SrCl2 as an efficient cocatalyst for acidic hydrolysis of methyl glycosides

Article abstract of DOI:10.1016/j.cclet.2013.11.028

SrCl₂ was found to be the most efficient cocatalyst for the acidic hydrolysis of methyl glycosides after 26 kinds of most representative metal salts were screened. The SrCl₂-cocatalyzed acidic hydrolysis of methyl glycosides is highl

Full text of DOI:10.1016/j.cclet.2013.11.028

Chinese Chemical Letters 25 (2014) 561–566
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Chinese Chemical Letters
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Original article
SrCl2 as an efficient cocatalyst for acidic hydrolysis of methyl
glycosides
Yong-Heng Shi a,b, Ya-Fei Xie b, Yu-Qiang Liu b, Qun-Chao Wei b, Wei-Ren Xu b,
b,
a,b,**
*
Li-Da Tang
, Gui-Long Zhao
a School of Pharmacy, Tianjin Medical University, Tianjin 300070, China
b Tianjin Key Laboratory of Molecular Design and Drug Discovery, Tianjin Institute of Pharmaceutical Research, Tianjin 300193, China
A R T I C L E I N F O
A B S T R A C T
Article history:
SrCl2 was found to be the most efficient cocatalyst for the acidic hydrolysis of methyl glycosides after 26
kinds of most representative metal salts were screened. The SrCl2-cocatalyzed acidic hydrolysis of
methyl glycosides is highlighted by short reaction times, less byproducts and high yields. A possible
mechanism for the SrCl2-cocatalyzed hydrolysis is also proposed.
Received 8 October 2013
Received in revised form 23 October 2013
Accepted 12 November 2013
Available online 21 November 2013
ß 2013 Li-Da Tang and Gui-Long Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical
Society. All rights reserved.
Keywords:
Cocatalyst
Hydrolysis
Lewis acid
Methyl glycoside
Strontium chloride
1. Introduction
acidic hydrolysis their hydroxyl groups are usually protected by
robust protecting groups, such as alkyl and benzyl groups [5–13],
Methyl glycosides constitute an important class of carbohy-
drate derivatives. Their hydrolysis can produce the corresponding
lactols, which are important intermediates for further modifica-
tions [1] or can be used to prepare glycosyl donors for the
construction of oligosaccharides [2–4].
because most other protecting groups usually cannot survive the
harsh hydrolysis conditions, we chose the methyl glycosides with
methyl and benzyl protecting groups for our study (Fig. 1). We
herein report an efficient cocatalyst, SrCl2, for the acidic hydrolysis
of methyl glycosides, which can be highlighted as inexpensive,
shortening reaction times, suppressing formation of byproducts
and improving yields.
Hydrolysis of methyl glycosides to their corresponding lactols is
always carried out under rather harsh conditions [5–13], which are
characterized by strongly acidic reaction media and high reaction
temperatures [5–8], and often suffers from poor yields [9–12] and
large quantities of byproducts which can cause great trouble in
purification in many cases [12,13]. Survey of literature revealed
that there are no currently available methods to solve these
problems. Prompted by these unsolved shortcomings in the
current methods, we were interested in finding a cocatalyst to
promote the hydrolysis while suppressing the formation of
byproducts so that relatively mild hydrolysis conditions could
be adopted and the aforementioned drawbacks could be solved.
Considering the fact that when methyl glycosides are ready for
2. Experimental
The melting points were measured with an XT-4 microscopic
melting apparatus and are uncorrected. The 1H NMR spectra
were recorded on a Bruker AV400 spectrometer with DMSO-d6
as solvent and TMS as internal standard. HPLC analyses were
performed with a Waters 2695 high-performance liquid chro-
matograph equipped with a Diamonsil C8 column (150 mm ꢀ
4.6 mm, 5
mm; held at 35 8C) and a UV detector (210 nm),
using MeCN/H2O (90/10, v/v) as mobile phase at a flow rate of
0.6 mL/min.
Methyl tetra-O-benzylglycosides 1–6 were prepared from their
corresponding unprotected methyl glycosides following a known
one-step procedure [14]. Compounds 7 and 8 were prepared from
*
Corresponding author.
**
Corresponding author at: School of Pharmacy, Tianjin Medical University,
Tianjin 300070, China.
methyl
a-D-glucopyranoside following known procedures [15,16],
E-mail addresses: tangld@tjipr.com (L.-D. Tang), zhao_guilong@126.com
(G.-L. Zhao).
and substrate 9 was prepared according to a known method [1]. All
1001-8417/$ – see front matter ß 2013 Li-Da Tang and Gui-Long Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.11.028

562
Y.-H. Shi et al. / Chinese Chemical Letters 25 (2014) 561–566
OBn
OBn
OBn
OBn
OBn
OBn
BnO
BnO
BnO
BnO
O
O
O
O
O
BnO
BnO
BnO
BnO
BnO
BnO
OMe
OMe
BnO
OBn
BnO
OBn
OMe
OMe
OMe
1
2
3
4
5
OMe
OBn
OBn
OBn
O
O
O
BnO
BnO
O
BnO
BnO
MeO
MeO
BnO
BnO
OMe
BnO
BnO
MeO
OMe
OMe
OMe
6
9
7
8
Fig. 1. Substrates used for the SrCl2-cocatalyzed acidic hydrolysis.
the substrates 1–8 are known compounds and their melting points
or 1H NMR data were in good agreement with those reported.
Procedure for screening of concentration of HCl and reaction
temperature: In a 50 mL round-bottomed flask, 1.00 g (1.8 mmol)
of 1 was dissolved in 6 mL of glacial acetic acid, and the solution
was stirred and heated to a specific temperature (Table 1). To the
mixture was added 1 mL of HCl with a specific concentration
(Table 1). The reaction was timed just after the addition of HCl, and
aliquots of the reaction mixture were subjected to HPLC analysis at
30 min intervals to follow the reaction course until a conversion of
>95% was reached.
Procedure for screening of the kind of metal salt: In a 50 mL
round-bottomed flask, 1.00 g (1.8 mmol) of 1 was dissolved in 6 mL
of glacial acetic acid, and the solution was stirred and heated to
70 8C. To the mixture was added 1 mL of 5 mol/L HCl, followed by
addition of 1.0 eq. of metal salt (Table 2). The reaction was timed
just after the addition of HCl and metal salt, and aliquots of the
reaction mixture were subjected to HPLC analysis at 30 min
intervals to follow the reaction course until a conversion of >95%
was reached.
intervals to follow the reaction course until a conversion of >95%
was reached.
Procedure for acidic hydrolysis of 1–9 to test the generality of
SrCl2 as cocatalyst: In a 50 mL round-bottomed flask, 1.00 g of
substrate 1–9 was dissolved in 6 mL of glacial acetic acid, and the
solution stirred and heated to 70 8C (Table 4). To the mixture was
added 1 mL of 5 mol/L HCl, followed by the addition of 0.10 eq. of
SrCl2ꢁ6H2O. The reaction was timed just after the addition of HCl
and SrCl2ꢁ6H2O, and aliquots of reaction mixture subjected to
HPLC analysis at 30 min intervals to follow the reaction course
until a conversion of >95% was reached. The reaction mixture was
quenched by adding 50 mL of ice-water, and the resulting mixture
was extracted with CH2Cl2 (20 mL ꢀ 3). The combined extracts
were washed with saturated aqueous NaHCO3 and brine, dried
over anhydrous Na2SO4 and evaporated on a rotary evaporator to
afford the crude lactols, which were purified by column
chromatography to yield the pure lactols 10–15. All the lactols
10–15 corresponding to 1–9 are known compounds, and their
melting points or 1H NMR were in good agreement with those
reported (Table 4).
Procedure for screening of equivalent of metal salt: In a 50 mL
round-bottomed flask, 1.00 g (1.8 mmol) of 1 was dissolved in 6 mL
of glacial acetic acid, and the solution was stirred and heated to
70 8C. To the mixture was added 1 mL of 5 mol/L HCl, followed by
addition of a specific equivalent of metal salt (Table 3). The reaction
was timed just after the addition of HCl and metal salt, and aliquots
of the reaction mixture were subjected to HPLC analysis at 30 min
3. Results and discussion
The corresponding lactols produced by hydrolysis of methyl
glycosides are versatile intermediates in organic chemistry
(Scheme 1). According to our earlier study [1] and the report of
Koto et al. [12], the hydrolysis of methyl glycosides needed rather
harsh reaction conditions, most of which involved heating the
methyl glycosides in glacial acetic acid at high temperatures with
aqueous strong acids, such as H2SO4 [10,11], HCl [7,8,12] and TfOH
[6]. These harsh reaction conditions could indeed result in quick
reactions; however, a main drawback was that the desired product
often included large quantities of byproducts, such as debenzy-
lated products [12,13], complicating the workup and purification
procedures and resulting, in many cases, in poor yields [9–12] due
notably to the reaction conditions [12] (Scheme 2). In the normal
acidic hydrolysis of methyl glycosides, the protons chelate the
glycosidic oxygen atoms aiding the cleavage of the glycosidic bond.
Since metal ions are also electron-deficient species which can also
chelate these oxygen atoms in an identical way to protons,
selection of a metal salt as a Lewis acid cocatalyst to accelerate the
hydrolysis step while suppressing the formation of byproducts was
desired to solve the problems in the reported methods associated
with the hydrolysis of methyl glycosides.
Table 1
Results for screening of HCl concentration and reaction temperature using 1 as
substratea.
Entry
HCl (mol/L)a
Temperature (8C)
Time (min)b
1
6
6
6
5
5
5
4
4
4
3
3
3
2
2
2
80
70
60
80
70
60
80
70
60
80
70
60
80
70
60
90
240
2
3
420
4
120
5
390
6
720
7
150
8
420
9
>720
240
10
11
12
13
14
15
660
>720
390
With high concentrations of HCl and high temperatures, the
acidic hydrolysis of methyl glycosides can proceed quickly, but
often leads to large quantities of byproducts and poor yields [12].
So, prior to the screening of metal salts as cocatalysts, suitable
reaction conditions to complete the hydrolysis of methyl glyco-
>720
>720
a
Reaction conditions: 1.00 g of 1, 1 mL of HCl, 6 mL of AcOH.
b
Reaction times corresponding to 95% conversion were measured by HPLC
analysis (C8 column, 210 nm, MeCN/H2O =90/10 (v/v)).

Y.-H. Shi et al. / Chinese Chemical Letters 25 (2014) 561–566
563
Table 2
Results for screening of metal salts as cocatalyst using 1 as substrate.
Entry
Metal salta
Time (min)b
Yield (%)c
Purity (%)d
Number of impurities
1
LiCl
240
240
240
240
540
300
210
300
210
210
180
420
240
210
210
270
180
120
390
210
270
360
270
150
330
240
73
71
70
73
74
82
77
81
90
55
60
71
52
69
73
73
66
51
75
88
77
85
83
85
81
68
85
84
82
84
81
87
83
87
94
67
71
83
66
78
81
84
76
66
84
92
86
90
89
91
88
79
4
4
6
3
4
2
4
3
2
9
9
5
9
5
5
4
7
9
4
2
4
2
2
2
3
6
2
NaCl
3
KCl
4
RbCl
5
CsCl
6
MgCl2
CaCl2
BaCl2
SrCl2
AlCl3
ZnCl2
SnCl2
SnCl4
CdCl2
HgCl2
PbCl2
BiCl3
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
TiCl4
CrCl3
MnCl2
FeCl3
CoCl2
NiCl2
CuCl2
Ce2(SO4)3
Ce(SO4)2
a
1.0 eq. of metal salt was used. Some metal salts used were in their hydrated forms: MgCl2ꢁ6H2O, BaCl2ꢁ2H2O, SrCl2ꢁ6H2O, SnCl2ꢁ2H2O, SnCl4ꢁ6H2O, CrCl3ꢁ6H2O, CoCl2ꢁ6H2O,
NiCl2ꢁ6H2O, CuCl2ꢁ2H2O, Ce2(SO4)3ꢁ8H2O, and Ce(SO4)2ꢁ4H2O.
b
Reaction times corresponding to 95% conversion were measured by HPLC analysis (C8 column, 210 nm, MeCN/H2O =90/10, v/v).
c
Yields of the desired product 10 were measured by HPLC analysis under conditions described above.
d
The purity of 10 herein was reflected by the purity of 10 in the crude reaction mixture determined by HPLC under conditions described above.
sides within a reasonable time should be found because if the
hydrolysis rate proceeds too fast or too slow, it will be difficult to
clearly detect the effect of the cocatalysts. We chose the commonly
encountered methyl 2,3,4,6-tetra-O-benzyl-a-D-glucopyranoside
1 (Scheme 2) as the substrate for the initial screening of the two
important hydrolysis parameters, i.e. the concentration of the acid
and the reaction temperature. It should be noted that HCl, as the
acid, and metal chlorides, as the Lewis acid cocatalyst, were used
throughout our study because most metal chlorides are soluble in
water whereas many of their sulfate counterparts are not. The
results are shown in Table 1. The concentration of HCl ranged from
2 mol/L to 6 mol/L, and the reaction temperatures ranged from
Table 3
Results for screening of equivalent of cocatalysts using 1 as substrate.
Entry
Metal salta
Quantity (eq.)b
Time (minc
Yield (%)d
Purity (%)e
Number of impurities
1
SrCl2
0.01
0.05
0.1
240
210
150
150
150
180
210
240
300
270
270
240
240
210
210
240
210
180
180
150
150
150
150
150
71
77
91
90
90
89
90
88
70
70
73
78
80
82
88
76
82
84
83
80
82
84
85
81
81
84
95
92
93
92
94
91
82
83
86
84
87
88
92
84
90
90
89
86
88
90
91
87
6
4
2
2
2
2
2
2
4
4
3
3
2
2
2
3
2
2
2
3
3
2
2
3
2
SrCl2
3
SrCl2
4
SrCl2
0.2
5
SrCl2
0.4
6
SrCl2
0.8
7
SrCl2
1.0
8
SrCl2
2.0
9
MnCl2
MnCl2
MnCl2
MnCl2
MnCl2
MnCl2
MnCl2
MnCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
0.01
0.05
0.1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.2
0.4
0.8
1.0
2.0
0.01
0.05
0.1
0.2
0.4
0.8
1.0
2.0
a
Some metal salts used were in the hydrated forms: SrCl2ꢁ6H2O and CuCl2ꢁ2H2O.
The quantities were based on the substrate 1.
b
c
Reaction times corresponding to 95% conversion were measured by HPLC analysis (C8 column, 210 nm, MeCN/H2O = 90/10, v/v).
d
e
Yields of the desired product 10 were measured by HPLC analysis under conditions described above.
Purity of 10 was herein reflected by the purity of 10 in the crude reaction mixture as determined by HPLC under conditions described above.

564
Y.-H. Shi et al. / Chinese Chemical Letters 25 (2014) 561–566
Table 4
Hydrolysis of a variety of methyl glycosides 2–9 with 0.1 eq. of SrCl2 under optimized reaction condition.
c
Entry
1
Substrate
OBn
Product
Reaction time (min)a
120
Yield (%)b
90
Identification
White solid. M.p. 150–151 8C (lit. [17] 150–152 8C)
OBn
BnO
BnO
O
O
BnO
BnO
OMe
OBn
BnO
OMe
2
3
60
60
88
89
Oil. 1H NMR data were in agreement with those reported [18]
Oil. 1H NMR data were in agreement with those reported [18]
OBn
OBn
BnO
BnO
BnO
BnO
O
O
OMe
OMe
OBn
OBn
OBn
OBn
O
OBn
OBn
O
BnO
BnO
BnO
BnO
OMe
OMe
4
150
91
Oil. 1H NMR data were in agreement with those reported [19]
Oil. 1H NMR data were in agreement with those reported [19]
OBn
OMe
O
O
BnO
BnO
MeO
MeO
BnO
MeO
OBn
OMe
OMe
5
6
150
150
90
92
O
BnO
BnO
O
BnO
BnO
BnO
OBn
OMe
OH
OBn
OBn
White solid. M.p. 128–129 8C. 1H NMR data were in agreement
with those reported [20]
BnO
BnO
BnO
BnO
O
O
OBnOH
OBnOH
7
180
90
87
92
Oil. 1H NMR data were in agreement with those reported [16]
OBn
OBn
O
OBn
OBn
O
BnO
BnO
BnO
BnO
OH
OH
8
White solid. M.p. 92–94 8C (lit. [1] 92–94 8C)
OBn
O
BnO
BnO
O
MeO
MeO
OBnOH
OH
OMe
a
Reaction times corresponding to 95% conversion were measured by HPLC analysis (C8 column, 210 nm, MeCN/H2O =90/10, v/v).
Isolated yields.
Structural confirmation of the products. Melting points or 1H NMR spectra of the products were compared with those reported.
b
c
OR
4
O
R O
3
etc.
R O
2
O
OR
OR
4
4
OR
1
+
O
H
O
R O
R O
3
3
OR
OR
R O
R O
4
H O
4
2
2
2
OMe
OH
OR
OR
1
1
NH
CCl
O
O
R O
R O
3
3
etc.
R O
R O
2
2
O
X
OR
3
OR
1
1
Scheme 1. Examples for hydrolysis of methyl glycosides and the representative applications of lactols thus produced.
60 8C to 80 8C. As can be observed clearly in Table 1, the reaction
rate responded much more dramatically to the temperature
change than to the change of HCl concentration. The reaction rate
decreased significantly below 70 8C and increased quickly above
70 8C, therefore, 70 8C was selected as the optimum temperature.
At 70 8C, the reaction times ranged from 240 min to >720 min, and
finally 390 min was believed to the optimum reaction time at
5 mol/L of HCl and most suitable for further screening of metal
salts as cocatalyst (entry 5 in Table 1).
With in hand the optimum conditions for the screening of metal
salts, i.e. 5 mol/L HCl at 70 8C, we proceeded to screen 26
representative metal salts as cocatalyst using 1 as substrate
(Table 2). As shown in Table 2, with soft alkali metal ions (entries
1–5) the hydrolysis exhibited a slight improvement compared with
that without any metal salt (entry 5 in Table 1); however, they
were marked by longer reaction times and moderate yields and
impurity profiles. The hard metal ions (entries 10, 11, 13, 17, 18
OBn
OBn
O
O
HCl
BnO
BnO
BnO
BnO
byproducts
AcOH/H2O
heat
OH
OBn
BnO
OMe
10
1
Scheme 2. Hydrolysis of 1 to its corresponding lactol 10 and the formation of
byproducts that follows.

Y.-H. Shi et al. / Chinese Chemical Letters 25 (2014) 561–566
OBn
565
OBn
OBn
OBn
H+
Sr2+
-MeOH
-Sr2+
O
O
O
BnO
BnO
O
BnO
BnO
BnO
BnO
BnO
BnO
H
OBnOMe
1 and 2
OBn
O
O
H
BnO
O
Bn
Sr2+
H2O
-H+
OBn
O
BnO
BnO
OH
OBn
10
Fig. 2. Proposed mechanism of SrCl2-cocatalyzed hydrolysis exemplified by 1 and 2.
and 26) could considerably accelerate the reaction, but, unfortu-
nately, could lead to the formation of significant levels of
byproducts, and dramatically decreased yields. Based on the
comprehensive evaluation in terms of reaction time, yield and
purity, three metal salts, SrCl2 (entry 9), MnCl2 (entry 20) and CuCl2
(entry 24), were selected for further investigation and indicated
that moderate hardness of the metal ions is best suited for the our
purpose. Neither too soft nor too hard metal ions were appropriate
for the hydrolysis, with the former ones having insufficient
cocatalytic efficiency while the latter ones exhibiting too powerful
cocatalytic efficiency that led to decomposition of the products as
well.
Since the preliminary screenings used 1.0 eq. of metal salts as
shown in Table 2, we determined to study the effect of the quantity
of the cocatalyst on the cocatalytic efficiency (Table 3). The
reactions were carried out with the three designated metal salts
using 1 as substrate and optimum reaction conditions, i.e. 5 mol/L
HCl at 70 8C. As shown in Table 3, as the quantity of cocatalyst
increased, the reaction rates with all three cocatalysts gradually
increased to reach plateaus. The reaction rate with SrCl2 and MnCl2
also displayed slight decreases at specific large quantities (entries
8 and 16). The smallest quantities of these three cocatalysts
corresponding to the best reaction rates were then determined as
0.1 eq. for SrCl2, 0.8 eq. for MnCl2, and 0.2 eq. for CuCl2. Further
examination of the data found that 0.1 eq. of SrCl2 was the best
because it corresponded to the shortest reaction time (150 min)
4. Conclusion
SrCl2 was found to be the most efficient cocatalyst for the acidic
hydrolysis of methyl glycosides among 26 metal salts screened as
Lewis acid. The SrCl2-cocatalyzed acidic hydrolysis of methyl
glycosides was highlighted by short reaction times, less bypro-
ducts and high yields. Based on this work, the best conditions for
acidic hydrolysis of methyl glycosides included heating the methyl
glycosides in a mixed solvent consisting of 5 mol/L HCl/acetic acid
(1/6, v/v) with 0.1 eq. of SrCl2 as cocatalyst at 70 8C. This strategy
solved the problems of the hydrolysis of methyl glycosides,
particularly the formation of large quantities of byproducts and
poor yields, where there are no acceptable methods currently
available.
Acknowledgments
The authors are very grateful to the Natural Science Foundation
of China (No. 21302141) and the Key Projects of Tianjin Science and
Technology Support Plan (No. 10ZCKFSH01300) for financial
support.
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2
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