To the reaction mixture
were added 4-nitrobenzyl bromide (3.24 g, 15.0 mmol) and Et3N (5.0 mL) in suc-
cession and allowed to stir for 3 hr at ambient temperature. The solvent was
evaporated and the resulting syrup was diluted with CH2Cl2 (100 mL). The
organic layer was washed with water, dried (Na2SO4), and concentrated
under reduced pressure. Purification of the crude reaction product over SiO2
using hexane-EtOAc (3:1) furnished pure 4-nitrobenzyl 2,3,4,6-tetra-O-
acetyl-1-thio-b-D-glucopyranoside (2c; 4.6 g; 92 %). Yellow oil. [a]2D5 2378
(c 1.0, CHCl3); IR (Neat): 1752, 1522, 1348, 1225, 1042 cm21 1H NMR
;
(CDCl3): d 8.20–8.16 (d, J ¼ 8.5 Hz, 2 H), 7.53–7.49 (d, J ¼ 8.5 Hz, 2 H),
5.20–5.12 (t, J ¼ 9.0 Hz each, 1 H), 5.10–5.0 (m, 2 H), 4.38 (d, J ¼ 8.0 Hz, 1
H), 4.30–4.21 (dd, J ¼ 9.0 and 4.5 Hz, 1 H), 4.18–4.10 (dd, J ¼ 12.0 and
3.8 Hz, 1 H), 4.07–3.81 (dd, J ¼ 13.2 Hz each, 2 H), 3.73–3.65 (m, 1 H), 2.10,
2.05, 2.03, 2.01 (4 s, 12 H); 13C NMR (CDCl3): d 170.7, 170.3, 169.7 (2C),
147.6, 145.3, 130.3 (2C), 124.1 (2C), 82.2, 76.4, 73.9, 70.1, 68.6, 62.5, 33.1,
21.0, 20.9, 20.8 (2C); MS (FAB): m/z 500 [M þ 1]; Anal. Calcd. for
C21H25NO11S (499): C, 50.50; H, 5.04. Found: C, 50.25; H, 5.30.
ACKNOWLEDGMENTS
Instrumentation facilities from SAIF, CDRI is gratefully acknowledged. P.T.
and G.A. thank CSIR, New Delhi, for providing Junior and Senior fellowships,
respectively. This project was partly funded by the Department of Science and
Technology (DST), New Delhi (SR/FTP/CSA-10/2002), India.
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