A simple approach to 3,6-branched galacto-oligosaccharides and its application to the syntheses of a tetrasaccharide and a hexasaccharide related to the arabinogalactans (AGs)

Article abstract of DOI:10.1016/S0040-4039(02)01717-3

The preparation of 1,2:5,6-di-O-isopropylidene-α-D-galactofuranose was improved by increasing the ratio of DMF to acetone and using a solid supported catalyst. Employing the easily accessible 1,2:5,6-di-O-isopropylidene-α-D-galactofuranose as the starting glycosyl acceptor, a method which is particularly suitable for the regio- and stereoselective syntheses of 3,6-branched galacto-oligosaccharides was developed. A tetrasaccharide and a hexasaccharide related to the arabinogalactans (AGs) from plants were readily prepared using this strategy.

Full text of DOI:10.1016/S0040-4039(02)01717-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7349–7352
A simple approach to 3,6-branched galacto-oligosaccharides and
its application to the syntheses of a tetrasaccharide and a
hexasaccharide related to the arabinogalactans (AGs)
Jun Ning,a,* Hairong Wangb and Yuetao Yia
aResearch Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
bDepartment of Chemistry, Tsinghua University, Beijing 100084, China
Received 20 June 2002; revised 9 August 2002; accepted 16 August 2002
Abstract—The preparation of 1,2:5,6-di-O-isopropylidene-a-
D
-galactofuranose was improved by increasing the ratio of DMF to
-galactofuranose as
acetone and using a solid supported catalyst. Employing the easily accessible 1,2:5,6-di-O-isopropylidene-a-
D
the starting glycosyl acceptor, a method which is particularly suitable for the regio- and stereoselective syntheses of 3,6-branched
galacto-oligosaccharides was developed. A tetrasaccharide and a hexasaccharide related to the arabinogalactans (AGs) from
plants were readily prepared using this strategy. © 2002 Elsevier Science Ltd. All rights reserved.
Increased appreciation of the role of carbohydrates in
biological and pharmaceutical science has resulted in a
revival of interest in carbohydrate chemistry. However,
compared with other biopolymers such as peptides and
nucleic acids, the role of the saccharide structure in
function has been minimally studied. This can be
attributed mainly to the difficulty of synthesizing sac-
charides. Therefore, the central problem in carbohy-
drate field is how to prepare oligosaccharides efficiently
and simply. Although over the past few decades, con-
siderable progress1 has been made in oligosaccharide
synthesis, there is still a long way to go to find a general
route for saccharide synthesis. Maybe, owing to this
structural complexity, the preparation of saccharides
will never achieve the same levels as the preparation of
peptides and nucleic acids. But special methods which
are suitable for a certain type of oligosaccharide synthe-
sis can be developed.
3,6-Branched
yl-(1“6)-[a-
pyranosyl and 3,6-di-O-(b-
D
-Galp residues like b-
-arabinofuranosyl-(1“3)]-b-
-galactopyranosyl)-b-D-
D
-galactopyranos-
L
D
-galacto-
D
galactopyranosyl are common structural components of
arabinogalactans (AGs) from plants including tradi-
tional herbal medicines such as Lycium barbarum L.,2
Bupleurum falcatum,3 Atractylodes lancea3 and Plantago
major L.4 In addition to important roles in cell differen-
tiation and development, they are involved in cell–cell
interactions, elongation growth of the cell wall, and
defense systems in plants.5 AGs possess various phar-
macological activities.3 Yamada’s research results show
that AGs from A. lancea have intestinal immune system
modulating activity in mice, and this effect is attributed
to the b-3,6-
D
-galactan moiety in AGs.3 Although the
-Galp residues in AGs is
presence of 3,6-branched b-
D
well defined, the exact structure of these saccharides
remains to be established. For detailed characterization
of AGs, especially, for further elucidation of the molec-
ular structure responsible for their biological activity, it
would be necessary to synthesize 3,6-branched galacto-
oligosaccharides. Reports on the synthesis of 3,6-
branched galacto-oligosaccharides are very limited.6
Here we disclose a special strategy for the synthesis of
this kind of oligosaccharide, which involves an
improved preparation of 1,2:5,6-di-O-isopropylidene-a-
Scheme 1. Reagents and conditions: (a) DMF/acetone (4/1),
Dry Hydrogen Resin, reflux, 15 h, 50%.
D
-galactofuranose and its use as the starting glycosyl
acceptor. Syntheses of a tetrasaccharide and a hexasac-
charide related to the AGs are presented as typical
examples using the strategy developed.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01717-3

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J. Ning et al. / Tetrahedron Letters 43 (2002) 7349–7352
So far, 1,2:5,6-di-O-isopropylidene-a-
D
-galactofuranose
synthesis of 2, attempting to increase the reaction reflux
temperature in order to obtain more furanose diketal 2.
To simplify the purification procedure, an anhydrous H+
form cation exchange resin called Dry Hydrogen Resin
(obtained from Nankai University, Ion Exchange and
Adsorbent Resin Factory, Tianjin, China) was used as
the solid supported catalyst (Scheme 1). As a result of
our reaction conditions, the ratio of 1,2:5,6-di-O-iso-
2 has found only limited use in synthetic carbohydrate
chemistry because it is not easily accessible. For example,
2 has been obtained from
D-glucose derivatives in three
-galactose in a maximum 22%
or six steps,7 or from
D
yield.8 Recently, Rauter’s group found that by using
zeolite HY as the catalyst, the furanose diketal 2 was
formed in 40% yield, together with the pyranose diketal
3, obtained only in 20% yield, and this significantly
improved the preparation of 2.9 Some reports showed
that high temperature is a factor known to favor furanose
formation in solutions of reducing sugars.10 Inspired by
this, we raised the ratio of DMF to acetone to 4 in our
propylidene-a-
D
-galactofuranose to 1,2:3,4-di-O-iso-
-galactopyranose in the reaction product
propylidene-a-
D
can reach more than 4, and the desired 1,2:5,6-di-O-iso-
propylidene-a-
D-galactofuranose can be easily crystal-
lized from the resultant residue in 50% yield.
,
Scheme 2. Reagents and conditions: (a) TMSOTf (0.01 equiv.), 4 A MS, CH2Cl2, rt, 2–4 h. (82% for 6, 84% for 7, 85% for 11,
87% for 12, 86% for 13, 84% for 17, 91% for 18. (b) 90% AcOH, 40°C, 2 h, 100%. (c) (i) 80% AcOH, reflux, 3 h; (ii)
Ac2O/pyridine, rt, 10 h; (iii) THF/CH3OH, 1.5N NH3, rt, 2–3 h; (iv) CH2Cl2, CCl3CN (2.0 equiv.), K2CO3 (2.0 equiv.), rt, 12 h,
71% for 14, 72% for 15 (for four steps). (d) FeCl3 (2 equiv.) in CH2Cl2, rt, 20 min, 90% for 16. (e) (i) 80% AcOH, reflux, 3 h;
(ii) CH2Cl2/CH3OH saturated with ammonia, rt, 36 h, 92% for 19, 93% for 20.

J. Ning et al. / Tetrahedron Letters 43 (2002) 7349–7352
7351
Couplings of
2
with perbenzoyl arabinofuranosyl
4. Yamada, H. In Bioactive Carbohydrate Polymers, Pro-
ceedings of the Phythochemical Society of Europe;
Paulsen, B. S., Ed.; Kluwer Academic Publishers, 2000;
Vol. 44, pp. 37–46.
5. (a) Sussex, I. M. Cell 1989, 56, 225; (b) Kreuger, M.; van
Host, G. J. Planta 1993, 189, 243; (c) Kreuger, M.; van
Host, G. J. Planta 1994, 197, 135; (d) Egertsdotter, U.;
Von Arnold, S. Physiol. Plant 1995, 93, 334; (e) Roberts,
K. Curr. Opin. Cell. Biol. 1990, 2, 920; (f) Showalter, A.
M.; Varner, J. E. In The Biochemistry of Plants; Stumpf,
P. K.; Conn, E. E., Eds.; Academic: New York, 1989;
Vol. 15, p. 485.
6. (a) Gu, G.; Yang, F.; Du, Y.; Kong, F. Carbohydr. Res.
2001, 336, 99; (b) Du, Y.; Pan, Q.; Kong, F. Carbohydr.
Res. 2000, 323, 28.
7. (a) Jarosz, S.; Krajewski, J. W.; Zamojski, A.; Duddeck,
H.; Kaiser, M. Bull. Pol. Acad. Sci. Chem. 1985, 33, 181;
(b) De Jongh, D. C.; Biemann, K. J. Am. Chem. Soc.
1964, 86, 67.
8. (a) Morgenlie, S. Acta Chem. Scand. 1973, 27, 3609; (b)
Morgenlie, S. Acta Chem. Scand. Ser. B 1975, 29, 367.
9. Rauter, A. P.; Ramoa-Riberio, F.; Fernanders, A. C.;
Figueiredo, J. A. Tetrahedron 1995, 51, 6529.
10. (a) Acree, T. E.; Shallenberger, R. S.; Lee, C. Y.; Einset,
J. W. Carbohydr. Res. 1969, 10, 355; (b) Acree, T. E.;
Shallenberger, R. S.; Mattick, L. R. Carbohydr. Res.
1968, 6, 498.
trichloroacetimidate 46 and perbenzoyl galactopyranos-
yl trichloroacetimidate 511 in the presence of
TMSOTf (0.01 equiv.) as catalyst, followed by selective
5,6-O-deacetonation afforded b-(1“3)-linked disaccha-
rides 8 and 9, respectively, as solids in high yields
(80–85% for the two steps) (Scheme 2). Condensation
of 5 with 8 and 9 catalyzed by TMSOTf regio- and
stereoselectively gave the 3,6-branched trisaccharides 11
and 12, respectively, in excellent yields (85–90%). Simi-
larly, the 6%%-O-trityl trisaccharide 13 was obtained by
coupling
2,3,4-tri-O-benzoyl-6-O-trityl-a-D-galacto-
pyranosyl trichloroacetimidate 1012 with 8 in an excel-
lent yield (87%). Removal of the 1,2-O-isopropylidene
group of 11 and 12 in 80% HOAc followed by acetyla-
tion with acetic anhydride in pyridine, selective 1-O-
deacetylation with ammonia in THF/CH3OH, and
subsequent treatment with trichloroacetonitrile in the
presence of K2CO3 afforded the trisaccharide glycosyl
donors 14 and 15 in good yields (71–74% over the four
steps). Selective 6-O-detritylation of 13 in CH2Cl2 with
FeCl3 gave the trisaccharide acceptor 16 in a high yield
(90%).13 Coupling of 14 with 16 using TMSOTf as the
catalyst regio- and stereoselectively afforded the
blocked hexasaccharide 17 in a high yield (84%), while
condensation of 15 with 1,2:3,4-di-O-isopropylidene-a-
D
-galactopyranose 3 gave the blocked tetrasaccharide
18 in an excellent yield (91%).14 Deisopropylidenation
of 17 and 18 in 80% HOAc, followed by deacetylation
in an ammonia-saturated solution in 1:1 CH2Cl2/
CH3OH, furnished the free hexasaccharide 19 and tetra-
saccharide 20, which are related to AGs, as amorphous
white solids in 92% and 93% yields, respectively (for the
two steps).
11. Rio, S.; Beau, J. M.; Jacquinet, J. C. Carbohydr. Res.
1991, 219, 71.
12. Yu, B.; Xie, J.; Deng, S.; Hui, Y. J. Am. Chem. Soc.
1999, 121, 12196.
13. Ding, X.; Wang, W.; Kong, F. Carbohydr. Res. 1997,
303, 445.
14. All new compounds gave satisfactory elemental analysis
results. Selected physical data for some key compounds
1
In all of the syntheses, easily accessible materials and
cheap reagents were used and the reactions were carried
out smoothly in high yields. Several intermediates were
not separated but were used directly in further reactions
thereby simplifying the procedures substantially.
are as follows: For 8: [h]D +46 (c 2.1, CHCl3). H NMR
(400 MHz, CDCl3): l 5.96 (d, 1H, J=4.0 Hz, H-1), 5.59
(dd, 1H, J=1.0, 4.7 Hz, H-3%), 5.49 (d, 1H, J=1.0 Hz,
H-2%), 5.45 (s, 1H, H-1%), 4.81 (dd, 1H, H-5a%), 4.74 (d,
1H, J=4.0 Hz, H-2), 4.64 (dd, 1H, H-5b%), 4.61 (m, 1H,
H-4%), 4.40 (d, 1H, H-3), 4.18 (dd, 1H, H-4), 3.88 (m, 1H,
H-5), 3.75 (dd, 1H, H-6a), 3.68 (dd, 1H, H-6b), 1.54, 1.33
(2 s, 6H, C(CH3)2); Anal. calcd for C35H36O13: C, 63.25;
H, 5.46. Found: C, 63.18; H, 5.51. For 9: [h]D +62 (c 1.5,
CHCl3). 1H NMR (400 MHz, CDCl3): l 5.99 (d, 1H,
J=3.4 Hz, H-4%), 5.78 (dd, 1H, J=8.0, 10.4 Hz, H-2%),
5.68 (d, 1H, J=4.1 Hz, H-1), 5.61 (dd, 1H, J=10.4, 3.4
Hz, H-3%), 5.01 (d, 1H, J=8.0 Hz, H-1%), 4.66 (dd, 1H,
H-6a%), 4.50–4.45 (m, 3H, H-6b%, H-3, H-2), 4.37 (m, 1H,
H-5%), 4.19 (dd, 1H, H-4), 3.89 (m, 1H, H-5), 3.70 (d, 2H,
H-6a, 6b), 1.48, 1.24 (2 s, 6H, C(CH3)2); Anal. calcd for
C43H42O15: C, 64.66; H, 5.30. Found: C, 64.75; H, 5.26.
In summary, a special strategy which is peculiarly suit-
able for the preparation of 3,6-branched galacto-
oligosaccharides has been developed.
Acknowledgements
This work was supported by the Beijing Natural Sci-
ence Foundation (6021004) and National Natural Sci-
ence Foundation of China (59973026 and 29905004).
1
For 11: [h]D +28.1 (c 1.6, CHCl3); H NMR (400 MHz,
References
CDCl3): l 5.97 (d, 1H, J=3.2 Hz, H-4), 5.85 (d, 1H,
J=4.1 Hz, H-1), 5.78 (dd, 1H, H-2), 5.61–5.58 (m, 2H, 2
H-3), 5.45 (d, 1H, J=1.0 Hz, H-2), 5.36 (s, 1H, H-1),
4.95 (d, 1H, J=7.9 Hz, H-1), 1.38, 1.28 (2 s, 6H,
(CCH3)2); Anal. calcd for C69H62O22: C, 66.66; H, 5.03.
Found: C, 66.97; H, 5.00. For 12: [h]D +34.9 (c 2.3,
1. (a) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H.
Science 2001, 291, 1523; (b) Sears, P.; Wong, C. H.
Science 2001, 291, 2344.
2. Peng, X.; Tian, G. Carbohydr. Res. 2001, 331, 95.
3. Yamada, H. In Bioactive Carbohydrate Polymers, Pro-
ceedings of the Phythochemical Society of Europe;
Paulsen, B. S., Ed.; Kluwer Academic Publishers, 2000;
Vol. 44, pp. 16–24.
1
CHCl3); H NMR (400 MHz, CDCl3): l 6.01 (m, 2H, 2
H-4), 5.83 (dd, 1H, H-2), 5.78 (dd, 1H, H-2), 5.66 (dd,
1H, H-3), 5.62 (dd, 1H, H-3), 5.55 (d, 1H, J=4.1 Hz,
H-1), 4.50 (d, 1H, J=8.0 Hz, H-1), 4.96 (d, 1H, J=7.9

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J. Ning et al. / Tetrahedron Letters 43 (2002) 7349–7352
Hz, H-1), 1.28, 1.14 (2 s, 6H, (CCH3)2); Anal. calcd for
4.87 (d, 1H, J=7.6 Hz, H-1), 1.39, 1.29 (2 s, 6H,
(CCH3)2); 13C NMR (100 MHz, CDCl3): l 166.1, 165.7,
165.1, 165.0, 165.0, 164.9 (6 PhCO), 112.8 (C(CH3)2),
104.9, 104.7, 101.6 (3C-1), 26.4, 25.9 (C(CH3)2). Anal.
calcd for C62H58O21: C, 65.37; H, 5.13. Found: C, 65.27;
H, 5.08. For 17: [h]D +55.3 (c 3.6, CHCl3); 13C NMR
(100 MHz, CDCl3): l 169.2, 169.1 (2 CH3CO), 113.0
(C(CH3)2), 107.2, 104.7, 104.5, 101.0, 100.7, 100.5 (6C-1),
26.5, 26.0 (2 C(CH3)2), 20.1, 18.7 (2 CH3CO). Anal. calcd
for C132H118O44: C, 65.83; H, 4.94. Found: C, 65.39; H,
5.02. For 18: [h]D +78.8 (c 1.8, CHCl3); 1H NMR (400
MHz, CDCl3): l 5.99 (d, 1H, H-4), 5.92 (d, 1H, H-4),
5.81 (dd, 1H, H-2), 5.68 (dd, 1H, H-2), 5.58–5.54 (m, 2H,
2 H-3), 5.46 (d, 1H, J=4.9 Hz, H-1), 5.12 (dd, 1H, H-2),
4.98 (d, 1H, J=8.0 Hz, H-1), 4.86 (d, 1H, J=7.7 Hz,
H-1), 4.25 (d, 1H, J=7.9 Hz, H-1), 2.18, 1.57 (2 s, 6H, 2
CH3CO), 1.42, 1.35, 1.29, 1.28 (4 s, 12H, 2 (CCH3)2); 13C
NMR (100 MHz, CDCl3): l 169.3, 168.2 (2 CH3CO),
108.9, 108.0 (2 C(CH3)2), 101.6, 101.0, 100.9, 95.8 (4C-1),
25.6, 25.4, 24.6, 23.7 (2 C(CH3)2), 20.2, 19.7 (2 CH3CO).
Anal. calcd for C90H86O31: C, 64.98; H, 5.21. Found: C,
65.16; H, 5.28. For 19: [h]D −9.6 (c 2.0, H2O); 13C NMR
(100 MHz, D2O): 109.73, 109.61, 104.37, 104.26, 103.97
(C-1B, 1C, 1D, 1E, 1F), 98.1 (C-1A for b), 94.3 (C-1A for a).
ES MS. calcd for C34H58O29: 930.81 [M]. Found: 953.8
(M+Na)+. For 20: [h]D −19.2 (c 1.6, H2O); 13C NMR
(100 MHz, D2O): 104.0, 103.8, 102.86 (C-1B, 1C, 1D),
97.25 (C-1A for b), 93.09(C-1A for a). ES MS. calcd for
C24H42O21: 666.58 [M]. Found: 689.6 (M+Na)+.
C77H68O24: C, 67.15; H, 4.98. Found: C, 67.27; H, 5.03.
For 13: [h]D +38.3 (c 1.5, CHCl3); H NMR (400 MHz,
1
CDCl3): l 6.06 (d, 1H, J=3.0 Hz, H-4), 5.82 (d, 1H,
J=4.1 Hz, H-1), 5.63–5.57 (m, 3H, H-2, 2 H-3), 5.42 (d,
1H, J=1.4 Hz, H-2), 5.34 (s, 1H, H-1), 4.83 (d, 1H,
J=7.3 Hz, H-1), 1.33, 1.27 (2 s, 6H, (CCH3)2); Anal.
calcd for C81H72O21: C, 70.43; H, 5.25. Found: C, 70.31;
H, 5.30. For 14: [h]D +57.3 (c 1.0, CHCl3); 1H NMR (400
MHz, CDCl3) l 8.34 (s, 1H, CNHCCl3), 6.45 (d, 1H,
J=3.7, H-1), 5.96 (d, 1H, H-4), 5.72 (dd, 1H, H-2), 5.64
(dd, 1H, H-3), 5.53 (dd, 1H, H-3), 5.40 (d, 1H, J=1.2
Hz, H-2), 5.39 (s, 1H, H-1), 5.31 (dd, 1H, H-2), 4.93 (d,
1H, J=7.7 Hz, H-1), 2.08, 1.86 (2 s, 6H, 2 CH3CO); 13C
NMR (100 MHz, CDCl3): l 169.9, 169.1 (2 CH3CO),
160.4 (OC(NH)CCl3), 107.0, 100.3, 93.4 (3 C-1), 90.4
(OC(NH)CCl3), 20.0, 19.9 (2 CH3CO). Anal. calcd for
C72H62Cl3NO24: C, 60.41; H, 4.36. Found: C, 60.19; H,
4.31. For 15: [h]D +21.8 (c 1.0, CHCl3); 1H NMR (400
MHz, CDCl3) l 8.23 (s, 1H, CNHCCl3), 6.33 (d, 1H,
J=3.7, H-1), 5.97 (d, 1H, H-4), 5.93 (d, 1H, H-4), 5.75
(dd, 1H, H-2), 5.68 (dd, 1H, H-2), 5.56 (dd, 1H, H-3),
5.54 (dd, 1H, H-3), 5.16 (dd, 1H, H-2), 4.95 (d, 1H,
J=7.7 Hz, H-1), 4.92 (d, 1H, J=8.0 Hz, H-1), 2.17, 1.46
(2 s, 6H, 2 CH3CO); Anal. calcd for C80H68Cl3NO26: C,
61.37; H, 4.38. Found: C, 61.49; H, 4.31. For 16: [h]D
+51.9 (c 1.2, CHCl3); 1H NMR (400 MHz, CDCl3): l
5.90 (d, 1H, J=4.1 Hz, H-1), 5.81 (dd, 1H, H-2), 5.77 (d,
1H, J=3.0 Hz, H-4), 5.60 (dd, 1H, H-3), 5.54 (dd, 1H,
H-2), 5.47 (d, 1H, J=1.4 Hz, H-2), 5.40 (s, 1H, H-1),