Full text of DOI:10.1055/s-0036-1591525

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
letter
A
en
G. Sun et al.
Letter
Synlett
Substoichiometric FeCl3 Activation of Propargyl Glycosides for the
Synthesis of Disaccharides and Glycoconjugates
R2
R1
Guosheng Suna
Yue Wua
Anqi Liua
Saifeng Qiua
Wan Zhanga
Zhongfu Wangb
R2
R1
R3
O
OH
R3
BnO
O
R4O
O
FeCl3 (20–30 mol%)
CH3CN, 60 °C
BnO
BnO
R4O
+
O
BnO
O
R4O
R4O
O
R4O
OMe
R1 = CH2OBn, R2 = H, R3 = OBn
or
R
1 = CH2OBn, R2 = OBn, R3 = H
or
1 = H, R2 = H, R3 = OBn
R4 = Bz or Bn
R4O
OMe
15 examples
66–91% yields
Jianbo Zhang*a
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3-
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X
R
a School of Chemistry and Molecular Engineering,
East China Normal University, Shanghai, 200241,
P. R. of China
Jbzhang@chem.ecnu.edu.cn
b School of Life Sciences, Northwest University, Xi’an,
710069, P. R. of China
Received: 05.10.2017
Accepted after revision: 03.12.2017
Published online: 03.01.2018
sides, which have the advantages of simple structure, easy
preparation and storage.18 However, most of the methods
for the activation of propargyl glycosides were limited to
the use of toxic or noble catalysts such as Hg(OTf)2, AuCl3,
AuBr3, or AuCl3 with AgSbF6, which makes them difficult to
be widely used in the synthesis of glycosides.19–27 Beyond
that, some reactions also depended on the use of 4 Å molec-
ular sieves. Sureshkumar et al. reported the AuBr3 promot-
ed glycosylation reaction between propargyl 1,2-orthoe-
sters and various aglycones in the presence of 4 Å powdered
molecular sieves and delivered the product in satisfactory
yield.21,28 Therefore, a new and green catalytic system to ac-
tivate propargyl glycosides is still needed. Compared with
above catalysts, iron is inexpensive and harmless. Ferric
salts have been widely used for a variety of organic reac-
tions because of their superior and unique catalytic proper-
ties, and iron(III) chloride was found to be an effective Lew-
is acid for activating alkynes under extremely mild
conditions.29–39 Very recently, we reported the iron-cata-
lyzed synthesis of α-2,6-dideoxy-O-glycosides.40,41 In this
article, we report that FeCl3 as an efficient and affordable
catalyst effectively activates propargyl glycosides to lead to
disaccharides and glycoconjugates under mild conditions.
To test the feasibility of our idea, examination and opti-
mization of the reaction parameter were explored using
propargyl 2,3,4,6-tetra-O-benzyl-α-D-glucopyranoside (1a)
as the glycosyl donor and methyl 2,3,4-tri-O-benzoyl-α-D-
glucopyranoside (2a) as the acceptor by varying different
common ferric salts and conditions (Scheme 1, Table 1).
Initially, we examined the glycosylation in the presence
of 0.3 equivalents of FeCl3 in dry acetonitrile as solvent
which led to only 30% conversion after 15 h (Table 1, entry
1). We next checked the reactivity of FeCl3·6H2O under the
same set of reaction conditions, but disappointingly its re-
activity was no better than that with FeCl3 (Table 1, entry
DOI: 10.1055/s-0036-1591525; Art ID: st-2017-w0736-l
Abstract Glycosides as glycosyl donors using FeCl3 have been de-
scribed. Under optimal reaction conditions, three kinds of propargyl
glycosides were found to react with steroids and sugar-derived glycosyl
acceptors to afford the corresponding disaccharides and glycoconju-
gates in good to excellent yields (66–91%). Meanwhile, the method can
also realize one-pot synthesis of disaccharides, making it an effective,
affordable, and green glycosylation procedure.
Key words propargyl glycosides, FeCl3, disaccharides, glycoconju-
gates, glycosylation
Oligosaccharides and glycoconjugates (glycolipids and
glycoproteins) widely exist in a large number of biomole-
cules.1–3 Many biological studies of these compounds at the
molecular level have shed light on the biological signifi-
cance in anticancer activity and molecular recognition for
transmission of biological information.4–7 However, it is dif-
ficult to obtain them from natural materials because of low
concentrations and microheterogeneous forms.8,9 Synthesis
of such molecules is an important area of modern research
to procure them in pure forms and in good quantities.10
Disaccharides are the simplest oligosaccharides and the
synthesis of them from monosaccharides is the basis for the
synthesis of complex oligosaccharides. From the point of
chemical synthesis, one of the most useful procedures to
achieve disaccharides is glycosylation reaction. The glycosyl
donor generates an intermediate oxocarbenium ion using a
suitable catalyst to further react with an acceptor to form a
glycosidic linkage.11–13 In the past decade, chemists have
developed a variety of glycosyl donors and much interest
has been devoted to the investigation of glycosylation pro-
tocols based on activation of the C–C triple bonds.14–17 One
of the most important used sets of them is propargyl glyco-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
G. Sun et al.
Letter
Synlett
2). Further, iron(II) salts like FeCl2 and Fe2(SO4)3 were found
to be unreactive under the same reaction conditions (Table
1, entries 3 and 4). Based on these results, we decided to
optimize other reaction conditions. Encouragingly, with the
increase of the amount of the glycosyl donor 1a, reactions
afforded the product in moderate to good yields in the pres-
ence of 0.3 equivalents of FeCl3 in acetonitrile (Table 1,
entries 5–8). The stoichiometric amount of catalyst was
also studied, and we found that 0.3 equivalents were suffi-
cient to promote the reaction (Table 1, entries 7, 9, and 10).
It was found that the reaction can also be carried out in 1,2-
dichloroethane and moderate yields of disaccharide 3a
were obtained (Table 1, entry 11). Dichloromethane and
tetrahydrofuran, which are also widely used in glycosyla-
tion, were not suitable in this case (Table 1, entries 12 and
13). Further optimization revealed that the reaction did not
OBn
OBn
BnO
BnO
O
O
BnO
BnO
BnO
BnO
O
O
1a
1b
OAc
O
O
AcO
AcO
BnO
BnO
BnO
O
AcO
1d
OH
O
1c
OH
O
O
BzO
BzO
BnO
BnO
BzO
BnO
2b
OMe
OMe
2a
OH
O
OBn
HO
BzO
O
O
O
BnO
BnO
BzO
BzO
O
O
O
OBn
BnO
OMe
O
O
BnO
2c
2d
BnO
glycosyl donor 1a
O
catalyst
BnO
H
O
+
solvent
T/°C
H
H
OH
O
O
BzO
H
BzO
BzO
H
HO
OMe
HO
BzO
OMe
2e
2f
glycosyl acceptor 2a
disaccharide 3a
Figure 1 Glycosyl donors and acceptors
Scheme 1 Iron-catalyzed glycosylation of propargyl glycosides
Table 1 Optimization of the Glycosylation Reaction Conditions
Entry
Cat. (0.3 equiv)
1a/2a (equiv)
Solvent
Temp (° C)
Time (h)
Yield for 3a (%)a
1
2
FeCl3
1:1.2
1:1.2
1:1.2
1:1.2
1:2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DCE
60
60
60
60
60
60
60
60
60
60
60
40
60
40
80
15
15
12
15
15
15
15
10
10
10
15
12
12
12
10
30
FeCl3·6H2O
FeCl2
21
3
NRb
NR
74
4
Fe2(SO4)3
FeCl3
5
6
FeCl3
1.2:1
2:1
51
7
FeCl3
83
8
FeCl3
4:1
77
9
FeCl3(0.1 equiv)
FeCl3(0.5 equiv)
FeCl3
2:1
20
10
11
12
13
14
15
2:1
85
2:1
58
FeCl3
2:1
DCM
NR
trace
NR
76
FeCl3
2:1
THF
FeCl3
2:1
CH3CN
CH3CN
FeCl3
2:1
a Isolated yield.
bNR = no reaction.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

C
G. Sun et al.
Letter
Synlett
proceed at all below 40 °C and the byproducts production
increased slowly at 80 °C although the rate of reaction im-
proved considerably (Table 1, entries 14 and 15).
1d because of the disarmed effect which will decrease the
reactivity of glycosyl donors significantly. Glycosylation be-
tween these propargyl glycosides and sugars was per-
formed, respectively, under optimized conditions to obtain
the corresponding disaccharides in good to excellent yields.
The utility of propargyl glycosides was gauged in the per-
spective of glycoconjugate formation using aglycones com-
prising cholesterol (2e) and dehydroepiandrosterone (2f).
Glucosyl propargyl glycoside 1a behaved as glycosyl donor
in all the reactions giving the corresponding compounds
3j,k (Figure 2). We also extended the scope of this method
to galactosyl 1b and xylosyl 1c propargyl glycosides result-
ing in the formation of glycosides 3l,m and 3n,o in moder-
ate yields. The glycosylation reaction between glycosyl do-
nor and steroids resulted in lower yields presumably due to
the poor solubility of steroids in acetonitrile. The structures
We next examined the scope and generality of the pres-
ent reagent system with a wide range of glycosyl donors
1a–d and acceptors 2a–f (Figure 1). Initially, we explored
the utility of propargyl glycosides for disaccharide synthe-
sis. As shown in Figure 2, the FeCl3-promoted glycosylation
reaction between glucosyl donor 1a and various sugar-
based aglycones comprising benzoyl-protected sugars 2a,c
and benzyl-protected sugar 2b gave the respective disac-
charides 3a–c. Unfortunately, this method does not apply to
acid-sensitive acceptors such as 2d. It is pertinent to men-
tion that the current glycosylation strategy was extended to
galactosyl and xylosyl propargyl glycosides 1b,c to obtain
disaccharides 3d–f and 3g–i, respectively (Figure 2). How-
ever, this method does not apply to acyl-protected donor
1
of the products 3a–o were determined by H NMR and 13C
OBn
OBn
OBn
BnO
OBn
OBn
BnO
OBn
OBn
O
O
O
BnO
BnO
O
O
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BnO
BzO
O
O
O
O
O
O
O
O
BzO
BzO
BnO
BnO
O
BzO
BzO
O
BnO
BnO
BzO
BzO
BnO
OMe
BzO
OMe
OMe
BzO
BnO
OMe
OMe
3da,15 h, 84%, α:β = 1:1
3a,15 h, 83%b, α:β = 1:3c
3b,15 h, 72%, α:β = 1:1.5
3c,15 h, 85%, α:β = 1:2.2
3e,18 h, 66%, α:β = 1:1.8
OBn
OBn
BnO
O
O
BnO
BnO
O
O
BnO
BnO
BnO
BnO
BnO
O
BnO
BnO
BnO
BzO
BzO
O
O
O
O
O
BzO
BzO
O
BzO
BzO
O
BzO
BzO
BnO
BnO
BzO
OMe
OMe
BnO
OMe
OMe
3i,12 h, 85%, α:β = 1.4:1
3f,12 h, 90%, α:β = 1.5:1
3g,15 h, 91%, α:β = 1.5:1
3h, 13 h, 73%, α:β = 1:1.8
O
H
H
OBn
H
H
OBn
OBn
H
OBn
O
O
BnO
BnO
O
BnO
BnO
H
H
BnO
H
O
O
OBn
OBn
O
OBn
3j, 24 h, 77%, α:β = 1:2.3
3k, 24 h, 70%, α:β = 1:2.6
3l, 36 h, 72%, α:β = 1:1.5
H
O
O
H
H
OBn
H
OBn
O
O
BnO
BnO
H
H
H
O
BnO
OBn
HO
O
OBn
3m, 36 h, 75%, α:β = 1:2.4
3n, 24 h, 80%, α:β = 1:1
3o, 24 h, 77%, α:β = 1:1.4
Figure 2 Glycosylation results of propargyl glycosides 1a–c with acceptors 2a–f. aThe amount of FeCl3 for the donors of 1b and 1c was 20 mol%.
b Isolated yields. c α/β Ratio was determined by 1H NMR analysis or mass ratio.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

D
G. Sun et al.
Letter
Synlett
NMR spectroscopy and mass spectrometry, also by compar-
In summary, we have described for the first time that
FeCl3 is an effective, green, and inexpensive promoter cata-
lyst for the activation of propargyl glycosides.55,56 The reac-
tions proceed cleanly to obtain disaccharides and glycocon-
jugates in moderate to excellent yields at mild conditions
without the need for molecular sieves. Meanwhile, the re-
action system can also realize one-pot synthesis of disac-
charides, indicating it as an economic and useful glycosyla-
tion procedure. Further studies toward this direction are
under way in our laboratory and will be reported in due
course.
ison with the reported data.42–44 The ratio between α-iso-
1
mer and β-isomer was determined by H NMR analysis or
their isolated yields.
One-pot reaction is economical and environmentally
friendly and becoming a promising method in organic syn-
thesis.45–53 It is interesting that we can also realize the one-
pot synthesis of disaccharides when propargyl glycosides
reacted with glycosyl intermediates which contain a pro-
tecting group (triphenylmethyl, Tr) in the sugars using the
optimized reaction conditions. 2,3,4,6- tetra-O-benzyl-α-D-
galactopyranoside 1b reacted with methyl 2,3,4-tri-O-ben-
zoyl-6-triphenylmethyl-α-D-glucopyranoside (2g) in the
presence of FeCl3 to get the disaccharide 3d in 81% yield af-
ter 15 h (Scheme 2).
Funding Information
This project was financially supported by Natural Science Foundation
of Shanghai (11ZR1410400), Shanghai college student innovative
training program (201510269085) and large instruments Open Foun-
BnO
BnO
OBn
O
dation of East China Normal University (20151043 & 20162015).
N
atuarlSecince
F
o
u
n
d
oaitnofS
h
a
n
g
h
a
1i(
1
Z
R
1
4
1
0
4
0
0
S)
h
a
n
g
h
a
iecolglestu
d
e
n
nti
n
o
vaitertnian
i
g
progra
m
2(
0
1
5
1
0
2
6
9
0
8
5
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C
h
n
i
a
N
orm
a
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y
2(
0
1
5
1
0
4
3
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n
i
a
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orm
a
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0
1
6
2
0
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5)
OBn
OBn
BnO
BnO
O
O
Acknowledgment
BnO
1b (2.0 equiv)
FeCl3 (20 mol%)
O
+
O
We thank the analytic center of East China Normal University for data
measurement.
BzO
BzO
CH3CN, 60 °C,15 h
81%
OTr
BzO
O
OMe
3d
BzO
BzO
BzO
OMe
Supporting Information
2g (1.0 equiv)
Supporting information for this article is available online at
Scheme 2 One-pot synthesis of disaccharides
https://doi.org/10.1055/s-0036-1591525.
S
u
p
p
o
nrtogI
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
The proposed reaction mechanism may be explained as
follows (Scheme 3).25,54 A π complexation of FeCl3 with
alkyne (compound 2) should generate oxonium cation 5.
This intermediate is simultaneously trapped with aglycone
to furnish the O-glycoside 6.
References and Notes
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O
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O
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O
1
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5
Scheme 3 The possible mechanism for the glycosylation
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

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Letter
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(55) Typical Experimental Procedure
Typically, to a mixture of 2,3,4,6-tetra-O-benzyl-α-D-glucopyra-
noside (1a, 0.1 mmol, 58 mg) and methyl 2,3,4-tri-O-benzoyl-α-
D-glucopyranoside (2a, 0.05 mmol, 25 mg) in a round-bottom
flask (5 mL) under nitrogen atmosphere. The FeCl3 (10 mg, 0.06
mmol) catalyst and anhydrous CH3CN solvent (6 mL) was added
to another round-bottom flask. Take the solution of FeCl3(1.5
mL) to the former round-bottom flask, and the reaction was
stirred at 60 °C for 15 h. After completion of the reaction (moni-
tored by TLC), the organic phase was condensed under vacuum
to get crude product, which was purified by silica gel column
chromatography (PE/EtOAc = 6:1) to get 3a in 83% yield. All new
compounds were characterized by 1H NMR,13C NMR, and MS.
Spectral and analytical data were in good agreement with the
desired structures.
(56) Selected Spectral Data – Compound 3a
Colorless oil, α-anomer:1H NMR (500 MHz, CDCl3): δ = 8.00 (dd,
J = 8.3, 1.2 Hz, 2 H), 7.97 (dd, J = 8.3, 1.2 Hz, 2 H), 7.91–7.85 (m, 2
H), 7.55–7.49 (m, 2 H), 7.46–7.28 (m, 24 H), 7.22 (m, 1 H), 7.15
(dd, J = 7.6, 1.7 Hz, 2 H), 6.16 (t, J = 9.4 Hz, 1 H), 5.55 (t, J = 9.9 Hz,
1 H), 5.24 (q, J = 3.5 Hz, 2 H), 4.93 (d, J = 11.0 Hz, 1 H), 4.84 (d, J =
11.0 Hz, 1 H), 4.80 (d, J = 11.0 Hz, 1 H), 4.78 (d, J = 12.5 Hz, 1 H),
4.76 (d, J = 3.5 Hz, 1 H), 4.65 (d, J = 12.2 Hz, 1 H), 4.56 (d, J = 12.1
Hz, 1 H), 4.47 (d, J = 11.0 Hz, 1 H), 4.40 (d, J = 12.1 Hz, 1 H), 4.36–
4.31 (m, 1 H), 3.98 (t, J = 9.3 Hz, 1 H), 3.90–3.84 (m, 2 H), 3.67–
3.62 (m, 2 H), 3.60 (dd, J = 11.0, 2.1 Hz, 1 H), 3.56 (dd, J = 9.7, 3.5
Hz, 1 H), 3.52 (dd, J = 10.7, 1.9 Hz, 1 H), 3.46 (s, 3 H).
β-Anomer: 1H NMR (500 MHz, CDCl3): δ = 8.00–7.85 (m, 6 H),
7.52–7.12 (m, 29 H), 6.17 (t, J =9.8 Hz, 1 H), 5.47 (t, J =9.9 Hz, 1
H), 5.25 (dd, J =10.2, 3.6 Hz, 1 H), 5.20 (d, J =3.6 Hz, 1 H), 5.05 (d,
J =10.8 Hz, 1 H), 4.91 (d, J =10.9 Hz, 1 H), 4.80 (d, J =10.8 Hz, 1 H),
4.76 (d, J = 10.9 Hz, 1 H), 4.68 (d, J =10.9 Hz, 1 H), 4.53 (d, J =11.5
Hz, 1 H), 4.50 (d, J =11.6 Hz, 1 H), 4.47 (d, J =7.8 Hz, 1 H), 4.43 (d,
J =12.2 Hz, 1 H), 4.41–4.34 (m, 1 H), 4.12 (dd, J =10.8, 2.0 Hz, 1
H), 3.81 (dd, J = 10.9, 7.6 Hz, 1 H), 3.66–3.63 (m, 2 H), 3.61 (d, J =
6.0 Hz, 1 H), 3.58 (d, J = 9.1 Hz, 1 H), 3.46–3.43 (m, 2 H), 3.37 (s,
3 H). ESI-MS: m/z calcd for C62H60O14 Na [M + Na+]: 1051.39;
found: 1051.25.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E