A novel polycondensation method for the synthesis of a two-faced β-1,4-glucan

Article abstract of DOI:10.1007/s00706-009-0173-z

The stereospecific synthesis of a chitosan derivative repeating 2-azido-3,6-di-O-benzyl-2-deoxy-β-d-glucopyranosyl-(1 → 4)-3,6-di-O-benzyl-2-deoxy-2-phthalimido-d-glucopyranose, which has two distinguishing faces, was achieved by polycondensation of the sole starting disaccharide, trichloroacetimidoyl 2-azido-3,6-di-O-benzyl-2-deoxy-β-d- glucopyranosyl-(1 → 4)-3,6-di-O-benzyl-2-deoxy-2-phthalimido-d- glucopyranoside in a short and efficient way.

Full text of DOI:10.1007/s00706-009-0173-z

Monatsh Chem (2009) 140:1257–1260
DOI 10.1007/s00706-009-0173-z
ORIGINAL PAPER
A novel polycondensation method for the synthesis of a two-faced
b-1,4-glucan
Toshinari Kawada Æ Yuko Yoneda Æ
Kensaku Shimizu Æ Chiori Itoh
Received: 31 March 2009 / Accepted: 2 August 2009 / Published online: 1 September 2009
Ó Springer-Verlag 2009
Abstract The stereospecific synthesis of a chitosan
derivative repeating 2-azido-3,6-di-O-benzyl-2-deoxy-b-D-
glucopyranosyl-(1 ? 4)-3,6-di-O-benzyl-2-deoxy-2-phth-
alimido-D-glucopyranose, which has two distinguishing
faces, was achieved by polycondensation of the sole
starting disaccharide, trichloroacetimidoyl 2-azido-3,6-di-
O-benzyl-2-deoxy-b-D-glucopyranosyl-(1 ? 4)-3,6-di-O-
(1), and reported the tetrasaccharide synthesis. During the
synthesis of the tetrasaccharide, the phthalimido donor
enables a stereospecific b-glycosidation (trans-glycosida-
tion), which encourages us to use the repeating disaccharide
unit 1 as a sole starting material for polycondensation. This
way an one-step synthesis of b-1,4-glucan with a large
degree of polymerization is achievable.
benzyl-2-deoxy-2-phthalimido-D-glucopyranoside in
a
In this report, we present the conversion of the repeating
disaccharide unit 1 into the sole starting disaccharide 4,
which in turn is used for a subsequent stereospecific
polycondensation reaction.
short and efficient way.
Keywords Carbohydrates ꢀ Protecting group ꢀ
Glycosides ꢀ Polysaccharides
Results and discussion
Introduction
The essence of this polycondensation applied is a poly-
glycosylation reaction, which is based on a well-known
glycosylation method. Thus, the imidate method [4], one of
the most popular methods of glycosylation, was chosen,
since the trichloroacetimidoyl group could be introduced
and transformed under mild conditions [5]. Normally, the
glycosyl imidate (glycosyl donor) reacts with a suitable
glycosyl acceptor, having a free hydroxyl group. However,
if the glycosyl imidate itself contains a free hydroxyl
group, it should act as both glycosyl donor as well as
glycosyl acceptor. Consequently, the chain reaction would
be expected to afford a polysaccharide.
The glucopyranose residues of a b-1,4-glucan present their
surfaces in an alternating ‘‘obverse–reverse’’ mode. These
two faces could be distinguished by introduction of a
functional group at the same carbon of each alternative
glucose unit. However, it seems to be difficult to attach a
functional group at the same carbon of each alternative
glucose unit directly into a polysaccharide chain [1].
In the previous papers [2, 3], we proposed a synthetic
method for such a b-1,4-glucan having two faces using the
repeating disaccharide unit tert-butyldimethylsilyl 4-O-acetyl-
2-azido-3,6-di-O-benzyl-2-deoxy-b-D-glucopyranosyl-(1 ? 4)
-3,6-di-O-benzyl-2-deoxy-2-phthalimido-b-D-glucopyranoside
The repeating disaccharide unit 1 was converted into a
sole starting disaccharide 4, which possesses both charac-
teristics, a glycosyl acceptor moiety (40-OH) and a glycosyl
donor residue (1-O-trichloroacetimidoyl group; Scheme 1).
The 40-O-acetyl group of the repeating disaccharide unit 1
and the tert-butyldimethylsilyl (TBDMS) group were
cleaved subsequently. However, if the TBDMS group was
cleaved prior to acetyl deprotection, the alkaline reaction
T. Kawada (&) ꢀ Y. Yoneda ꢀ K. Shimizu ꢀ C. Itoh
Graduate School of Life and Environmental Science,
Kyoto Prefectural University, Shimogamo, Sakyo-ku,
Kyoto 606-8522, Japan
e-mail: kawada@kpu.ac.jp
123

1258
T. Kawada et al.
Scheme 1
OBn
O
OBn
O
N3
N3
NaOMe
MeOH
BnO
HO
BnO
AcO
O
O
OTBDMS
OTBDMS
NH
BnO
BnO
O
OBn
O
OBn
NPhth
NPhth
1
2
TBAF, THF-AcOH
OBn
O
OBn
N3
N3
O
CCl3CN, DBU
CCl3
CH2Cl2
O
O
BnO
HO
BnO
HO
O
BnO
O
BnO
OH
O
OBn
NPhth
BF3-Et2O, CH2Cl2
OBn
NPhth
4
3
OBn
OBn
O
N3
N3
O
BnO
BnO
HO
O
O
O
BnO
BnO
OH
O
O
NPhth
NPhth
5
OBn
OBn
n
conditions used for de-acetylation caused many side reac-
tions leading to a dramatic decrease in yield.
peaks corresponding to the molecular adduct with Na?
[M ? Na]?, accompanied with minor peaks ionized with
K? [M ? K]?. Other minor peaks correspond to products
obtained by hydrogenation [M (N3 ? NH2)] of one or two
azido group(s) to amino group(s) during MALDI-TOF-MS
measurement.
It is indispensable to prepare a mono-imidate for con-
ducting the polycondensation reaction. As shown in
Scheme 1, disaccharide 3 was treated with trichloroaceto-
nitrile (6.0 eq.) using 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU, 0.2 eq.) [6] as a base for 10 min to give a mono-
imidate 4 in 67% yield. An increase in reaction time led to
the formation of significant amounts of 1,40-di-imidate,
albeit the yield of mono-imidate 4 was also slightly
increased.
GPC analysis shows that the numerical average of the
degree of polymerization is 4.5, and the degree of polydi-
spersion (Mw/Mn) is 1.96.
As mentioned above, the synthesis of chitosan deriva-
tives containing two kinds of protecting groups at C-2
position in an alternate way was successfully achieved by
polycondensation of the repeating disaccharide imidate 4.
Thus, ‘‘obverse and reverse’’ faces of the synthesized
chitosan derivative could be distinguished due to phtha-
limido and azido groups at C-2; both could be easily
converted into free amino and/or acetamido groups [3].
Benzyl groups also could be cleaved after suitable chemi-
cal processing at C-1, if appropriate.
Mono-imidate 4 was polycondensed using boron tri-
fluoride diethyl etherate (BF3-Et2O) as catalyst in
anhydrous dichloromethane. After the reaction was fin-
ished within 5 min, the reaction mixture was neutralized by
the addition of a saturated aqueous solution of sodium
bicarbonate, washed with water and brine, and the solvent
was evaporated to give polycondensed product 5, which
was used for chemical analysis without any purification.
The stereochemistry of the product 5 was determined by
13C NMR. Signals derived from the newly generated gly-
cosidic bonds appeared at d = 97 ppm, which indicates the
expected b-configuration. The anomeric carbon at the
reducing end gives a signal at d = 93 ppm, which suggests
that even the free hydroxyl group is fixed in the b-con-
figuration presumably affected by the neighboring
phthaloyl group. The internal glycosidic bond neighboring
with the azido group, which originally exists in the
repeating disaccharide unit 1, shows signals of around
d = 102 ppm for the corresponding anomeric carbon
atoms. The NMR studies show that the polycondensation
reaction proceeded in a stereospecific manner, and the
product 5 is a b-1,4-glucan.
Experimental
General methods
Nuclear magnetic resonance (NMR) spectra were measured
with tetramethylsilane as an internal standard with a JEOL
ARX-500. The assignments of the signals were determined
1
using H–1H correlated spectroscopy and/or 13C–1H het-
eronuclear multiple-quantum correlation technique.
Coupling constants (J) are given in Hz. MALDI-TOF MS
spectra were recorded on Bruker Reflex III using 2,5-
dihydroxy benzoic acid as a matrix. GPC analysis was done
with an Asahipak GSM-700H. Anhydrous dichloromethane
and tetrahydrofuran were prepared by distilling from
phosphorus pentoxide and sodium/benzophenone, respec-
tively. Flash column chromatography was performed
on silica gel (Wakogel FC-40). Preparative thin-layer
The molecular weight was determined by matrix-assis-
ted laser desorption/ionization time-of-flight mass
spectroscopy (MALDI-TOF-MS). A series of chito-oligo-
saccharide derivatives up to the dodecasaccharide
derivative were detected. Each oligosaccharide gave major
123

A novel polycondensation method for the synthesis of a two-faced b-1,4-glucan
1259
residue was purified by flash column chromatography
(EtOAc/toluene, 1:2, v/v) to yield 1.17 g (78%) 3 as
colorless crystals. Rf = 0.40 (EtOAc/n-hexane, 1:1, v/v);
0
chromatography (TLC) was done on silica gel plates
(Kieselgel 60 F254, Merck). Results of elemental analyses
agreed favorably with calculated values. Unless otherwise
indicated, the usual workup for each reaction mixture
consists of extraction with ethyl acetate, washing with
brine, drying over sodium sulfate, and evaporation in
vacuo.
1H NMR (270 MHz, CDCl3): d = 2.95 (d, 1H, J4 OH
=
2.3 Hz, 40-OH), 3.12 (dd, 1H, J2 ,3 = 9.7 Hz,
0
0
J3 ,4 = 8.7 Hz, H-30), 3.17 (m, 1H, H-50), 3.29 (dd,
0
0
1H, J1 ,2 = 8.1 Hz, J2 ,3 = 9.7 Hz, H-20), 3.48 (dd, 1H,
0
0
0
0
J5 ,6 a = 5.6 Hz, J6 a,6 b = 10.1 Hz, H-60a), 3.53–3.66 (m,
3H, 1-OH, H-60b, H-40), 3.68 (ddd, 1H, J4,5 = 9.9 Hz,
J5,6a = 1.5 Hz, J5,6b = 3.3 Hz, H-5), 3.81 (dd, 1H,
J5,6a = 1.5 Hz, J6a,6b = 10.9 Hz, H-6a), 3.95 (dd, 1H,
J5,6b = 3.3 Hz, J6a,6b = 10.9 Hz, H-6b), 4.04–4.11 (m,
0
0
0
0
tert-Butyldimethylsilyl 2-azido-3,6-di-O-benzyl-2-deoxy-b-
D-glucopyranosyl-(1 ? 4)-2-deoxy-3,6-di-O-benzyl-2-
phthalimido-b-D-glucopyranoside (2, C54H62N4O11Si)
To a solution of 426 mg tert-butyldimethylsilyl 4-O-acetyl-
2-azido-3,6-di-O-benzyl-2-deoxy-b-D-glucopyranosyl-(1 ? 4)
-2-deoxy-3,6-di-O-benzyl-2-phthalimido-b-D-glucopyrano-
side (1, 0.25 mmol) in 5 cm3 MeOH 15 mm3 sodium
methoxide in MeOH (28%, 0.07 mmol) was added drop-
wise at 0 °C. The reaction mixture was stirred at room
temperature overnight, and then neutralized with DOWEX
50W-X8 (H?) resin. The resin was filtered off, and the
filtrate was concentrated in vacuo and purified by flash
column chromatography (EtOAc/toluene, 2:3, v/v) to yield
404 mg (97%) 2 as a colorless glass. Rf = 0.43 (EtOAc/n-
hexane, 1:2, v/v); 1H NMR (270 MHz, CDCl3): d = -0.11
(s, 3H, SiMe), 0.04 (s, 3H, SiMe), 0.66 (s, 9H, t-Bu) 2.93 (d,
2H, H-2, H-4), 4.29 (d, 1H, J1 ,2 = 8.1 Hz, H-10), 4.33 (dd,
1H, J = 10.7 Hz, J = 8.4 Hz, H-3), 4.36–4.49 (m, 4H,
CH2Ph), 4.71 (d, 1H, CH2Ph), 4.75 (d, 1H, CH2Ph), 4.83 (s,
2H, CH2Ph), 5.32 (t, 1H, J1,2 = 8.2 Hz, J1,OH = 8.2 Hz,
H-1), 6.78–6.84 (m, 3H, Ph), 6.92–6.96 (m, 2H, Ph), 7.21–
7.42 (m, 15H, Ph), 7.59–7.70 (m, 4H, Phth) ppm;
13C NMR (67.8 MHz, CDCl3): d = 57.5 (C-2), 66.1
(C-20), 68.0 (C-6), 70.6 (C-60), 73.0 (C-50), 73.2 (C-40),
73.5, 73.6, 74.4 (3 9 CH2Ph), 74.7 (C-5), 75.0 (CH2Ph),
76.7 (C-3), 77.9 (C-4), 82.6 (C-30), 92.8 (C-1), 100.9
(C-10), 123.1 (Phth), 126.8–128.4 (Ph), 131.4 (Phth), 133.6
(Phth), 137.3, 137.6, 138.0, 138.4 (4 9 Ph), 167.9 (Phth)
ppm; MALDI-TOF-MS: m/z = 895.32 [M ? K]?, 879.35
[M ? Na]?, 869.35 [M (N3 ? NH2) ? K]?, 853.34 [M
(N3 ? NH2) ? Na]?.
0
0
1H, J4 ,OH = 2.0 Hz, 40-OH), 3.19 (dd, 1H, J2 ,3 = 9.7 Hz,
0
0
0
J3 ,4 = 8.6 Hz, H-30), 3.24 (ddd, 1H, H-50), 3.33 (dd, 1H,
0
0
J1 ,2 = 8.1 Hz, J2 ,3 = 9.7 Hz, H-20), 3.50 (dd, 1H,
0
0
0
0
J5 ,6 a = 5.6 Hz, J6 a,6 b = 10.1 Hz, H-60a), 3.61–3.68 (m,
3H, H-40, H-60b, H-5), 3.78 (dd, 1H, J5,6a = 1.5 Hz,
J6a,6b = 11.2 Hz, H-6a), 3.98 (dd, 1H, J5,6b = 3.3 Hz,
J6a,6b = 11.2 Hz, H-6b), 4.08–4.15 (m, 2H, H-2, H-4),
4.29 (dd, 1H, J = 10.9 Hz, J = 8.4 Hz, H-3), 4.43 (d, 1H,
0
0
0
0
Trichloroacetimidoyl 2-azido-3,6-di-O-benzyl-2-deoxy-b-
D-glucopyranosyl-(1 ? 4)-2-deoxy-3,6-di-O-benzyl-2-
phthalimido-b-D-glucopyranoside (4, C50H48Cl3N5O11)
To a solution of 366 mg 3 (0.43 mmol) in 18 cm3
anhydrous CH2Cl2 13 mm3 DBU (86 lmol) was added,
and the solution was stirred for 30 min. Trichloroacetoni-
trile (357 mg, 2.6 mmol) was added and stirred for another
10 min at room temperature. The reaction mixture was
directly applied on preparative TLC for purification
(EtOAc/n-hexane, 1:2, v/v) to yield 289.6 mg (67%) 4 as
colorless crystals. Rf = 0.76 (EtOAc/n-hexane, 1:2, v/v);
0
J1 ,2 = 8.1 Hz, H-10), 4.40–4.45 (m, 3H, CH2Ph), 4.56
(d, 1H, CH2Ph), 4.74 (2 9 d, 2H, CH2Ph), 4.85 (s, 2H,
CH2Ph), 5.32 (d, 1H, J1,2 = 7.9 Hz, H-1), 6.81–6.86 (m,
3H, Ph), 6.95–6.98 (m, 2H, Ph), 7.18–7.43 (m, 15H, Ph),
7.58–7.79 (m, 4H, Phth) ppm; 13C NMR (67.8 MHz,
CDCl3): d = -5.4 (SiMe), -4.1 (SiMe), 17.6 (SiCMe3),
25.4 (t-Bu), 57.8 (C-2), 66.2 (C-20), 68.1 (C-6), 70.8 (C-60),
73.0 (C-50), 73.37 (CH2Ph), 73.44 (C-5 or C-40), 73.7, 74.1
(2 9 CH2Ph), 74.8 (C-5 or C-40), 75.1 (CH2Ph), 76.5 (C-3),
78.0 (C-4), 82.6 (C-30), 93.3 (C-1), 101.0 (C-10), 123.0
(Phth), 126.8–128.4 (Ph), 131.5 (Phth), 133.5 (Phth),
137.3, 137.99, 138.03, 138.6 (3 9 Ph) ppm.
0
0
1H NMR (270 MHz, CDCl3): d = 2.88 (d, 1H, J4 OH
=
2.1 Hz, 40-OH), 3.14 (dd, 1H, J2 ,3 = 9.7 Hz,
0
0
J3 ,4 = 8.7 Hz, H-30), 3.19 (m, 1H, H-50), 3.32 (dd, 1H,
0
0
J1 ,2 = 8.1 Hz, J2 ,3 = 9.7 Hz, H-20), 3.51 (dd, 1H,
0
0
0
0
J5 ,6 a = 5.6 Hz, J6 a,6 b = 10.1 Hz, H-60a), 3.60–3.67
(m, 2H, H-40, H-60b), 3.84 (ddd, 1H, J4,5 = 9.9 Hz,
0
0
0
0
2-Azido-3,6-di-O-benzyl-2-deoxy-b-D-glucopyranosyl-
(1 ? 4)-2-deoxy-3,6-di-O-benzyl-2-phthalimido-b-D-glu-
copyranose (3, C48H48N4O11)
To a stirred solution of 1.7 g 2 (1.68 mmol) in 17 cm3
anhydrous THF 46 mm3 acetic acid (0.8 mmol) and
3.2 cm3 tetra-n-butylammonium fluoride solution (TBAF,
1.0 M in THF, 3.2 mmol) were added at 0 °C. The reaction
mixture was stirred for 1 h, and then worked up. The
J5,6a = 1.3 Hz,
J5,6b = 3.0 Hz,
H-5),
3.88
(dd,
1H, J5,6a = 1.3 Hz, J6a,6b = 11.4 Hz, H-6a), 4.03 (dd,
1H, J5,6b = 3.0 Hz, J6a,6b = 11.4 Hz, H-6b), 4.22 (dd, 1H,
J3,4 = 7.9 Hz, J4,5 = 9.9 Hz, H-4), 4.34–4.54 (m, 7H, H-2,
H-3, H-10, CH2Ph), 4.74–4.85 (m, 4H, CH2Ph), 6.39 (d,
1H, J1,2 = 8.4 Hz, H-1), 6.80–6.84 (m, 3H, Ph), 6.94–6.98
(m, 2H, Ph), 7.23–7.43 (m, 15H, Ph), 7.64–7.67 (m, 4H,
Phth), 8.54 (s, 1H, NH) ppm.
123

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T. Kawada et al.
Acknowledgments We are grateful to Dr. Hiroshi Kamitakahara,
Graduate School of Agriculture, Kyoto University, Japan, for
MALDI-TOF-MS measurements. This research was conducted with
the support of a grant-in-aid for Scientific Research (18658133) from
Japan Society for the Promotion of Science (JSPS).
[4-(2-Azido-3,6-di-O-benzyl-2-deoxy-b-D-glucopyranosyl)-
(1 ? 4)-2-deoxy-3,6-di-O-benzyl-2-phthalimido-b-D-
glucopyranoside-(1 ?]n (5)
Carefully dried 4 (135 mg, 0.135 mmol) was dissolved in
10 cm3 anhydrous CH2Cl2 and cooled to -78 °C. Then
4.9 mm3 boron trifluoride diethyletherate (BF3-Et2O)
(0.027 mmol) was added, and the reaction mixture was
stirred for 5 min. Triethylamine (1 drop) was added for
neutralizing the acidic catalyst and quenching the reaction.
The reaction mixture was worked up, and the solvent was
evaporated to obtain an oily syrup (107 mg). MALDI-
TOF-MS: m/z = 5,070.92 [M12 ? Na]?, 4,248.57
[M10 ? K]?, 4,232.26 [M10 ? Na]?, 4,183.65 [M10
(2 9 N3 ? 2 9 NH2)]?, 3,410.25 [M8 ? K]?, 3,394.28
[M8 ? Na]?, 3,345.33 [M8 (N3 ? NH2)]?, 2,571.93
[M6 ? K]?, 2,555.96 [M6 ? Na]?, 2,507.01 [M6
(N3 ? NH2)]?, 853.34 [M2 (N3 ? NH2) ? Na]?.
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