N-benzoylglycine/thiourea cooperative catalyzed stereoselective O-glycosidation: Activation of O-glycosyl trichloroacetimidate donors

Article abstract of DOI:10.1016/j.catcom.2019.04.006

A new practical utility for β-stereoselective glycosylation via activation of O-glycosyl trichloroacetimidate donors using N-benzoylglycine/thiourea cooperative catalysis has been demonstrated. This method represents the first instance where amino acid derived N-benzoylglycine is used as a catalyst for O–glycosylation under mild reaction conditions at ambient temperature. NMR spectroscopy studies suggest that thiourea cocatalyst exhibit a cooperative behaviour that has a strong effect on the reaction rate, yield, and the β-selectivity.

Full text of DOI:10.1016/j.catcom.2019.04.006

Accepted Manuscript
N-benzoylglycine/thiourea cooperative catalyzed stereoselective
O-glycosidation: Activation of O-glycosyl trichloroacetimidate
donors
Atul Dubey, Rekha Sangwan, Pintu Kumar Mandal
PII:
S1566-7367(19)30114-1
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.04.006
CATCOM 5674
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 January 2019
3 April 2019
5 April 2019
Please cite this article as: A. Dubey, R. Sangwan and P.K. Mandal, N-benzoylglycine/
thiourea cooperative catalyzed stereoselective O-glycosidation: Activation of O-glycosyl
trichloroacetimidate donors, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.04.006
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ACCEPTED MANUSCRIPT
N-Benzoylglycine/thiourea cooperative catalyzed stereoselective O-
Glycosidation: Activation of O-glycosyl trichloroacetimidate donors
Atul Dubey,a Rekha Sangwana b and Pintu Kumar Mandala b*
a Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, BS-10/1,
Sector 10, Jankipuram Extension, Sitapur Road, P.O. Box 173, Lucknow, 226 031, India.
bAcademy of Scientific and Innovative Research, New Delhi 110001, India
* Corresponding author; Email: pintuchem06@gmail.com
Abstract
A new practical utility for -stereoselective glycosylation via activation of O-glycosyl
trichloroacetimidate donors using N-benzoylglycine/thiourea cooperative catalysis has been
demonstrated. This method represents the first instance where amino acid derived N-
benzoylglycine is used as a catalyst for O–glycosylation under mild reaction conditions at
ambient temperature. NMR spectroscopy studies suggest that thiourea cocatalyst exhibit a
cooperative behaviour that has a strong effect on the reaction rate, yield, and the -selectivity.
Keywords: Organocatalyst, stereoselective, glycosylation, trichloroacetimidate, Schreiner’s
thiourea
1. Introduction
Glycosylation to construct most essential glycosidic bond is present in a crucial class
of biomolecules such as oligosaccharides, glycoconjugates, glycoproteins, glycopeptides,
peptidoglycans, glycolipids, and lipopolysaccharides, which play important roles in numerous
biological processes [1-2]. Owing to their high importance for the synthesis of fundamental
glycosidic bonds with high efficiency and selectivity, chemists have studied glycosylation
chemistry for over a century [1-2]. However, it is still not an easy task for making

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stereoselective glycosidic linkages in several class of biomolecule. Considering their
importance, several efficient protocols have been established for the stereoselective
glycosylation [3-6].
Despite such a great success for the construction of stereoselective traditional glycosylations,
the development of reagent/catalyst-controlled glycosylations [7] offers many advantages
over traditional methods, as these strategies have the potential to allow the high efficiency
and selectivity. Despite the broad application of organocatalysis within asymmetric synthesis
[8], it is still uncommonly employed in the area of carbohydrate chemistry, especially for
glycosylation reactions.
Moreover, the use of cocatalysts with organocatalysts known as ‘cooperative catalysis’ [9] is
an exciting field whereby enhance the catalytic activity and selectivity. In particular,
cooperativity between Brønsted acids and hydrogen-bonding cocatalysts such as thiourea
derivatives has broad area of application [10]. In literature, the Schreiner group [11], the
Jacobsen group [12], and others [13] have been used the cooperativity between Brønsted acids
and hydrogen-bonding cocatalysts, as for instance thiourea derivatives for successfully
applied to asymmetric catalysis. In the hope of extending the application of organocatalysts, a
small molecule metal-free hydrogen-bond catalyst Schreiner’s thiourea [14] have been
recently employed for glycosylation [5]. Literature survey revealed only very limited
examples that evolved recently make use of cooperative catalysis for stereoselective
glycosylation. The work done by Schmidt group has successfully applied the synergistic
catalysts thiourea derivatives with phosphorus acids for stereoselective O-glycosylation
(Figure 1) [5]. Similarly, Galan et al. established a method for the preparation of 2-
deoxyglycosides from glycals under the influence of cooperative catalysis (chiral phosphoric
acids/thiourea derivatives) (Figure 1) [15]. More recently, Kumar’s group also demonstrated

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glycosylation using a synergistic catalytic system of electron-deficient pyridinium salts/aryl
thiourea (Figure 1) [16].
Figure 1: Representative precedents in thiourea catalyzed glycosylations.
Encouraged by reported literature, we decided to focus our attention on the synthesis of
stereoselective glycosides via cooperative catalysis approach. Besides the advantages of these
methods, drawbacks related to chiral cocatalyst accessibility and stereoselectivity of the
glycoside formation still remain challenging. Hence we postulated that synergistic
Schreiner’s thiourea/N-benzoylglycine activation could provide a more efficient and practical
glycosylation strategy for the preparation of O-glycoside with excellent yields and -
selectivity from highly reactive O-glycosyl trichloroacetimidate donor (Figure 1).

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This method represents the first instance where environmentally benign amino acid
derivative, such as N-benzoylglycine [17] used as a catalyst would interact with the soft
Lewis basic moiety of thiourea [18] to increase the HB-donating ability of thiourea [19], so
that the LG could be activated [20] with wide functional group tolerance.
A highly reactive glycosyl donor for instance, O-glycosyl trichloroacetimidates as efficient
glycosyl donors [21], generally requires a pKa value less than 5 for activation at room
temperature [22].
From the previous thiourea-based study we have taken very successful achiral Schreiner’s
thiourea compound 4 for our work [23-24]. It is known from the literature that N-
benzoylglycine exhibit pKa values of about 3.6 [25]. Based on this fact, we anticipated that
N-benzoylglycine the pKa value would be further diminished in the presence of hydrogen-
bonding cocatalysts such as thiourea derivatives acts as an acid amplifier [26]. N-
Benzoylglycine in the presence of Schreiner′s thiourea as cocatalyst exhibit cooperative
behavior which strongly effects the reaction rate and stereoselectivity. We report that
synergistic Schreiner’s thiourea/N-benzoylglycine activation provide a more facile, efficient
and practical glycosylation strategy for significant -stereoselectivity at room temperature but
in the presence of TMSOTf  selectivity depends on the reaction temperature [27].
However there is no report on catalyst activity of amino acid derivative N-benzoylglycine for
O- glycosylation. It is a first report where application of a natural amino acid derivative, N-
benzoylglycine with Schreiner’s thiourea for significant -stereoselectivity glycosidic bond
formation at room temperature. Their reactivities and synthetic diversities provide new tool
for the development of highly stereoselective glycosides. Hence, the application of the
synergistic catalyst system consisting of N-benzoylglycine/thiourea derivatives for glycosidic
bond formation will be an exciting addition to the literature (Figure 1).

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2. Results and discussion
2.1 Catalyst screen in the glycosylation reaction
The basic attempts start with the screening of ability of different commercially available
substituted glycine derivatives as a catalyst for improving the stereoselective glycosylation.
We first screened glycosyl reactions employing 5 mol % of N-acetylglycine (3a, pKa = 3.64)
[25] as catalysts with suitable donor -D-galactopyranosyl trichloroacetimidate (1) [28] and
1-pentanol as acceptor 2a in the absence and presence of Schreiner’s thiourea (4, 5 mol %) in
dry CH2Cl2 at room temperature. It was observed that formation of -mixtures of 5a takes
place with slow reaction rate after 48 hours (Table 1, entry 1).
However, when catalyst 3a with 4 was added (entry 2), the result was totally different, in
much shorter reaction time, the exclusive formation of glycoside 5a in 7:1 ratio was
observed; thus, result demonstrate the strong influence of thiourea 4 on reaction rate, yield,
and stereoselectivity. Then we only use Schreiner’s thiourea (4) on its own without any
catalyst which did not promote the glycosylation reaction at all (entry 3). To attain faster
reaction rates, glycosylation reaction was done with 10 mol % of N-acetylglycine 3a and 10
mol % of thiourea 4, the reaction rate increases without losing the high -selectivity (entry 4).
Even in the same condition for longer time yield of the desired product had no effect (Table
1, entry 5). To achieve higher yield and faster reaction rates, the acidic behaviour of N-
benzoylglycine (3b, pKa = 3.64) [25] was investigated (entries 6, 7). In the presence of 4 (10
mol %) glycosylation reaction proceeded cleanly to give product 5a in good yields with
excellent -selectivity (β:α >30:1) after 2 h. (Table 1, entry 6) but no improvement of yield
and selectivity was observed even after longer time (entry 7). On the other hand when
electron donating group (4-methylbenzoyl)glycine catalyst (3c, pka = 3.74) [25] in
combination with 4 proved to be detrimental to reaction rate, yield, and stereocontrol, with

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reactions needing 8 h for completion, practically exclusive formation of desired glycoside 5a
was observed with lesser yield  10:1, Table 1, entry 8). Next, we decided to explore the
reaction conditions using 3b via increasing the acid loading to 20 mol % of 4 had no effect on
the outcome of the model reaction (entry 9). After analyzing the effect of acidic profile of
glycine derivatives, we decided to explore the reaction conditions by changing the reaction
solvent system to ether, acetonitrile, tetrahydrofuran, toluene or dichloroethane (Table 1,
entries 10, 11, 12 and 13) which had an adverse effect on the reaction rate, yield, and
selectivity. Performing the reaction under inverse addition conditions had no impact on the
selectivity of glycoside formation (Table 1, entry 14) [29].
Table 1. Initial catalyst screen in the glycosylation of donor 1 and acceptor 2a.a
Entry 1a Catalyst (3a-c) Cocatalyst 4 Solvent Time (h)
Yieldb (ratio)c
1
2
3a
3a
-
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
48
10
48
8
5% (1:1)
56% (7:1)









3
n.r., n.d.d
4e
5e
6
3a
3a
3b
3b
3c
3b
67% (7:1)
24
2
68% (7:1)
88% >(30:1)
87% >(30:1)f
69% >(10:1)
88% >(30:1)f
7
8
8
8
9g
2

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10
11
12
13
14h
3b
3b
3b
3b
3b
CH3CN
THF
10
9
43% >(20:1))
23% >(15:1)





Toluene
DCE
24
6
Trace
82% >(30:1)f
CH2Cl2
3
86% >(30:1)f
aReaction conditions: 1 (1.0 equiv), 2a (1.2 equiv), 3a–c (5 mol %), 4 (5 mol %), solvent (5 mL), at room
b
temperature under nitrogen atmosphere. Yield determined after purification by flash silica gel chromatography.
d
e
cAnomeric ratios were determined by 1H NMR spectroscopy. n.r.–no reaction, n.d.–no decomposition. 10
f
mol% of catalyst 3a and 4 were employed to increase the reaction rate. Detection limit of the minor isomer. g20
h
mol% of catalyst 3band 4 were employed. Inverse addition.
2.2 Scope of substrates
Having the optimum reaction conditions in hand results with 3b as catalyst and 4 as
cocatalyst, our attention was then turned to exploring the scope of the cooperative catalytic
system on coupling reactions 1 [28] with a range of acceptors nucleophiles 2b-l (Table 2).
The efficacy of this reaction was examined with aliphatic, aromatic and sugar alcohols. In all
cases, reactions proceeded smoothly within 2-4 h and in excellent yields with -selectivity, as
1
determined by the characteristic anomeric signals in the H- and 13C NMR spectra.
Glycosylations with less sterically hindered primary alcohols (2b-f) afforded their
corresponding glycoside products with high  selectivity in 78% to 88% yield within 2 h
(Scheme 1, entries 1-5). It is important to note that, halogenated primary alcohol such as 3-
chloropropanol (2d), 3-bromopropanol (2e) gave exclusively β-glycoside 5d and 5e in 85%
and 88 % yield respectively (Scheme 1, entries 3, 4). Furthermore, with decreasing
nucleophilicity of the acceptor hydroxy group (through steric), as in aromatic acceptors 2g–h
produced their corresponding glucosides 5g–h in 76–79% yields with moderate to good -
selectivity.

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Then, moving from primary to secondary alcohols the reaction rate and the high preference
for -product formation decreased and the -anomer could also be detected (entries 8 and 9).
Acceptor 2i and 2j under the optimized reaction conditions gave their corresponding
glycosides 5i and 5j with moderate selectivity (Table 2, entries 8, 9).
Interestingly, the reaction with the bulky primary alcohol, 3,5-dimetyladamant-1-yl-methanol
(2k) produced preferentially or exclusively -glycoside 5k in 76% yield (Scheme 1, entry
10). As expected, studies with additional carbohydrate acceptor such as 1,2:3,4-di-O-
isopropylidene-D-galactose (2l), the corresponding -selective disaccharide glycoside 5l can
be successfully obtained as major products with this method (Table 2, entry 11) and the acid
sensitive group survived well [30].
Table 2. Acceptor (2a-l) scope in glycosylation reaction with donor 1 in the presence of 3b
as catalyst and 4 as cocatalyst.

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To further demonstrate the scope of this method, we next investigated the stereoselectivity for
different glycosyl donors [31] and acceptors as tabulated in scheme 2. First glycosylation was
studied with -D-glucopyranosyl trichloroacetimidate (6) with a variety of glycosyl
acceptors, e.g., 2b and 2g under the optimized reaction condition (10 mol% of catalysts 3b
and 4) gave their corresponding glucosides 11 and 12 respectively, with preferentially or
exclusively -selectivity, having good yield (Scheme 1).

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Scheme 1. Glycosylation of donors 6–10 with variety of acceptors in the presence of 3b as
catalyst and 4 as cocatalyst.a
Under the optimized reaction conditions, 4,6-O-benzylidine-2,3- di-O-benzyl-α-D-
galactopyranosyl trichloroacetimidate (7) was treated with different acceptors such as 2b and
2e, gave their corresponding glycoside 13 and 14 respectively, with -selectivity in good
yields. But, the moderate stereoselective outcome was noticed in the case of glycosylation of
4,6-O-benzylidine-2,3- di-O-benzyl-α-D-glucopyranosyl trichloroacetimidate (8) glycosyl

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donor with aliphatic primary hydroxyl group 2b (scheme 1). A particularly interesting case is
disaccharide trichloroacetimidate 9 with a variety of glycosyl acceptors, e.g., 2a, 2b, 2c, and
2j under the optimized reaction condition gave their corresponding glycosides 16–19,
respectively in good yields with exclusively -glycoside. In case of 2,3,4,6-tetra-O-acetyl--
D-glucopyranosyl trichloroacetimidate (10) [32] was treated with 2e under the optimized
conditions, exclusively -glycoside glycosides 20 was obtained (Scheme 1).
Scheme 2. Schreiner’s thiourea/N-benzoylglycine -catalyzed O-glycosylation.

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Furthermore, we explore the utility of thiourea catalyzed glycosylation by using different
glycosyl donor 6-10 & 21 with a series of differentially protected glycosyl acceptor like
primary and secondary (2l, 2m [33], 2n, & 2o [34] bearing benzylidene and isopropylidene
protecting groups. For example, when acceptor 2n was treated with donors 6 under the
optimized conditions, glycoside 28 was procured in good yields with excellent -selectivity
(Scheme 2). Our attention then turned to explor the scope of regioselectivity to note that
glycosylation with protected acceptor 2o with glycosyl donors 10 under the optimized
condition lead to highly regioselective glycoside products 23 in good yields with good
selectivity (Scheme 2).
According to the literature, the TMSOTf catalyzed activation of O-glycosyl
trichloroacetimidates donor leading to temperature-dependent -selectivity which has ion-
pair formation, thus favoring an SN2-type reaction at low temperature and SN1-type at high
temperature [27].
To study the reaction pathway, some additional NMR studies were performed involving the
shift of the -NH and –OH signals and shifts of proton signals of bis(trifluoromethyl)phenyl
1
thiourea residues. In H NMR spectroscopy, the mixtures of the cocatalyst 4 with donor 1,
cocatalyst 4 with acceptor 2a and mixture of cocatalyst 4, donor 1, acceptor 2a was studied
(Supporting information Figure S1, S2, S3, S4).
When the NMR studies were performed with mixtures of 1+2a+4, peaks were also shifted but
no effect was observed for 1:1 mixture of 3b+4 and also for 3b+2a due to the low solubility
of catalyst 3b. However, acceptor 2a was added to the reaction mixture of 3b+4 to increased
shifts of the aryl protons of 4 due to hydrogen-bond-mediated interaction between the catalyst
3b and cocatalyst 4 [5].

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From NMR study [5] and a literature survey, the following aspects can be made:
- The Schreiner’s thiourea is the key for a reaction rate increase and the -selectivity.
- The acceptor 2a, catalyst 3b, and cocatalyst 4 form hydrogen-bond-mediated complex [35]
which seems to anticipate the interaction with donor 1 through intramolecular reaction lead to
product formation.
- SN2-type glycoside bond formation which is even facilitated at room temperature and in the
absence of anchimeric assistance.
3. Conclusions
In summary, overall the result contained herein demonstrates
a
practical, highly
stereoselective, and efficient glycosylation method for the preparation of -glycosides using a
cooperative N-benzoylglycine/thiourea promoter system. The reaction is widely applicable to
a range of glycosyl donors, nucleophile acceptors proceeds with excellent selectivity as well
as high yield, and tolerant of most common protecting groups. The reaction condition can be
considered as an attractive alternative to the existing procedures. Thiourea as cocatalyst
exhibits a cooperative behavior that has a strong effect on the reaction rate, yield, and the
selectivity of glycosidations. The described reaction of stereoselective O-glycosylation of
trichloroacetimidate donor using a cooperative N-benzoylglycine/thiourea promoter system
should be efficient and indicate substantial improvement to the known synthesis methods.
4. Experimental section
Trichloroacetimidate donor 1, 6, 7, 8, 9, 10 and 21 (1.0 equiv), acceptor (2a-p) (1.2 equiv)
and cocatalyst aryl thiourea 4 (10 mol%) were dissolved in dry DCM (5 ml) and stirred at
room temperature under a nitrogen atmosphere for 10 min. Catalyst 3 (10 mol %) was added
and the resulting reaction mixture was stirred at room temperature until total consumption of
glycosyl donor, then quenched with aqueous NaHCO3 and extracted with CH2Cl2. The

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organic layer was washed with water, dried over Na2SO4, and organic layer concentrated in
vacuo. The crude product was purified by flash column chromatography to afford the desired
glycosides 5, 12-20, and 22-28. The ratios of the newly formed glycosidic bonds were
1
determined by the H NMR integration.
Acknowledgments
A. D. & R. S. thanks CSIR, New Delhi for providing Research Fellowship. The author
gratefully acknowledges financial support by DST-SERB (Scheme No.EMR/2017/001791),
New Delhi and SAIF Division of CSIR-CDRI for providing the spectroscopic and analytical
data. CDRI communication no. 110/2019/PKM.
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 -stereoselective glycosylation via activation of O-glycosyl trichloroacetimidate
donors
 N-Benzoylglycine in the presence of Schreiner′s thiourea as cocatalyst exhibit
cooperative behavior
 SN2-type glycoside bond formation at room temperature in the absence of anchimeric
assistance
 Widely applicable to a range of glycosyl donors, nucleophile acceptors, proceeds with
excellent selectivity