Regioselective esterification of various D-glucopyranosides: Synthesis of a fully protected disaccharide unit of hyaluronic acid

Article abstract of DOI:10.1055/s-2003-40338

A highly regioselective esterification of various D-glucopyranosides with triethylamine and acid anhydrides in excellent yields is described here. Its application toward the synthesis of a fully protected disaccharide unit of hyaluronic acid is also highlighted.

Full text of DOI:10.1055/s-2003-40338

1364
LETTER
Regioselective Esterification of Various D-Glucopyranosides: Synthesis of a
Fully Protected Disaccharide Unit of Hyaluronic Acid
R
egioselective Est
i
erifica
t
n
ion of Vario
-
D
-Glu
A
copyranosides n Lu, Chien-Hung Chou, Cheng-Chung Wang, Shang-Cheng Hung*
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
Fax +886(2)27831237; E-mail: schung@chem.sinica.edu.tw.
Received 28 May 2003
Dedicated to Professor Ray Lemieux, in honor of his many contributions to organic chemistry.
to the inductive effect of the two oxygen atoms at the ano-
Abstract: A highly regioselective esterification of various D-glu-
copyranosides with triethylamine and acid anhydrides in excellent
yields is described here. Its application toward the synthesis of a ful-
ly protected disaccharide unit of hyaluronic acid is also highlighted.
meric center, the C2-oxide formed in triethylamine solu-
tion reacts predominantly with various anhydrides to
furnish the esters in high selectivity. As expected from the
steric considerations, a clear-cut preference was observed
for 6-O-protection during acylation of the 4,6-diol 13, to
give the corresponding 6-benzoate 14 (entry 9) and 6-ac-
etate 15 (entry 10) in 83% and 79% yields, respectively.
Key words: carbohydrates, esterifications, hyaluronic acid
Regioselective protection of individual hydroxy groups is
of crucial importance in carbohydrate chemistry.1 For the
preparation of selectively protected glycosyl acceptors or
donors toward the synthesis of oligosaccharides or glyco-
conjugates, acetyl (Ac) and benzoyl (Bz) are generally
used as electron-withdrawing protecting groups to block
single hydroxyls. Some strategies have been reported for
regioselective introduction of acyl groups via direct treat-
ment with benzoyloxybenzotriazole2 (BzOBT, commer-
cially non-available), or selective activation of hydroxy
groups through stannylene acetals3 as well as enzymes.4
These methodologies mostly have their advantages and
disadvantages, which may give low selectivity and yields,
involving tedious purification of regioisomers. To tackle
this problem, we have employed a very simple combina-
tion of triethylamine with acid anhydrides as mild esteri-
fication reagents to study the regioselectivity of various
D-glucopyranosides.
Table 1 Regioselective Esterification of Various D-Glucopyrano-
sides with Triethylamine and Acid Anhydrides at Room Temperature
Entry
Glucopyranoside
Product
Yield (%)
Ph
O
Ph
O
O
O
O
O
HO
HO
RO
HO
OMe
OMe
1
2
2: R = Bz
3: R = Ac
93
80
1
Ph
Ph
O
O
O
O
O
O
HO
HO
HO
RO
O
O
3
4
5: R = Bz
6: R = Ac
78
64
4
4-OMePh
HO
4-OMePh
O
O
O
O
O
O
HO
HO
RO
OMe
OMe
Table 1 illustrates the results of regioselective acetylation
and benzoylation on a variety of D-glucopyranosyl diols.5
Initially, benzoylation of methyl 4,6-O-benzylidene-a-D-
glucopyranoside 1 with 1.4 equivalents of benzoic anhy-
dride and 9 equivalents of triethylamine in dichlo-
romethane at room temperature led to the corresponding
2-benzoate 26,2 in excellent yield (92%, entry 1) as a sole
product. Similar phenomenon was observed when Ac2O
was used as an acetylating agent, affording the desired 2-
acetate 37 in 80% yield (entry 2). It should be noted that a
random esterification occurs if pyridine is used in place of
triethylamine. In entries 3-8, the a-allyl glucopyranoside
4, p-methoxybenzylidene acetal 7, and p-bromoben-
zylidene acetal 10 were selected to examine the compati-
bility of substituted groups at the anomeric, O4, and O6
positions, and the corresponding 2-esters 5,8 6,9 8, 9, 11,
and 12 were obtained in good yields, respectively. Owing
5
6
7
8: R = Bz
9: R = Ac
86
69a
4-BrPh
4-BrPh
O
O
O
O
O
O
HO
HO
HO
RO
OMe
OMe
7
8
10
11: R = Bz
12: R = Ac
RO
88
65a
HO
O
O
O
HO
BnO
HO
STol
OH
STol
BnO
BnO
BnO
9
10
13
14: R = Bz
15: R = Ac
83
79
Ph
O
Ph
O
O
O
O
OR
HO
HO
N3
N3
11
12
16
17: R = Bz
18: R = Ac
93
92
a Compounds 7 and 10 were recovered in 13% and 18% yields,
respectively.
Synlett 2003, No. 9, Print: 11 07 2003.
Art Id.1437-2096,E;2003,0,09,1364,1366,ftx,en;S03703ST.pdf.
© Georg Thieme Verlag Stuttgart · New York

LETTER
Regioselective Esterification of Various D-Glucopyranosides
1365
Since D-glucosamine is a typical component of numerous catalyzed by HA synthase could lead to a polymer
biomolecules, for example, glycosaminoglycans, blood (n = 1,500).14 Chemical methods having either a D-
group antigens, N-glycoproteins and GPI anchors, it was glucosamine15 or a D-glucuronic acid15d,16 residue at the
thought worthwhile to examine the regioselective dis- reducing end have been investigated.
crimination of the hydroxy groups at C1 and C3. In entry
11, 2-azido-2-deoxy-4,6-O-benzylidene-D-glucopyranose
From the basic structure of the disaccharide repeating-unit
in HA, a free hydroxy group at the C3 position of D-glu-
cosamine residue is required for further glycosylation.
With the key synthon 17 in hands, our approach to the
16, generated from D-glucosamine hydrochloride in two
straightforward steps,10 was subjected to benzoylation un-
der these conditions. It was heartening to see the forma-
synthesis of HA-disaccharide is outlined in Scheme 1. It
tion of the corresponding b-anomeric benzoate 17 as a
starts from the glycosyl bromide 19, which can be conve-
sole isomer (93%) in a highly regio- and stereoselective
niently prepared from commercially available D-glucurol-
manner. Its absolute configuration was unambiguously
actone in three steps.17 Silver trifluoromethanesulfonate-
determined through the X-ray single crystal analysis.11
activated coupling of the donor 19 with the alcohol 17 in
Similarly, acetylation of 16 (entry 12) furnished the b-
the presence of 2,6-di-t-butyl-4-methylpyridine (DBMP)
anomeric acetate 18 in 92% yield. The high regio- and
yielded the single orthoester 20 (80%) without isolation of
stereoselectivity in this case is perhaps induced by a close
any desired product. On the other hand, hydrolysis of
compound 19 with silver carbonate in acetone and water
(97%) followed by treatment with trichloroacetnitrile em-
interplay of various factors. The higher reactivity of the
anomeric hydroxy group stems out from its higher acidity
in mild basic conditions, to gererate higher proportion of
ploying 1,8-diazabicyclo[4.3.0]undecane (DBU) as a base
anomeric alkoxide that reacts preferentially with bulky
provided the corresponding trichloroacetimidate 21
acylating agents resulting in observed regioselctivity.
(73%), which was subjected to couple with the glycosyl
Along with this, the kinetic streoelectronic effect12 and
acceptor 17 to give the desired disaccharide 2218 in 81%
1,3-diaxial repulsion orient the oxide toward the equatori-
al position, giving the b-isomer, exclusively.
yield. The b-configuration of the newly formed glycosidic
bond is determined according to the trans-diaxial cou-
Hyaluronic acid (HA), an ubiquitous glycosaminoglycan pling constant (J1,2 = 8.0 Hz) of anomeric proton in the
found in almost all tissues, possesses unique viscoelastic D-glucuronate unit. Reaction of compound 22 with thio-
and rheological properties.13 It plays significant roles in a acetic acid afforded the expected N-acetyl derivative 2318
diverse set of biological processes including cell adhe- (75%), a fully protected HA-disaccharide unit.
sion, hemopoiesi and angiogenesis.12 HA is a negatively
In conclusion, we have successfully developed a highly
regioselective acetylation and benzoylation of the D-glu-
copyranosyl 2,3-diols at O2, D-glucopyranosyl 4,6-diols
at O6, and D-glucosamine-derived 1,3-diol at O1, using a
charged linear polysaccharide consisting of b-1,4-linked
repeating disaccharide units of b-1,3-linked D-glucuronic
acid and N-acetyl-D-glucosamine. The literature has doc-
umented some strategies to prepare HA-related mole-
very simple and mild reagent combination. The prepara-
cules. Enzymatic synthesis from UDP-2-acetamido-2-
tion of a fully protected HA-disaccharide via assembly of
deoxy-a-D-glucopyranose and UDP-a-D-glucuronic acid
OMe
O
OMe
O
O
O
AgOTf
AcO
AcO
AcO
AcO
+
17
DBMP, MS4A, CH2Cl2,
–15 °C, 4 h, 80%
AcO
O
Br
O
Ph
O
O
O
O
OBz
19
Me
N3
20
1. Ag2CO3, acetone/H2O = 2/1, 0 °C, 1 h, 97%
2. CCl3CN, DBU, –10 °C, 2 h, 73%
OMe
OMe
O
O
O
Ph
O
O
cat. TMSOTf
O
O
AcO
AcO
AcO
AcO
O
+
17
OBz
CH2Cl2, MS4A,
4 h, r.t., 81%
AcO
OAc
N3
O
CCl3
NH
22
21
HO
OH
O
OMe
O
O
Ph
O
O
O
AcSH
r.t., 30 h, 75%
O
O
HO
O
AcO
AcO
O
HO
O
OBz
O
OAc
NHAc
NHAc
OH
hyaluronic acid
23
Scheme 1
Synlett 2003, No. 9, 1364–1366 ISSN 1234-567-89 © Thieme Stuttgart · New York

1366
X.-A. Lu et al.
LETTER
(14) De Luca, C.; Lansing, M.; Martini, I.; Crescenzi, F.; Shen,
G.-J.; O’Regan, M.; Wong, C.-H. J. Am. Chem. Soc. 1995,
117, 5869.
(15) (a) Slaghek, T. M.; Nakahara, Y.; Ogawa, T. Tetrahedron
Lett. 1992, 33, 4971. (b) Slaghek, T. M.; Nakahara, Y.;
Ogawa, T.; Kamerling, J. P.; Vliegenthart, J. F. G.
the key building block 17 with the trichloroacetimidate 21
followed by transformation of N3 into NAc group is also
carried out efficiently. Applications of the disaccharides
22 toward the synthesis of HA-related oligosaccharides
are currently under investigation.
Carbohydr. Res. 1994, 255, 61. (c) Halkes, K. M.; Slaghek,
T. M.; Hyppönen, T. K.; Kruiskamp, P. H.; Ogawa, T.;
Kamerling, J. P.; Vliegenthart, J. F. G. Carbohydr. Res.
1998, 309, 161. (d) Yeung, B. K. S.; Hill, D. C.; Janicka, M.;
Petillo, P. A. Org. Lett. 2000, 2, 1279.
Acknowledgment
We thank Mr. Yuh-Sheng Wen for X-ray single crystal structural
analysis and the National Science Council of Taiwan R. O. C. (NSC
91-2113-M-001-004 and NSC 91-2323-B-001-006) for financial
support.
(16) (a) Slaghek, T. M.; Hyppönen, T. K.; Ogawa, T.; Kamerling,
J. P.; Vliegenthart, J. F. G. Tetrahedron Lett. 1993, 34,
7939. (b) Slaghek, T. M.; Hyppönen, T. K.; Ogawa, T.;
Kamerling, J. P.; Vliegenthart, J. F. G. Tetrahedron:
Asymmetry 1994, 5, 2291. (c) Carter, M. B.; Petillo, P. A.;
Anderson, L.; Lerner, L. E. Carbohydr. Res. 1994, 258, 299.
(d) Coutant, C.; Jacquinet, J.-C. J. Chem. Soc., Perkin Trans.
1 1995, 1573. (e) Blatter, G.; Jacquinet, J.-C. Carbohydr.
Res. 1996, 288, 109.
References
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(18) The selected physical data of key compounds is listed.
Compound 22: [a]D25 –103.1 (c 1.0, CHCl3). Mp 209–
210 °C. IR (CHCl3): n = 2956 (w), 2115 (s), 1751 (s), 1635
(m), 1374 (m), 1219 (s), 912 (w)cm–1. 1H NMR (400 MHz,
CDCl3): d = 8.09 (d, J = 7.2 Hz, 2 H, ArH), 7.63 (t, J = 7.2
Hz, 1 H, ArH), 7.49 (t, J = 7.6 Hz, 2 H, ArH), 7.44–7.37 (m,
5 H, ArH), 5.81 (d, J = 8.3 Hz, 1 H, H-1), 5.53 (s, 1 H,
benzylidene), 5.25–5.14 (m, 2 H, H-3¢, H-4¢), 5.05 (t, J = 8.3
Hz, 1 H, H-2¢), 4.84 (d, J = 8.0 Hz, 1 H, H-1¢), 4.35 (dd,
J = 10.3, 4.8 Hz, 1 H, H-5), 3.84–3.73 (m, 5 H, H-2, H-3, H-
4, H-6, H-5¢), 3.64–3.60 (m, 1 H, H-6¢), 3.61 (s, 3 H, OMe),
2.08 (s, 3 H, OAc), 2.01 (s, 3 H, OAc), 1.99 (s, 3 H, OAc).
13C NMR (100 MHz, CDCl3): d = 170.08 (C), 169.34 (C),
169.30 (C), 166.77 (C), 164.37 (C), 136.76 (C), 134.09
(CH), 130.08 (CH), 129.17 (CH), 128.65 (CH), 128.31
(CH), 125.90 (CH), 101.54 (CH), 100.84 (CH), 93.68 (CH),
80.01 (CH), 79.39 (CH), 72.42 (CH), 72.08 (CH), 71.55
(CH), 69.25 (CH), 68.21 (CH2), 67.06 (CH), 65.02 (CH),
52.69 (CH3), 20.55 (CH3), 20.42 (CH3). HRMS (FAB, MH+)
calcd for C33H36N3O15: 714.2146. Found: 714.2111. Anal.
Calcd for C33H35N3O15: C, 55.54; H, 4.94; N, 5.89. Found: C,
55.41; H, 4.64; N, 5.55. Compound 23: [a]D25 –66.2 (c 0.9,
CHCl3). Mp 188–189 °C. IR (CHCl3): n = 3417 (w), 2955
(m), 1756 (s), 1751 (s), 1735 (s), 1654 (m), 1249 (s), 1084
(s), 753 (m)cm–1. 1H NMR (400 MHz, CDCl3): d = 8.06 (d,
J = 7.5 Hz, 2 H, ArH), 7.60 (t, J = 7.5 Hz 1 H, ArH), 7.48–
7.44 (m, 4 H, ArH), 7.41–7.37 (m, 3 H, ArH), 6.29 (d,
J = 7.9 Hz, 1 H, H-1), 5.87 (d, J = 7.9 Hz, 1 H, NH), 5.52 (s,
1 H, benzylidene), 5.23–5.15 (m, 2 H, H-3¢, H-4¢), 5.00 (t,
J = 7.7 Hz, 1 H, H-2¢), 4.90 (d, J = 7.7 Hz, 1 H, H-1¢), 4.45
(t, J = 8.7 Hz, 1 H, H-3), 4.36 (dd, J = 8.9, 3.3 Hz, 1 H, H-
6), 3.95–3.89 (m, 2 H, H-2, H-4), 3.82–3.72 (m, 3 H, H-5, H-
5¢, H-6), 3.62 (s, 3 H, OMe), 1.99 (s, 3 H, OAc), 1.98 (s, 3 H,
OAc), 1.96 (s, 3 H, OAc), 1.94 (s, 3 H, OAc). 13C NMR (100
MHz, CDCl3): d = 170.42 (C), 170.05 (C), 169.43 (C),
169.39 (C), 167.08 (C), 164.81 (C), 136.97 (C), 133.82
(CH), 130.09 (CH), 129.12 (CH), 128.65 (CH), 128.60
(CH), 128.29 (CH), 126.07 (CH), 101.51 (CH), 99.67 (CH),
92.38 (CH), 79.76 (CH), 77.32 (CH), 72.09 (CH), 71.56
(CH), 69.24 (CH), 68.57 (CH2), 66.63 (CH), 55.43 (CH),
52.68 (CH3), 23.29 (CH3), 20.53 (CH3), 20.43 (CH3). HRMS
(FAB, MH+) calcd for C35H40NO16: 730.2347. Found:
730.2360. Anal. Calcd for C35H39NO16: C, 57.61; H, 5.39; N,
1.92. Found: C, 57.58; H, 5.33; N, 1.85.
(4) Schelhaas, M.; Waldmann, H. Angew. Chem., Int. Ed. Engl.
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(5) Following is the general procedure for the regioselective
esterification of various D-glucopyranosyl diols. To a
solution of the diol in CH2Cl2 (10 mL/g) was added acid
anhydride (1.4 equiv) at r.t. under nitrogen. After stirring for
10 min, Et3N (9 equiv) was added to the mixture, and the
whole solution was kept stirring overnight. MeOH (5 equiv)
was added to quench the reaction, and the resulting solution
was evaporated under reduced pressure. The syrup was
dissolved in EtOAc, and the mixture was sequentially
washed by water, aq sat. NaHCO3(twice), and brine. The
organic layer was dried over MgSO4, filtered, and
concentrated in vacuo. The residue was purified by either
recrystallization in ethanol or flash column chromatography
on silica gel to give the desired product.
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(11) Colorless crystals from chloroform/hexane, C20H19N3O6,
fw = 397.14, crystal dimensions: 0.43 × 0.31 × 0.19 mm3,
crystal system: orthorhombic, space group: P212121, unit-
cell dimensions: a = 8.7073(7), b = 11.0684(21),
c = 19.784(6) Å, V = 1906.7(7) Å3, Z = 4, rcalcd = 1.384
gcm–3, wavelength = 0.7107 Å, F(000) = 831.85, mu = 0.10
mm–1, 2q(max) = 50.0. The deposition number at the
Cambridge Crystallographic Data Centre is CCDC 162431.
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Synlett 2003, No. 9, 1364–1366 ISSN 1234-567-89 © Thieme Stuttgart · New York