Synthesis of an antimetastatic tetrasaccharide β-d-Gal-(1 → 4)-β-d-GlcpNAc-(1 → 6)-α-d-Manp-(1 → 6)-β-d-Manp-OMe

Article abstract of DOI:10.1016/j.cclet.2012.10.022

An antimetastatic tetrasaccharide T₁, β-d-Gal-(1 → 4)-β-d-GlcpNAc-(1 → 6)-α-d-Manp-(1 → 6)-β-d-Manp-OMe, was synthesized with two approaches. The first approach was a conventional method employing thioglycoside and Koenigs-Knorr glycosylation r

Full text of DOI:10.1016/j.cclet.2012.10.022

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1371–1374
www.elsevier.com/locate/cclet
Synthesis of an antimetastatic tetrasaccharide b-D-Gal-(1 ! 4)-b-D-
GlcpNAc-(1 ! 6)-a-D-Manp-(1 ! 6)-b-D-Manp-OMe
*
Kai Jun Liao, Xiao Feng Jin, Xiang Bao Meng, Chen Li, Zhong Jun Li
State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Beijing 100083, China
Received 8 September 2012
Available online 30 November 2012
Abstract
An antimetastatic tetrasaccharide T1, b-D-Gal-(1 ! 4)-b-D-GlcpNAc-(1 ! 6)-a-D-Manp-(1 ! 6)-b-D-Manp-OMe, was syn-
thesized with two approaches. The first approach was a conventional method employing thioglycoside and Koenigs–Knorr
glycosylation reaction in 24% overall yield. The second one was a novel route through the azidoiodo-glycosylation strategy by using
2-iodo-2-deoxylactosyl azide as the donor in 36% overall yield.
# 2012 Zhong Jun Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Antimetastatic tetrasaccharide; Thioglycoside; Koenigs–Knorr method; Azidoiodo-glycosylation; 2-Iodo-2-deoxyl-actosyl azide; b-
Methyl mannoside
Laminin is the main non-collagenous glycoprotein found in the basement membrane. The interaction of cancer
cells with laminin was identified as a key event in tumor invasion and metastasis [1–5]. Studies suggest that laminin
carbohydrates may also participate in such cellular responses [6–8]. The carbohydrate residues of laminin must be
available to enable the cells to spread or to extend neurite [3]. It was previously shown that the tetrasaccharide
fragment, b-D-Gal-(1 ! 4)-b-D-GlcpNAc-(1 ! 6)-a-D-Manp-(1 ! 6)-b-D-Manp-OMe (T1, Fig. 1), inhibits the
attachment of tumor cells (S180) to laminin [9].
Several oligosaccharides containing N-acetyllactosamine fragment have been synthesized in our laboratory using
the conventional methods [10,11]. Due to the long steps and low yields, an efficient synthesis of T1 is still not
available. There is a need to develop a novel and efficient synthetic route to prepare a relatively large quantity of this
tetrasaccharide to study its biological activities.
Lafont et al. [12] reported a novel and convenient azidoiodo-glycosylation approach to form the 2-acetamido-
2-deoxylactoside in moderate yields employing 2-iodolactosyl azide as the donor. The same laboratory later
reported [13,14] that with this method, the construction of 1,2-trans-glycosidic linkage (b-configuration only) and
2-acetamido group was accomplished in one pot under mild conditions with satisfactory yield. We herein report
the synthesis of T1 using the thioglycoside and Koenigs–Knorr glycosylation method and the azidoiodo-
glycosylation method.
* Corresponding author.
E-mail address: zjli@bjmu.edu.cn (Z.J. Li).
1001-8417/$ – see front matter # 2012 Zhong Jun Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.10.022

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K.J. Liao et al. / Chinese Chemical Letters 23 (2012) 1371–1374
OH
OH
O
OH
O
HO
O
OH
O
O
HO
OH
HO
HO
AcHN
OH
O
O
HO
HO
OMe
T
1
Fig. 1. Structure of T1.
The tetrasaccharide fragment T1 was synthesized via a 2 + 2 route, and the retrosynthetic analysis is shown in
Scheme 1. In this model, the formation of 1,2-trans-b-glycosides of 2-acetamido-2-deoxylactosyl moiety (intermediate
5) and the construction of the b-mannosidic bond (intermediate 11) were recognized as the key and difficult steps.
The b-mannopyranosidic linkage is a common structure motif of glycoproteins. Many synthetic methods are
developed for the construction of this linkage with respective advantages and shortcomings [15–17]. Due to the high b/
a ratio and low reagent cost, the classic Koenigs–Knorr method employing the insoluble silver carbonate promoter is
commonly used to constructed the b-mannopyranosidic linkage (Scheme 2).
To prepare tetrasaccharide T1, the acetyl lactosamine 5 was first converted to bromide by treating with HBr–HOAc.
After coevaporation with toluene for several times, the bromide was then coupled with the acceptor 11 catalyzed by
AgOTf at 0 8C. The benzyl group of tetrasaccharide 12 was converted to acetyl group, followed by the conversion of
the trichloroethoxy carbonyl group to acetyl group with zinc dust and HOAc [15]. In the end, the acetyl group was
removed with Na, MeOH to afford the tetrasaccharide T1 (Scheme 3). Largely because of the low yield of the
Koenigs–Knorr reaction, the overall yield of this conventional method is low.
To improve the yield, we attempted to apply the azidoiodoglycosylation approach to the synthesis of
tetrasaccharide T1. The acceptor was the mannoside 11. The donor 3,6-di-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-b-D-
galactopyranosyl)-2-deoxy-2-iodo-a-D-mannopyranosyl azide 13 was easily prepared from hexa-O-acetyllactal by
iodoacetoxylation and subsequent substitution with azide [13].
Treatment of the glycosyl donor 2-iodoglycosyl azide 13 with triphenylphosphine (1.1 equiv.) and the acceptor 11
(1.5 equiv.) (Scheme 4) in 1,2-dichloroethane from 0 to 45 8C afforded the corresponding dimannoside 3,6-di-O-acet-
yl-4-O-(2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl)-2-aminophosphonium-2-deoxy-b-D-glucopyranoside iodide
[14]. This intermediate was then allowed to react with compound 11, followed by anion exchange with OHꢀ,
deacetylation with NaOCH3/CH3OH and reacetylation with Ac2O/pyridine, to afford compound 14 in an overall yield
of 80.9%. Following the debenzylation of 14 on Pd(OH)2 under 0.4 MPa H2 and per-O-acetylation, compound 15 was
obtained for the full structure determination. Finally, deacetylation of 15 with a catalytic amount of sodium methoxide
in methanol afforded the product T1 in 90% yield [20].
OBz
OBz
O
BzO
STol
OAc
OBz
OBz
OBz
O
4
OBn
O
OBn
O
BzO
O
OH
OH
O
AcO
OH
O
HO
AcO
TrocHN
OBz
TrocHN
OAc
HO
O
OH
O
O
HO
5
OH
HO
HO
AcHN
3
OH
O
O
OBn
O
T1
HO
BnO
OBn
O
AcO
HO
OMe
BnO
HO
BnO
BnO
OBn
O
O
OBn
O
AcO
BnO
BnO
STol
BnO
7
OCH3
11
BnO
OBn
O
HO
OAc
6
BnO
OCH3
BnO
9
Scheme 1. Retrosynthetic analysis for tetrasaccharide T1.

K.J. Liao et al. / Chinese Chemical Letters 23 (2012) 1371–1374
1373
OBn
AcO
O
a
BnO
BnO
OBn
O
OBn
O
AcO
BnO
BnO
AcO
BnO
BnO
STol
7
8
d
OBn
O
OBn
O
OBn
OAc
HO
BnO
BnO
O
AcO
6
c
O
BnO
BnO
BnO
BnO
b
OCH
OCH
3
OCH
3
3
10
9
OBn
O
HO
BnO
BnO
e
OBn
O
O
BnO
BnO
OCH
3
11
Scheme 2. Synthesis of the acceptor 11. Reagents and conditions [18,19]: (a) TolSH, Et2OꢁBF3, CH2Cl2, 0 8C, 71.1%; (b) i. HBr, AcOH, 10 h; ii.
˚
˚
CH3OH, Ag2CO3, CH2Cl2, 4 A MS, ꢀ30 8C, 6 h, 43.0% (over two steps); (c) Na, CH3OH, rt, 99%; (d) NIS, AgOTf, Et2O, 4 A MS, Ar, ꢀ78 8C, 18 h,
54.4%; (e) Na, CH3OH, rt, 99%.
OBz
OBz
OBn
O
OBz
O
OBz
O
BzO
O
OBn
O
BnO
O
AcO
OBz
a
O
TrocHN
BzO
O
BnO
AcO
OBn
O
OBz
BnO
TrocHN
OAc
O
BnO
BnO
OCH3
12
5
OH
OH
O
OH
O
HO
O
OH
O
b
O
HO
AcHN
HO
HO
HO
OH
O
O
HO
OCH3
T1
HO
Scheme 3. Synthesis of tetrasaccharide T1. Reagents and conditions: (a) i. HBr–HOAc; ii. 11, AgOTf, CH2Cl2, 0 8C, 77.8%; (b) i. H2, Pd-C; ii.
Ac2O, Py; iii. Zn, HOAc; iv. Na, MeOH, 77%.
AcO
AcO
OAc
O
OAc
O
OBn
O
HO
BnO
BnO
AcO
AcO
OAc
O
O
OAc
O
O
OR
O
AcO
AcO
I
O
OBn
O
AcHN
O
RO
a
+
c
RO
T1
OAc
BnO
AcO
OCH3
OR
O
O
BnO
N3
14 R= OBn
15 R= OAc
RO
OCH3
b
RO
13
11
˚
Scheme 4. Synthesis of the tetrasaccharide: (a) i. PPh3, ClCH2CH2Cl, 4 A MS, rt, 12 h; ii. anion resin, ethanol; iii. Na, MeOH; iv. Ac2O-pyridine
(1:2), 80% (over four steps, 36% of 11 retrieved); (b) i. Pd(OH)2/C, H2, MeOH; ii. Ac2O-pyridine (1:2), 81% (over two steps); (c) Na, MeOH, 90%.
In conclusion, the tetrasaccharide T1 was synthesized in two approaches. The yield of the conventional method with
thioglycoside and Koenigs–Knorr glycosylation is low. By using the azidoiodo-glycosylation strategy, the yield was
significantly increased (36%).
Acknowledgments
This work was supported by the National Basic Research Program of China (973 program, No. 2012CB822100)
and the National Natural Science Foundation of China (No. 20972012).

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K.J. Liao et al. / Chinese Chemical Letters 23 (2012) 1371–1374
References
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[16] H. Paulsen, O. Lockhoff, Chem. Ber. 114 (1981) 3102.
[17] P.J. Garegg, P. Ossowshi, Acta Chem. Scand. 337 (1983) 249.
[18] S.N. Lam, J. Gervay-Hague, J. Org. Chem. 70 (2005) 2387.
[19] N.V. Ganesh, N. Jayaraman, J. Org. Chem. 72 (2007) 5500.
[20] Methyl b-D-galactopyranosyl(1 ! 4)-2-acetamido-2-deoxy-b-D-glucopyranosyl(1 ! 6)-a-D-mannopyranosyl(1 ! 6)-b-D-mannopyranoside
(T1): Method 1. The product 12 (0.3 g, 0.154 mmol) in MeOH (30 mL) was hydrogenated at room temperature under atmospheric hydrogen
pressure in the presence of 10% Pd(OH)2/C. The suspension was stirred overnight. After filtration and concentration, the residue was per-
acetylated in Ac2O-pyridine (v/v, 1:2, 100 mL) for 6 h. Following the complete conversion, MeOH was added to quench the reaction and the
solution was diluted with CH2Cl2, neutralized with NaHCO3 (sat.), 1 mol/L HCl (aq.), dried, and evaporated. The residue was dissolved in
5 mL HOAc. To the solution was added freshly activated Zn-powder (300 mg); the mixture was stirred at room temperature for 5 h, then diluted
with toluene, filtered, and concentrated in vacuo. The residue was dissolved in 50 mL methanol, added catalytic amount of NaOMe, and stirred
for 10 h. The reaction solution was neutralized with Dowex H+, filtered and concentrated. The residue was purified by column chromatography
(MeOH-H2O) to obtain T1 (86 mg, 78%). Method 2: Compound 15 (0.38 g, 0.304 mmol) in MeOH (100 mL) was stirred for 10 h in the
presence of catalytic amount of NaOMe. The solubilization occurred after a few minutes. And the deacetylated product T1 was obtained by
filtration and concentration as syrup (0.17 g, 98.0%); [a]25 ꢀ128 (c 1.0, D2O); 1H NMR (300 MHz, D2O): d 4.40 (2H, H-1,10), 4.30 (d, 1H, J
D
7.8 Hz, H-100), 3.99 (d, 1H, J 9.9 Hz, H-1000), 3.84–3.17 (m, 28H, H-2, H-3, H-4, H-5, H-6, H-20, H-30, H-40, H-50, H-60, H-200, H-300, H-400, H-500,
H-600, H-2000, H-3000, H-4000, H-5000, H-6000), 1.88 (s, 3H, –NAc); 13C NMR (75 MHz, D2O): d 175.0 (COCH3), 103.4 (C-1), 102.0 (C-100), 101.7 (C-
1000), 100.1 (C-10), 75.9, 75.9, 75.3, 74.8, 73.6, 73.0, 71.8, 71.5, 71.1, 70.8, 70.3, 69.5, 69.0, 67.2, 67.1, 66.1, 61.6, 60.6, 57.4, 55.6 (C-2, C-3, C-
4, C-5, C-6,C-20, C-30, C-40, C-50, C-60, C-200, C-300, C-400, C-500, C-600, C-2000, C-3000, C-4000, C-5000, C-6000), 49.4 (OCH3), 22.8 (COCH3).
HRESIMS: calcd. for C27H47NO21 [M+Na]+: 744.2532. Found: m/z 744.2534.