N-bromosuccinimide-mediated conversion of allyl glycosides to glycosyl hemiacetals

Article abstract of DOI:10.1080/07328301003664887

A novel reaction condition has been developed for the hydrolysis of differentially functionalized allyl glycosides to their corresponding glycosyl hemiacetal derivatives in the presence of N-bromosuccinimide (NBS). The reaction condition is exceptionally fast, mild, and compatible with most of the functional groups used in the oligosaccharide synthesis, and yields were excellent. Copyright Taylor & Francis Group, LLC.

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N-Bromosuccinimide-Mediated
Conversion of Allyl Glycosides to Glycosyl
Hemiacetals
Rajib Panchadhayee a & Anup Kumar Misra a
a Division of Molecular Medicine, Bose Institute, A.J.C. Bose
Centenary Campus, P-1/12, C.I.T. Scheme VII-M, Kolkata, 700054,
India
Version of record first published: 30 Mar 2010
To cite this article: Rajib Panchadhayee & Anup Kumar Misra (2010): N-Bromosuccinimide-Mediated
Conversion of Allyl Glycosides to Glycosyl Hemiacetals, Journal of Carbohydrate Chemistry, 29:2,
76-83
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Journal of Carbohydrate Chemistry, 29:76–83, 2010
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Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328301003664887
N-Bromosuccinimide-
Mediated Conversion of Allyl
Glycosides to Glycosyl
Hemiacetals
Rajib Panchadhayee and Anup Kumar Misra
Division of Molecular Medicine, Bose Institute, A.J.C. Bose Centenary Campus,
P-1/12, C.I.T. Scheme VII-M, Kolkata 700054, India
A novel reaction condition has been developed for the hydrolysis of differentially func-
tionalized allyl glycosides to their corresponding glycosyl hemiacetal derivatives in the
presence of N-bromosuccinimide (NBS). The reaction condition is exceptionally fast,
mild, and compatible with most of the functional groups used in the oligosaccharide
synthesis, and yields were excellent.
Keywords Allyl glycoside; N-Bromosuccinimide; Glycosyl hemiacetal; Hydrolysis
INTRODUCTION
1-Hydroxy sugars or glycosyl hemiacetal derivatives are useful intermediates
for the synthesis of complex oligosaccharides.[1,2] They can be used directly in
the dehydrative glycosylation reactions[3] or can be converted to reactive gly-
cosyl donors[4–6] to be used in the oligosaccharides or natural product synthe-
sis.[7,8] Because of their usefulness, a number of reports appeared in the liter-
ature dealing with the preparation of glycosyl hemiacetal derivatives, which
include (a) removal of the anomeric acetyl group using hazardous hydrazine
salts[9] or organic bases[10] or acidic conditions[11]; (b) hydrolysis of alkyl gly-
cosides[12] and oxidative removal of 4-methoxyphenyl glycosides[13]; and (c) hy-
drolysis of thioglycoside derivatives using a variety of thiophilic reagents.[14–20]
Hydrolysis of thioglycoside derivatives has been found to be beneficial over
other methods due to the less toxic and mild reaction conditions as well as
compatibility of other functional groups present in the sugar skeleton.
Received December 29, 2009; accepted January 31, 2010.
Address correspondence to Anup Kumar Misra, Division of Molecular Medicine, Bose
Institute, A.J.C. Bose Centenary Campus, P-1/12, C.I.T. Scheme VII-M, Kolkata 700054,
India. E-mail: akmisra69@gmail.com
76

Conversion of Allyl Glycosides to Glycosyl Hemiacetals
77
Suitably functionalized allyl glycosides have been used as glycosyl accep-
tors in the synthesis of several complex oligosaccharides.[21] In a multistep
oligosaccharide synthetic strategy the anomeric allyl group has been used as
a temporary protecting group that can be removed after glycosylation to gen-
erate hemiacetal derivatives for their use in the next step. Conventionally, re-
moval of the allyl group is carried out using expensive palladium or rhodium
salts[22]; in a two-step reaction sequence involving the isomerization of the
O-allyl group to the more labile O-propenyl group in the presence of expen-
sive iridium salt[23]; or using irradiation[24] followed by hydrolysis of the re-
sulting O-propenyl group using N-bromosuccinimide (NBS). In some cases the
removal of the anomeric allyl group became exceptionally problematic because
of the functional groups present in the substrates and thereby required spe-
cial reaction conditions.[25,26] However, many of these methods for the removal
of the allyl group suffer from limitations such as use of expensive reagents,
incompatibility with acidic functional groups, relatively low yield, and some-
times harsh reaction conditions. In this context, it would be useful to develop
an economically convenient mild reaction protocol for the preparation of glyco-
syl hemiacetal derivatives having sensitive protecting groups. Although NBS
has been used to remove allyl ether under the special reaction conditions men-
tioned earlier, we disclose herein a novel metal-free reaction condition for the
preparation of functionalized glycosyl hemiacetal derivatives by direct NBS-
mediated hydrolysis of allyl glycosides, avoiding the use of expensive reagents
or stringent reaction conditions (Sch. 1).
Scheme 1: N-Bromosuccinimide (NBS)-mediated hydrolysis of allyl glycoside for the
preparation of glycosyl hemiacetal derivatives.
RESULTS AND DISCUSSION
During the synthesis of oligosaccharides[27] using suitably protected allyl gly-
coside as the glycosyl acceptor under the thioglycoside activation condition in
the presence of N-iodosuccinimide (NIS) and trifluoromethane sulfonic acid
(TfOH) combination, we isolated a considerable amount of unwanted product,
which was characterized as the hemiacetal derivative generated from the allyl
glycoside. Taking this clue from the earlier experiments, allyl glycoside (1) was
allowed to react separately with NBS and NIS in the presence and absence of
an acid in different solvents, for example, CH2Cl2, CH3CN-H2O (9:1), acetone-
H2O (10:1), THF. It was observed that the use of 1.1 equiv. of NBS in acetone-
H2O can form hemiacetal derivative in excellent yield from the corresponding

R. Panchadhayee and A.K. Misra
78
allyl glycosides without requiring any acid as activator at room temperature in
3 minutes. Although NIS was almost equally effective for this transformation,
we opted to use economically cheaper NBS. However, NBS has some limita-
tion because of its tendency for radical reactions. To expand the scope of the
reaction, a series of differentially protected allyl glycoside was transformed
into corresponding hemiacetal derivative in excellent yield (Sch. 1, Table 1)
following similar reaction conditions. Use of CH3CN-H2O (9:1) as solvent can
furnish similar yield of the product, but acetone-H2O has been used because
of its cheap availability. Most of the functional groups used in the protection of
hydroxy groups of the carbohydrate backbone (i.e., benzylidene, isopropylidene
acetal, benzyl, 4-methoxybenzyl, benzoyl, acetyl, tert-butyldiphenylsilyl, etc.)
remain unaffected under the reaction condition. Although NBS has been used
in the oxidative opening of benzylidene acetal under the Hanessian-Hullar re-
action condition, it has no effect on the benzylidene acetal using the present
reaction condition. A comparative study on the effect of solvent and halogenat-
ing agents on the formation of hemiacetal derivatives is presented in Table 2.
Using α- and β-allyl glycosides, similar results were obtained.
We presumed that bromonium ion (Br+) generated from NBS and the allyl
group are in a close proximity to form an addition product, which after hydrol-
ysis resulted in the glycosyl hemiacetal derivatives from the allyl glycoside.
In summary, a novel, metal-free mild reaction condition has been devel-
oped for the direct hydrolysis of allyl glycosides in the presence of NBS. This
finding can also explain the low-yielding glycosylation reactions using func-
tionalized allyl glycoside as glycosyl acceptor under iodonium ion-mediated
thioglycoside activation conditions. Use of readily available reagents, without
requirement of heavy metallic salts, high boiling solvents, or expensive Lewis
acid additives, makes this exceptionally fast reaction protocol for the prepara-
tion of glycosyl hemiacetal derivatives an attractive alternative to the existing
methods.
EXPERIMENTAL
General Procedure
All 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 2N H2SO4)-sprayed plates in hot plate. Silica gel
1
230–400 mesh was used for column chromatography. H and 13C NMR spec-
tra were recorded on Bruker Avance DRX 500 MHz using CDCl3 as solvents
and TMS as internal reference unless stated otherwise. Chemical shift value
is expressed in δ ppm. ESI-MS were recorded on a Micromass Quattro II triple

Conversion of Allyl Glycosides to Glycosyl Hemiacetals
79
Table 1: Hydrolysis of allyl glycosides using N-bromosuccinimide (NBS)a at room
temperature
Time
(min)
Yield (%)
(α/β)
Thioglycosides
Products
Ref
[14]
1
15
16
3
90 (2:1)
92 (3:1)
[14]
2
<2
[14]
[14]
[14]
[16]
3
4
5
6
17
18
19
20
3
5
5
5
90 (5:1)
88 (2:1)
82 (2:1)
80 (3:1)
[14]
[14]
7
8
21
22
<2
<2
90 (4:1)
85 (3:1)
9
23
24
3
5
86 (2:1)
90 (1:3)
—
[18]
10
11
12
25
26
5
82 (3:1)
—
—
3
80 (2:1)
(Continued on next page)

R. Panchadhayee and A.K. Misra
80
Table 1: Hydrolysis of allyl glycosides using N-bromosuccinimide (NBS)a at room
temperature (Continued)
Time
(min)
Yield (%)
(α/β)
Thioglycosides
Products
Ref
[16]
13
14
27
28
5
5
88 (3:1)
82 (2:1)
[16]
NPhth, N-phthalimido; MBn, 4-methoxybenzyl.
a1.1 equiv. of NBS was used in acetone-H2O (10:1).
quadrupole mass spectrometer. Commercially available grades of organic sol-
vents of adequate purity are used in many reactions.
Typical Experimental Protocol for The Preparation of Glycosyl
Hemiacetal Derivatives
To a solution of functionalized allyl glycoside (1 mmol) in acetone-H2O
(2 mL; 10:1, v/v) was added NBS (1.1 mmol) at 0 to 5◦C and the reaction mix-
ture was allowed to stir at the same temperature for the appropriate time
(Table 1). The reaction mixture was diluted with CH2Cl2 (20 mL) and the or-
ganic layer was washed with 5% Na2S2O3 and H2O, dried (Na2SO4), and evap-
orated to dryness. The crude product was purified over SiO2 using hexane-
EtOAc as eluant to give pure glycosyl hemiacetal derivative. An inseparable
anomeric mixture of hemiacetal derivative was formed in every case; the ratio
1
1
was determined from the integration of their H NMR spectra. H NMR and
13C NMR spectra of the known glycosyl hemiacetal derivatives matched with
the data reported in the cited references. Spectral data for new compounds,
Table 2: Optimization of the reaction condition for the formation of glycosyl
hemiacetal derivative (15) from allyl glycoside (1)
Entry
Substrate
Catalyst
Solvent
CH2Cl2
CH3CN-H2O
Acetone-H2O
CH3CN-H2O
Acetone-H2O
Acetone-H2O
Time (min)
Yield (%)
1
2
3
4
5
6
1
1
1
1
1
1
NBS
NBS
NBS
NIS
NIS
I2
30
5
85
88
90
85
90
82
3
15
15
45

Conversion of Allyl Glycosides to Glycosyl Hemiacetals
81
which were not reported earlier, are presented below. Although both α- and
β-anomers formed under the reaction conditions, spectral data for the major
isomer are presented for the sake of simplicity.
2,3-Di-O-acetyl-4,6-O-benzylidene-α-D-galactopyranose (23)
1H NMR (CDCl3, 300 MHz): δ 7.48–7.33 (m, 5 H, Ar-H), 5.46 (s, 1 H, PhCH),
5.32–5.29 (m, 1 H, H-2), 4.94 (dd, J = 10.4, 3.4 Hz, 1 H, H-3), 4.53 (d, J = 3.4
Hz, 1 H, H-1), 4.32 (br s, 1 H, H-4), 3.85–3.77 (m, 2 H, H-6a,b), 3.50–3.49 (m,
1 H, H-5), 2.09 (s, 6 H, 2 COCH3); ESI-MS: calcd. for C17H20O8: m/z 352.12;
found: m/z 335.1 [M-H2O+1].
2-O-benzoyl-3-O-benzyl-4,6-O-benzylidene-α-D-
galactopyranose (25)
1H NMR (CDCl3, 300 MHz): δ 8.02–7.17 (m, 15 H, Ar-H), 5.59–5.54 (m, 1
H, H-2), 5.46 (s, 1 H, PhCH), 4.67 (d, J = 12.4 Hz, 1 H, PhCH2a), 4.58 (d, J =
9.0 Hz, 1 H, H-1), 4.56 (d, J = 12.4 Hz, 1 H, PhCH2b), 4.20 (br s, 1 H, H-4),
4.03–4.01 (m, 1 H, H-3), 3.76–3.69 (m, 2 H, H-6a,b), 3.39–3.37 (m, 1 H, H-5);
ESI-MS: calcd. for C27H26O7: m/z 462.17; found: m/z 445.1 [M-H2O+1].
2-O-acetyl-4,6-O-benzylidene-3-O-(4-methoxybenzyl)-α-D-
galactopyranose (26)
1H NMR (CDCl3, 300 MHz): δ 7.51–6.80 (m, 9 H, Ar-H), 5.46 (s, 1 H, PhCH),
5.32–5.26 (m, 1 H, H-2), 4.58–4.43 (m, 2 H, MeOPhCH2), 4.52 (d, J = 3.0 Hz,
1 H, H-1), 4.30–4.27 (m, 1 H, H-3), 4.15 (br s, 1 H, H-4), 3.87–3.84 (m, 2 H, H-
6a,b), 3.79 (s, 3 H, OCH3), 3.57–3.55 (m, 1 H, H-5); ESI-MS: calcd. for C23H26O8:
m/z 430.16; found: m/z 413.1 [M-H2O+1].
ACKNOWLEDGEMENTS
R.P. thanks CSIR, New Delhi, for providing a Senior Research Fellowship. This
work was supported by Ramanna Fellowship (AKM), Department of Science
and Technology, New Delhi (SR/S1/RFPC-06/2006).
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