Electron-deficient pyridinium salts/thiourea cooperative catalyzed O-glycosylation via activation of O-glycosyl trichloroacetimidate donors

Article abstract of DOI:10.3762/bjoc.13.236

The glycosylation of O-glycosyl trichloroacetimidate donors using a synergistic catalytic system of electron-deficient pyridinium salts/aryl thiourea derivatives at room temperature is demonstrated. The acidity of the adduct formed by the 1, 2-addition of

Full text of DOI:10.3762/bjoc.13.236

Electron-deficient pyridinium salts/thiourea cooperative
catalyzed O-glycosylation via activation of O-glycosyl
trichloroacetimidate donors
Mukta Shaw1, Yogesh Kumar1, Rima Thakur2 and Amit Kumar*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 2385–2395.
1Department of Chemistry, Indian Institute of Technology Patna, Bihta
801106, Bihar, India and 2Department of Chemistry, National Institute
of Technology Patna, Patna 800005, Bihar, India
doi:10.3762/bjoc.13.236
Received: 27 July 2017
Accepted: 04 October 2017
Published: 09 November 2017
Email:
Amit Kumar* - amitkt@iitp.ac.in
Associate Editor: S. Flitsch
* Corresponding author
© 2017 Shaw et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
cooperative catalysis; electron-deficient pyridinium salts; O-glycoside;
regioselectivity; thiourea
Abstract
The glycosylation of O-glycosyl trichloroacetimidate donors using a synergistic catalytic system of electron-deficient pyridinium
salts/aryl thiourea derivatives at room temperature is demonstrated. The acidity of the adduct formed by the 1,2-addition of alcohol
to the electron-deficient pyridinium salt is increased in the presence of an aryl thiourea derivative as an hydrogen-bonding cocata-
lyst. This transformation occurs under mild reaction conditions with a wide range of O-glycosyl trichloroacetimidate donors and
glycosyl acceptors to afford the corresponding O-glycosides in moderate to good yields with predictable selectivity. In addition, the
optimized method is also utilized for the regioselective O-glycosylation by using a partially protected acceptor.
Introduction
The glycosidic linkage is the principal bond present in a crucial Nature extensively employs small organic molecules as cata-
class of biomolecules such as oligosaccharides and glycoconju- lysts for the acceleration of many important biochemical reac-
gates, where one sugar unit is linked with another sugar unit or tions, such as glycosyltransferase reactions, hydrolysis of strong
any other molecules (aglycons) [1-4]. Owing to their high amide bonds and others [11-13]. Taking inspiration from nature,
importance, several efficient protocols have been developed for in the last few decades chemists around the world have utilized
the stereoselective glycosylation in the past few decades [5-10]. organic molecules to accelerate many imperative organic trans-
However, the synthesis of fundamental glycosidic bonds with formations [14-17]. One of the major applications of
high efficiency and selectivity yet remains one of the major organocatalysis lies in the field of enantioselective synthesis,
challenges for organic chemists, in particular, carbohydrate where organocatalysts are considered as fundamental tools in
chemists.
the catalysis toolbox [18-22]. Moreover, the reactivity and
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selectivity of organocatalysts can be further amplified in the reaction medium enhances the acidity of the ammonium salt
presence of other cocatalysts known as “cooperative catalysis” due to doubling their dual hydrogen bonding ability with the
[23]. In particular, cooperativity between Brønsted acids and carboxylate and the alkoxy group of the ammonium salt. A thio-
hydrogen-bonding cocatalysts such as thiourea derivatives has urea derivative also enhances the nucleophilicity of the glycosyl
attracted much interest [24-29]. Despite the broad application of acceptor by imparting a partial negative charge on it. Hence, the
cooperative catalysis, it is still uncommonly employed in the application of the synergistic catalyst system consisting of elec-
area of carbohydrate chemistry, especially for glycosylation tron-deficient pyridinium salts/thiourea derivatives for glyco-
reactions, due to the prerequisite of having both catalysts being sidic bond formation will be an exciting addition to the litera-
compatible under the reaction conditions. The Schmidt group ture (Scheme 1).
has successfully applied the synergistic catalysts (thiourea de-
rivatives with phosphorus acids) for stereoselective O-glyco- Results and Discussion
side bond formation [30]. Similarly, Galan et al. reported a To check our hypothesis, a series of 1H NMR spectroscopic
method for the preparation of 2-deoxyglycosides from glycals studies were conducted by selecting commonly used
under the influence of cooperative catalysis (chiral phosphoric O-glucopyranosyl trichloroacetimidate 1α [39-41] as glycosyl
acids/thiourea derivatives) [31]. Encouraged by these reports donor and 3,5-di(methoxycarbonyl)-N-(cyanomethyl)pyri-
and our own research interest in developing stereoselective dinium bromide (3a) as a catalyst. For example, when glycosyl
glycosylation methods, we decided to focus our attention on the donor 1α was treated with catalyst 3a (10 mol %) at room tem-
synthesis of glycosides via cooperative catalysis. A highly reac- perature for 4 h (Table 1, entry 1) in CD2Cl2 solvent, it was ob-
tive glycosyl donor for instance, O-glycosyl trichloroacet- served that there is neither any interaction of 1α with catalyst 3a
imidate, generally requires a pKa value less than 5 for activa- nor decomposition of 1α (Figure 1b) as the peak position of 1α
tion at room temperature [32-36]. It is known from the litera- remained unchanged. The salt remains insoluble in CD2Cl2
ture that pyridinium salts exhibit pKa values of about 5.2 [37]. and hence did not show any peak in the 1H NMR spectra.
Of late, Berkessel et al. disclosed an elegant method, where dif- However, when the mixture of electron-deficient pyridinium
ferent electron deficient pyridinium salts (expected pKa values salt 3a and glycosyl acceptor 2a (1:1) was dissolved in
less than 5) were used as a catalyst for the activation of glycals CD2Cl2 and investigated by 1H NMR, an upfield shift of the
to provide stereoselective 2-deoxyglycosides with high yields -CH2- peak of 3a from δ 6.17 to δ 4.03 (Figure 2c) was ob-
[38]. Based on this fact, we anticipated that for electron-defi- served. This result clearly supports the possible formation of
cient pyridinium salts the pKa value would be further dimin- 1,2-adduct X, on the reaction of 3a with 2a which is
ished in the presence of hydrogen-bonding cocatalysts such as eventually responsible for the loss of aromaticity of the pyridin-
thiourea derivatives. The presence of Schreiner′s thiourea in the ium ring.
Scheme 1: Mechanistic hypothesis for work.
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Table 1: Optimization of reaction conditionsa.
entry
catalyst
cocatalyst 4
solvent
reaction time
yieldb (α/β ratioc)
1d
2
3a
3a
3a
3a
3b
3c
3a
3a
3a
3a
3a
3a
HBr
3a
–
–
–
+
+
+
+
+
+
+
+
+
–
–
DCM
DCM
DCM
DCM
DCM
DCM
DCM
ACN
4 h
24 h
4 h
2 h
6 h
8 h
5 h
4 h
7 h
24 h
3 h
5 h
8 h
4 h
n.r., n.d.e
56% (1:1)f
86% (1.1:1)
90% (2.2:1)
72% (2:1)
64% (1.7:1)
86% (2.1:1)
56 % (2.1:1)
37% (1:1)
trace
3g
4
5
6
7h
8
9
THF
10
11
12i
13
14k
toluene
DCE
80% (1.4:1)
82%(2:1)
tracej
DCM
DCM
DCM
n.r.
aReaction conditions: 1α (0.15 mmol), 2a (0.165 mmol), 3a–c (10 mol %), 4 (10 mol %), solvent (3 mL), at room temperature under nitrogen atmo-
sphere. bYield of isolated product. cAnomeric ratios were determined by 1H NMR spectroscopy. d1α was stirred with 10 mol % 3a for 4 h at room tem-
perature. en.r. – no reaction, n.d. – no decomposition. fReaction was not completed. g25 mol % of 3a was used. hPerformed at 0 °C. iInverse addition
condition. jA trace amount of glucosyl bromide was also formed. k25 mol % of 2,4,6-trimethylpyridine was added.
Based on the outcomes of 1H NMR spectroscopic studies, we The acidity of the ammonium salt may be enhanced by the
started optimizing the reaction conditions. Upon treatment of introduction of a cocatalyst such as an aryl thiourea derivative,
glycosyl donor 1α and glycosyl acceptor 2a in 1:1.1 molar ratio which has the ability to form a dual hydrogen bond with the
with 10 mol % of 3a in dry DCM at room temperature, the carboxylate and the alkoxy group of the ammonium salt [42-
desired O-glycoside 5a was isolated in 56% yield and with poor 44]. To ensure our postulation, a 1H NMR spectroscopic study
selectivity (Table 1, entry 2). The use of 25 mol % of 3a was re- was carried out with a mixture of glycosyl acceptor 2a, pyri-
quired to drive the reaction to completion with 86% yield dinium salt 3a (10 mol %) and aryl thiourea 4 (10 mol %) in
(Table 1, entry 3). This result, as envisaged, was indeed inter- CD2Cl2 at room temperature (Figure 3). The -Me peak of
esting and encouraging, which clearly indicates the ability of -CO2Me of the ammonium salt shifted from δ 4.09 to δ 3.97,
the electron-deficient pyridinium salt to activate the trichloro- which indirectly confirms the presence of hydrogen bonding be-
acetimidate donor. However, it took longer reaction time and tween -NH of the thiourea and the carbonyl carbon of -CO2Me
required higher catalyst loading (up to 25 mol %). This outcome of 3a (Figure 3d). Also, the magnification of the nucleophilici-
can be attributed to lower acidity of the ammonium salt formed ty of the glycosyl acceptor by the thiourea derivative results in
by 1,2-addition of the acceptor to the pyridinium salt. The shifting of the -OH peak from δ 2.70 to δ 3.62 (Figure 3d).
conjugate base formed after the release of a proton from the am-
monium salt may be quite stable to impart a negative charge to Once this understanding was gained from 1H NMR studies, aryl
the acceptor oxygen.
thiourea cocatalyst 4 (10 mol %) was added with 10 mol % of
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Figure 1: 1H NMR (a) glycosyl donor 1α and (b) a mixture of 1α and 10 mol % 3a in CD2Cl2 at room temperature.
Figure 2: 1H NMR (a) glycosyl acceptor 2a, (b) pyridinium salt 3a (in DMSO-d6) and (c) a mixture of 2a and 3a in 1:1 ratio in CD2Cl2 at room
temperature.
3a to the reaction mixture of 1α and 2a and pleasingly the reac- [(pentafluorophenyl)methyl]pyridinium bromide (3c, Table 1,
tion was completed within a short span of time, glycoside 5a entry 6). However, the results were not up to our expectation
was obtained with improved yield (90%) and selectivity (α:β and the desired glycoside was obtained in low yield. Once, we
2.2:1 ratio, Table 1, entry 4). Furthermore, several other cata- fixed the catalyst for glycosylation, further parameters were
lysts were also screened for glycosylation, such as 3,5- also optimized. Lowering the reaction temperature (room tem-
di(methoxycarbonyl)-N-[(ethoxycarbonyl)methyl]pyridinium perature to 0 °C), the product formation rate slowed down and
bromide (3b, Table 1, entry 5), 3,5-di(methoxycarbonyl)-N- the selectivity remains unchanged (Table 1, entry 7). Similarly,
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Figure 3: 1H NMR (a) glycosyl acceptor 2a, (b) pyridinium salt 3a (in DMSO-d6), (c) aryl thiourea and (d) a mixture of 2a, 3a (10 mol %) and
4 (10 mol %) in CD2Cl2 at room temperature.
changing the solvent system to acetonitrile, tetrahydrofuran, tol- glycosylation of 1α with several acceptors (Table 2). In all
uene, and dichloroethane had an adverse effect on the reaction cases, reactions proceeded smoothly within 2–6 h and in good
rate, yield and selectivity (Table 1, entries 8–11). Performing yields with moderate to good selectivity, as determined by the
the reaction under inverse addition conditions had no impact on 1H and 13C NMR spectra. Glycosylation with less sterically
the selectivity of glycoside formation (Table 1, entry 12) hindered primary alcohols, e.g., allyl alcohol (2b), benzyl
[39,40]. Aware of the fact that HBr, which is eventually gener- alcohol (2c), 4-methoxy benzyl alcohol (2d), and secondary
ated in situ during the course of reaction from ammonium salt alcohols, e.g., cyclohexanol (2e), cyclopentanol (2f) produced
X, might be the potential catalyst for the activation of the their corresponding glucosides 5b–f in 78–88% yields and with
glycosyl trichloroacetimidate donor, we conducted an addition- moderate α-selectivity (Table 2, entries 1–5). It is important to
al experiment. Glycosyl donor 1α was treated with 25 mol % of note that, when a halogenated primary alcohol such as
HBr instead of pyridinium salt 3a (Table 1, entry 13) where 2-bromoethanol (2g) was treated with glycosyl donor 1α, it
only a trace of glycoside 5a and some glycosyl bromide were gave exclusively β-glycoside 5g in 72% yield (Table 2, entry 6).
formed along with the hydrolyzed product. Hence, it could be However, 3-chloropropanol (2h) as acceptor procured gluco-
concluded that HBr is not the real catalyst in this cooperative side 5h in 70% yield with marginal selectivity (Table 2,
catalysis. Further, the addition of acid scavenger such as 2,4,6- entry 7). Similarly, the reaction with the bulky primary alcohol,
trimethylpyridine inhibits the formation of the glycoside, which 1-adamantanemethanol (2i) produced the desired glucoside 5i in
indirectly supports that the pyridinium salt is a decisive catalyst 75% yield and moderate selectivity (Table 2, entry 8). Further,
for the activation of the glycosyl trichloroacetimidate donor reactions with hindered acceptors, for instance, 1-adamantanol
(Table 1, entry 14). Therefore, the optimal reaction conditions (2j), (+)-menthol (2k), (–)-menthol (2l), cholesterol (2m) re-
for O-glycosylation are the following: the use of 3,5- quired longer reaction times with high catalyst loading and pro-
di(methoxycarbonyl)-N-(cyanomethyl)pyridinium bromide (3a) duced the desired glucosides 5j–m, respectively, in moderate to
as catalyst (10 mol %), thiourea derivative 4 as cocatalyst good yields (43–67%) and with good α-selectivity (Table 2,
(10 mol %) and dichloromethane as solvent at room tempera- entries 9–12). As expected, on treatment with a sugar-based
ture (Table 1, entry 4).
acceptor such as 1,2:3,4-di-O-isopropylidene-D-galactose (2n),
the corresponding glycoside 5n was produced in 72% yield with
With the optimized conditions in hand, our emphasis was then moderate selectivity (Table 2, entry 13) and the acid sensitive
focused on the exploration of the synergistic catalytic system on group survived well [34].
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Table 2: Acceptor scope in glycosylation reaction with donor 1αa.
entry
1
ROH
product
time (h)
yieldb
88%
α/β ratioc
2
1.2:1
2b
5b
2
2
88%
2:1
2c
5c
3
2
82%
3.3:1
2d
5d
4
2
78%
2.1:1
2e
5e
5
6
2
2
85%
72%
2.5:1
2f
5f
β only
2g
5g
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Table 2: Acceptor scope in glycosylation reaction with donor 1αa. (continued)
7
2
70%
1.1:1
2h
5h
8
6
75%
3.3:1
2i
5i
5j
9d
5
43%
2:1
2j
10d
4
61%
α only
2k
5k
11d
4
67%
5:1
2l
5l
12d
6
60%
10.1:1
5m
2m
13
3
72%
1:1.3
2n
5n
aReaction conditions: 1α (0.15 mmol), 2a–n (0.165 mmol), 3a (10 mol %), 4 (10 mol %), solvent (3 mL), at room temperature under nitrogen atmo-
sphere. bYield of isolated product. cAnomeric ratios were determined by 1H NMR spectroscopy. d20 mol % of 3a and 20 mol % of 4 was used.
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To further demonstrate the efficacy of this method other impor- and 7α under the optimized conditions lead to the regioisomer-
tant glycosides were synthesized with different donors as tabu- ic products 23 and 24 [46,47] in moderate yields with good
lated in Table 3. Glycosylation of D-galactopyranosyl trichloro- selectivity (Scheme 2).
acetimidate 6α with a variety of glycosyl acceptors, e.g., 2c, 2g,
2h, 2i and 2n under the optimized reaction conditions gave their Plausible mechanism
corresponding galactosides 9–13, respectively, with moderate To confirm the reaction pathway, few additional control experi-
selectivity (Table 3, entry 1–5) [40,41]. Similarly, the reaction ments were carried out (for details see Supporting Information
of D-mannopyranosyl trichloroacetimidate 7α with glycosyl File 1). When the reaction was performed with 1.0 equiv of 3a
acceptors 2c, 2g, 2h, and 2n produces their corresponding and acceptor 2a under standard inverse procedure conditions,
mannosides 14–18 (61–78% yields) with moderate selectivity the desired O-glycoside 5a was procured in 36% yield and 56%
(Table 3, entries 6–10) [40,41]. Gratifyingly, the highest stereo- of 1α was recovered through column chromatography. There-
selective outcome was observed when 4,6-O-benzylidine-2,3- fore, we conclude that the reaction would have followed an
di-O-benzyl-α-D-glucopyranosyl trichloroacetimidate 8α was intermolecular glycosylation reaction through an oxocarbenium
used as a glycosyl donor. For example, when 8α was treated ion. Combining all of these observations and results from earlier
with different donors such as 2i, 2m, 2n under the optimized literature reports, a plausible reaction mechanism for the elec-
conditions, glucosides 19–21 were procured in good yields with tron-deficient pyridinium salt/thiourea cocatalyzed glycosida-
excellent α-selectivity (Table 3, entry 11–13) [45].
tion is outlined in Figure 4. It is presumed that at first electron-
deficient pyridinium salt 3a undergoes 1,2-addition with the
The regioselective glycosylation is an important aspect in acceptor to produce ammonium salt X. The addition of the thio-
carbohydrate chemistry. It is pleasing to note that on reaction urea derivative as hydrogen-bonding cocatalyst could activate
with partially protected acceptor 22 with glycosyl donors 1α the glycosyl donor by increasing the acidity of ammonium salt
Table 3: Glycosylation of donors 6α–8α with variety of acceptorsa.
entry
donor
acceptor
product
time (h)
yieldb
α/β ratioc
1
2
6α
2c
2g
2h
2i
9
2
2
2
6
3
2
2
2
7
3
6
8
4
91%
74%
81%
73%
62%
74%
78%
77%
68%
61%
61%
54%
61%
1.7:1
2.1:1
1:4.6
2.6:1
1.4:1
1.2:1
3:1
10
11
12
13
14
15
16
17
18
19
20
21
3
4
5
2n
2c
2g
2h
2m
2n
2i
6
7α
8α
7
8
1.9:1
1:1.9
α only
α only
α only
α only
9d
10
11
12d
13
2m
2n
aReaction conditions: donor (0.15 mmol), acceptor (0.165 mmol), 3a (10 mol %), 4 (10 mol %), solvent (3 mL), at room temperature under nitrogen at-
mosphere. bYield of isolated product. cAnomeric ratios were determined by 1H NMR spectroscopy. d20 mol % of 3a and 20 mol% of 4 was used.
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Scheme 2: Synergistic electron-deficient pyridinium salt/aryl thiourea-catalyzed regioselective O-glycosylation.
Figure 4: Plausible reaction mechanism.
X to form an oxocarbenium intermediate B. Further, the nucleo- Conclusion
philic attack of the acceptor to the oxocarbenium ion B would In conclusion, we have disclosed an efficient and general
produce the desired glycoside 5. Higher α-selectivity may be at- protocol for the glycosylation of trichloroacetimidate glycosyl
tributed to the anomeric effect.
donors using the concept of cooperativity between an electron-
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1H NMR studies divulge that a 1,2-adduct formation between
the electron-deficient pyridinium salt and the glycosyl acceptor
plays a crucial role for the activation of the trichloroacetimidate
donors. The presence of thiourea derivatives further enhances
the reaction rate and selectivity due to its dual hydrogen bond-
ing ability. The reaction proceeds smoothly at room tempera-
ture with good to excellent yields and α-selectivity and is
applicable to a wide range of glycosyl donors as well as accep-
tors. The advantage of this methodology lies in the usage of an
environmentally benign catalyst, mild reaction conditions and
the regioselective formation of O-glycosides.
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This work was supported by SERB New Delhi – SB/FT/CS-
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thanks IIT Patna for providing a research fellowship. The author
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