Simple and mild stereoselective O-glycosidation using 1,2-anhydrosugars under neutral conditions

Article abstract of DOI:10.1016/j.tetlet.2019.01.049

The ring opening of α-D-1,2-anhydrohexapyranoses with phenols proceeded smoothly in ethyl acetate (neutral conditions) in the absence of metal ion catalysts or additives to stereoselectively furnish 1,2-cis-α-aryl glycosides as the major product and 1,2-t

Full text of DOI:10.1016/j.tetlet.2019.01.049

Accepted Manuscript
Simple and Mild Stereoselective O-Glycosidation using 1,2-Anhydrosugars
Under Neutral Conditions
Devaraj Somasundaram, Kalpattu K. Balasubramanain, Bhagavathy
Shanmugasundaram
PII:
S0040-4039(19)30085-1
https://doi.org/10.1016/j.tetlet.2019.01.049
TETL 50582
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 November 2018
19 January 2019
23 January 2019
Please cite this article as: Somasundaram, D., Balasubramanain, K.K., Shanmugasundaram, B., Simple and Mild
Stereoselective O-Glycosidation using 1,2-Anhydrosugars Under Neutral Conditions, Tetrahedron Letters (2019),
doi: https://doi.org/10.1016/j.tetlet.2019.01.049
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Graphical Abstract
Simple and Mild Stereoselective O-Glycosidation Leave this area blank for abstract info.
using 1,2-Anhydrosugars Under Neutral Conditions
Devaraj Somasundaram,a Kalpattu K Balasubramanian,b and
Shanmugasundaram Bhagavathy*a
OBn
OBn
H
OBn
H
H
phenols
O
alcohols
O
O
BnO
BnO
BnO
BnO
BnO
BnO
H
OAlkyl
H
EtOAc
EtOAc
OH
OH
O
(83-95%)
H
OAryl
(77-96%)
-anomer
(major product)
only -anomer
: = ~(90 : 10)

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Simple and Mild Stereoselective O-Glycosidation using 1,2-Anhydrosugars
Under Neutral Conditions
Devaraj Somasundarama , Kalpattu K Balasubramanainb and Bhagavathy Shanmugasundarama
,
aDepartment of Chemistry, B. S. Abdur Rahman Crescent Institute of Science and Technology, Vandalur, Chennai 600015, India
bDepartment of Biotechnology, Indian Institute of Technology Madras, Guindy, Chennai 600048, India
———
ARTICLE INFO
ABSTRACT
 Corresponding author. Tel.: +91-044-2275-1347; fax: +91-044-2275-1349; e-mail: bhagavathy@crescent.education
Article history:
Received
Received in revised form
Accepted
The ring opening of α-D-1,2-anhydrohexapyranoses with phenols proceeded smoothly in ethyl
acetate (neutral conditions) in the absence of metal ion catalysts or additives to stereoselectively
furnish 1,2-cis-α-aryl glycosides as the major product and 1,2-trans-β-aryl glycosides as the
minor product in good yields. Under similar conditions, this ring opening reaction with alcohols
afforded exclusively β-alkyl glycosides in excellent yields.
Available online
Keywords:
1,2-Anhydrosugars
Glycal epoxide
2009 Elsevier Ltd. All rights reserved.
Aromatic glycosylation
Aryl glycosides
Alkyl glycosides
electronic nature of the substitutents.13 Higher selectivity in
1. Introduction
favour of the a-anomer has been reported when the glycosidation
In recent years, aromatic O-glycosidation has received
considerable attention from both synthetic and medicinal
chemists because of their diverse biological activities1 and
pharmaceutical potentials.2 The recent review by Jacobsen and
co-workers has extensively discussed the various methods for the
synthesis of aryl glycosides.3 Glycosyl acetates, phosphates,
halides, trichloroacetimidates and sulphoxides are commonly
used as glycosyl donors.4 Free glycals as well as their derivatives
with good leaving groups at the allylic position have also been
used as glycosyl donors.5 Although a plethora of methods are
available in the literature for O-glycosidation, they suffer from
limitations such as low yields, poor stereoselectivity and the
requirement of catalysts, additives or promoters. Some of these
methods are not amenable for large scale production due to cost
of the reagents, poor atom economy, long reaction times, high
temperature conditions, difficulty in accessing the glycosyl donor
or tedious workup procedures.
was conducted using ZnCl2.13b Regio- and stereoselective
glycosylation of sugar 1,2-diols and 1,3-diols using catalytic
amounts of cyclic boronate esters derived from these diols is also
known.14 To date there have been no reports on the metal ion
free, catalyst free and additive/activator free stereoselective
synthesis of aryl glycosides at ambient temperature. Herein, we
describe a simple and mild stereoselective O-glycosidation
method using 1,2-anhdyrosugars under neutral conditions.
2. Results and Discussion
The substrates, 1,2-α-D-anhydroglucose 1 and 1,2-α-D-
anhydrogalactose
2 were obtained by the stereoselective
epoxidation of 3,4,6-tri-O-benzyl-D-glucal and 3,4,6-tri-O-
benzyl-D-galactal, respectively, via in situ generation of
dimethyldioxirane using oxone/acetone in a biphasic system as
described in the literature.15 The crude 1,2-anhydrosugars were
directly used for further reactions.
In the case of aryl glycosides it is not easy to obtain good
yields because of dimerisation,6 use of hazardous metal salts7 and
the formation of C-glycosides in the case of activated aromatic
compounds.8 The use of 1,2-anhydrosugars as glycosyl donor is
well known for the synthesis of alkyl and aryl glycosides.9
However, most of these cases require either metal salts of the
phenol10 or expensive catalysts/promoters.11 Few cases lead to a
mixture of products resulting in low yields with the requirement
of harsh reaction conditions and tedious work up.12
Our initial reaction of 1,2-anhydro-3,4,6-tri-O-benzyl-α-D-
glucopyranose 1 with p-cresol 3a in acetonitrile at ambient
temperature for 3 h proceeded smoothly to yield α-p-cresyl 3,4,6-
tri-O-benzyl-D-glucopyranoside 4a (1,2-trans-adduct) as the
major
product
and
β-p-cresyl
3,4,6-tri-O-benzyl-D-
glucopyranoside 5a (1,2-cis-adduct) as the minor product
(Scheme 1).
OBn
R'
R
OBn
R'
H
OBn
O
OH
R'
H
O
R
In the case of phenols as nucleophiles, the stereochemical
course of the ring opening reaction of 1,2-anhydrosugars is
largely influenced by an external additive or bases and the
H
+
R
H
BnO
O
O
EtOAc
BnO
R
OH
+
H
OH
BnO
O
H
R
O
(1.1 eq.)
R
1
2
R= OBn; R' = H
R= H; R' = OBn
minor product
3a-f
major product
5a-h R= OBn; R' = H
7a-f R= H; R' = OBn
4a-h R= OBn; R' = H
6a-f R= H; R' = OBn

2
Tetrahedron
3
4
5
1
1
1
1
1
1
3c
3d
3e
3f
3g
3h
2-Me
4-F
4-CN
4-OMe
4-NO2
2-
Naphthol
4-Me
H
2-Me
4-F
6
6
6
6
6
6
4c:5c
4d:5d
4e:5e
4f:5f
4g:5g
4h:5h
91
83
87
90
86
93
88:11
85:15
88:12
87:13
91:09
91:09
Scheme 1. Ring opening of α-D-1,2-anhydrosugars 1 and 2 with
6
7
8
phenols
As the solvent polarity is known to influence the
stereoselectivity in glycosidation reactions,16 we screened various
solvents (free of moisture) with different dielectric constants such
as CH2Cl2, acetonitrile, ethyl acetate (free of acetic acid),
acetone, toluene, DMF and DMSO. The ring opening of 1 with p-
cresol in the above solvents led predominantly to α-anomer 6a as
the major product and β-anomer 7a as the minor product (Table
1). Anomeric ratios were determined by HPLC analysis of the
crude product.
9
2
2
2
2
2
2
3a
3b
3c
3d
3e
3f
6
6
6
6
6
6
6a:7a
6b:7b
6c:7c
6d:7d
6e:7e
6f:7f
90
91
95
88
85
93
93:06
85:14
93:07
95:05
87:13
81:19
10
11
12
13
14
4-CN
4-OMe
aReagents and conditions: 1 or 2 (1.0 eq.), phenol (1.1 eq.), EtOAc (1mL), RT; bIsolated
yield after short wash column chromatography; cAnomeric ratio determined by HPLC
analysis of the crude product.
Among the solvents screened, acetone and ethyl acetate gave
the best results in terms of reaction time and yield. Since 1,2-
anhydrosugars are prone to slowly open by reacting with trace
amounts of moisture,17 we chose ethyl acetate18 over acetone to
generalize this glycosidation reaction. Although the reaction
required a much shorter reaction time in toluene, it was not
preferred over ethyl acetate as some of the solid phenols were not
easily soluble in toluene and required heating to higher
temperatures to obtain a homogenous solution.
Enthused by these findings, we explored this glycosidation
methodology for the synthesis of alkyl glycosides. Treatment of
1,2-anhydrosugar 1 with isopropanol in ethyl acetate at ambient
temperature for 3 h proceeded smoothly to yield exclusively β-
isopropylglucoside 8a (1,2-trans-adduct) in 80% yield. The
scope was demonstrated by extending it to a number of primary
and secondary alcohols. Treatment of 1,2-anhydrosugar 1 with
various alcohols (Table 3) in ethyl acetate at ambient temperature
proceeded smoothly to afford the corresponding β-alkyl
glucosides 8a-l in very good yields (Scheme 2, Table 3);19 no
traces of α-alkyl glycosides were observed in these reactions.
Table 1. Reaction of 1,2-anhydrosugar 1 with p-cresol
Entr
Solvent
Time
(h)
6.5
12
6
6
2.15
24
24
Yield
(%)a
83
76
85
94
87
no rxn
no rxn
α:βb
y
1
2
3
4
5
6
7
OBn
OBn
R'
R'
CH2Cl2
86:14
86:14
88:12
84:16
85:15
Alcohol
EtOAc
H
O
H
O
R
BnO
Acetonitrile
Acetone
Ethyl acetate
Toluene
DMF
DMSO
R
BnO
OR
H
OH
H
O
8a-l R= OBn; R' = H
9a-e R= H; R' = OBn
1
2
R= OBn; R' = H
R= H; R' = OBn
c
-
-
c
Scheme 2. Reaction of α-D-1,2-anhydrosugars 1 and 2 with
alcohols
aIsolated yield after column chromatography; bAnomeric ratio determined by HPLC;
cIncomplete reaction with only decomposed products.
This cis-α-selective aromatic glycosidation of 1,2-
anhydrosugar 1 was generalized with phenols 3a-f in ethyl
acetate (Scheme 1, Table 2).19 In all cases, the α-aryl glycoside
4a-h was formed as the major product and the β-anomer 5a-h as
the minor product. Phenols bearing electron donating substituents
as well as electron withdrawing substituents underwent this
glycosidation in good yields.
This reaction was also facile in the case of menthol (Table 3,
Entry 11). Even cholesterol was found to undergo glycosidation
but required slightly elevated conditions viz., reflux in toluene for
2 h (Table 3, entry 12), which compared favourably to the harsh
conditions and low yield reported in the literature.21 Allylic
alcohols and propargyl alcohol (Table 3, entries 6-8) were also
suitable substrates. Furthermore, this glycosidation was equally
facile in the case of 1,2-anhydro-α-D-galactopyranose 2. The
The glycosidation with galacto-epoxide 2 also yielded α-aryl
galactoside 6a-f as the major product and β-aryl galactoside 7a-f
as the minor product. In the case of phenols 3d, 3e and 3g, trace
amounts (<5%) of the hydrolyzed product17 (1,2-diol) due to
moisture was also observed. The workup of this reaction requires
only evaporation of the solvent. All the aryl glycosides are
known compounds and the spectral data are in accordance with
those reported in the literature.20
reaction of
2 with alcohols in ethyl acetate gave the
corresponding β-alkyl galactosides 9a-e in good yields. All β-O-
alkyl glycosides are known compounds and the spectral data are
in accordance with those reported in the literature.20,22
Table 3. Reaction of 1 and 2 with alcohols in ethyl acetatea
Ent
ry
1
Subst Time Pro Yieldb
rate (h) duct (%)
R-OH
α:βc
Isopropanol
Methanol
Ethanol
1
1
1
1
1
6
6
6
9
6
8a
8b
8c
8d
8e
96 Only β
96 Only β
95 Only β
86 Only β
82 Only β
2
3
4
5
n-Butanol
t-Butanol
Table 2. Ring opening reaction of 1 and 2 with phenols in ethyl
acetatea
Entr Subst Phenol
R
Time Product Yieldb Ratio
6
Allylic alcohol
1
8
8f
88 Only β
y
rate
(h)
α:β
(%)
α:βc
7
8
Cinnamyl alcohol
Propargyl alcohol
1
1
6
8
8g
8h
88 Only β
77 Only β
1
2
1
1
3a
3b
4-Me
H
6
6
4a:5a
4b:5b
94
95
84:16
89:11

3
9
Cyclohexanol
Benzylalcohol
1
1
1
1
2
2
2
6
6
8i
8j
82 Only β
95 Only β
88 Only β
77 Only β
88 Only β
92 Only β
95 Only β
In summary, we have developed a mild and stereoselective
method for the glycosidation of phenols and alcohols under
neutral conditions by the ring opening of 1,2-anhydrosugars
which are readily available from the epoxidation of glycals. This
approach directly provides easy access to aryl glycosides with a
free hydroxyl group at the C-2 position of the sugar moiety that
can be exploited for the synthesis of carbohydrates containing a
1,2-linkage, as found in many natural glycoconjugates, such as
saponins.25
10
11 Menthol
8
2d
8k
8l
12 Cholesterol
13 Methanol
14 Isopropanol
15 Ethanol
5
9a
9b
9c
6
6
16
Allylic alcohol
2
6
9d
85 Only β
Acknowledgments Dr. K. K. Balasubramanian thanks INSA for
the award of INSA Senior Scientist and financial support.
17 Benzyl alcohol
2
5
9e
88 Only β
aReagents and conditions: 1 or 2 (1.0eq.), alcohol (1.1eq.), EtOAc (1 mL), RT; bIsolated
yield after short column chromatography; cDetermined by NMR; dReaction was facile
only when heated at reflux in toluene at 110 °C for 2 h.
References and notes
Boron,23a aluminium,23a zirconium23b and zinc23c-e are known to
bring about cis-stereoselective ring opening of α-1,2-
anhydrosugars by phenols and C-nucleophiles. Mechanistically,
intramolecular ligand transfer from the metal to the anomeric
carbocation has been proposed to account for the observed
stereochemistry. In contrast, metal phenoxides give β-aryl
glycosides via an SN2 pathway.13
1. (a) Pratt, M. R.; Bertozzi, C. R. Chem. Soc. Rev., 2005, 34, 58; (b)
Boeckler, G. A.; Gershenzon, J.; Unsicker, S. B. Phytochemistry,
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P.; Wen, Y-P. The Journal of Microbiology, 2007, 45, 499; (d)
Buskas, T.; Ingale S.; Boons, G. J. Glycobiology, 2006, 16, 113;
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2000, 100, 4495. (f) Bertozzi C. R.; Kiessling, L. L. Science, 2001,
291, 2357; (g) Yang, Y.; Zhang, X.; Yu, B. Nat. Prod. Rep., 2015,
32, 1331.
The contrasting stereochemical preference exhibited by
alcohols and phenols under neutral conditions in the absence of
any catalysts, additives or promoters in our ring opening reaction
indicates that the pKa of the aglycone hydroxyl group may have a
role in directing the stereochemical course of the reaction.
Alcohols, which are weak acids compared to phenols, probably
react via an SN2 mechanism to provide β-alkyl glycosides; while
in the case of phenols, which are relatively stronger acids than
alcohols, the mechanism (Scheme 3) could proceed by an SN1
pathway via the formation of an oxocarbenium ion that would
lead to mixtures of α- and β-aryl glycosides with 1,2-trans-α-aryl
glycosides as the major product. The anomeric stabilisation in the
case of α-aryl glycosides may be a reason for the formation of α-
isomers as the major product. The trans stereoselectivity
observed in the case of benzoic acid24 is difficult to account for
with the present data. We have also observed only the formation
of β-glycosyl benzoate when 1 was treated with benzoic acid
(Scheme 3) under our conditions.
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1372.
OBn
OBn
H
OBn
H
OBn
H
H
O
phenol
O
O
O
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
H
O
O
O
O
O
O
O
1
H
H
H
4b
-isomer
(major product)
Scheme 1. Plausible mechanism for the ring opening reaction of
1
The driving force is presumably protonation of the epoxide
with the phenolic hydrogen that leads to weakening of the
anomeric C-O epoxide bond of the anhydrosugar further assisted
by mesomeric interaction of the ring oxygen. This is supported
by the fact under identical conditions no reaction was observed in
the case of cyclohexene epoxide with either isopropanol or
phenol. Our studies reveal that the pka of the aglycone alone
cannot be the deciding factor to account for the contrasting
stereoselectivity observed in the case of alcohols, phenols and
carboxylic acids. Further investigation is needed to understand
the mechanistic aspects dictating the stereochemical course of
this ring opening reaction.
11. Li, Y.; Tang, P.; Chen, Y.; Yu, B. J. Org. Chem. 2008, 73, 4323.
12. (a) Hickinbottom, W. J. J. Chem. Soc., 1928, 3140; (b) Brigl,
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and hence was initially distilled over anhydrous K2CO3
followed by distillation over CaH2. However, HPLC grade
ethyl acetate with zero moisture content can also be directly
used for this reaction.
19. To a solution of phenol or alcohol (1.1 equiv.) in freshly
distilled dry EtOAc (1 mL), 1,2-anhydrosugar 1 or 2 (100 mg,
1.0 equiv.) was added and stirred at ambient temperature under
an inert atmosphere for the specified time. Upon reaction
completion (TLC), the reaction was concentrated in vacuo and
purified by a short wash column chromatography.
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1000.
HIGHLIGHTS
Title: Simple and Mild Stereoselective O-
Glycosidation using 1,2-anhydrosugars
Under Neutral Conditions
 Stereoselective aromatic glycosylation
using 1,2-anhydrosugars.
 O-Glycosylation without the use of any
additives/catalysts.
 Access to O-glycosides with free
hydroxyl group at C-2 position.
Regards
Dr. S. Bhagavathy
26.