Stereoselective α-galactofuranosylation and synthesis of di- and tetrasaccharide subunits of cell wall polysaccharides of Talaromyces flavus

Article abstract of DOI:10.1016/j.tetlet.2008.05.118

Stereoselective α-galactofuranosylation employing 2′-carboxybenzyl glycosides as galactosyl donors has been established. The tetrabenzyl-protective group on the galactosyl donor was essential for the α-galactosylation of secondary alcohol acceptors. The p

Full text of DOI:10.1016/j.tetlet.2008.05.118

Tetrahedron Letters 49 (2008) 4734–4737
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stereoselective
a-galactofuranosylation and synthesis of di- and
tetrasaccharide subunits of cell wall polysaccharides of Talaromyces flavus
*
Ju Yuel Baek, Yong Jae Joo, Kwan Soo Kim
Center for Bioactive Molecular Hybrids and Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
-galactofuranosylation employing 20-carboxybenzyl glycosides as galactosyl donors has
-galac-
Article history:
Stereoselective
a
Received 2 April 2008
Revised 22 May 2008
Accepted 26 May 2008
Available online 13 June 2008
been established. The tetrabenzyl-protective group on the galactosyl donor was essential for the
a
tosylation of secondary alcohol acceptors. The present method was successfully applied to the synthesis
of di- and tetrasaccharide subunits of cell wall polysaccharides of Talaromyces flavus.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
a
-Galactofuranosylation
Talaromyces flavus
Oligosaccharides
Glycosylation
Talaromyces flavus is a soil-inhabiting fungus and has been
known to suppress Verticulum wilt of eggplant and tomato,1 and
parasitizes Rhizoctonia solani2 and Sclerotinia sclerotiorum.3 Like
the cell wall of other fungi,4 that of T. flavus was known to be rich
of polysaccharides but contains unusually high proportion of galac-
tofuranose.5 The water-soluble cell wall polysaccharides were iso-
lated from T. flavus and the following structure of the disaccharide
as galactofuranosyl donors but they are not generally applicable for
-galactofuranosylation with range of glycosyl acceptors.
Recently, an indirect method employing 2,3-anhydroglucofurano-
a
a
syl thioglycosides as glycosyl donors has been utilized as alterna-
tives for the synthesis of
reported an example of the direct stereoselective construction of
the
-galactofuranosyl linkage employing the 20-carboxylbenzyl
a
-galactofuranosides.9 We have also
a
repeating unit was identified: ?6)-[
(1? (A) (Fig. 1).6 Interesting structural feature of this polysaccha-
ride is the occurrence of the -galactofuranosyl moiety, which is
a
-
D
-Galf-(1?2)]-
a
-
D
-Manp-
(CB) glycoside method during the total synthesis of immuno-
modulatory glycolipids, agelagalstatin.10 Herein, we report the
a-
D
stereoselective construction of the a-galactofuranosyl linkage
often a problematic subunit to incorporate stereoselectively. Due
to the potential of T. flavus as a biocontrol agent and the synthetic
and synthesis of the disaccharide subunit (A, n = 1) and the tetra-
saccharide subunit (A, n = 2) of cell wall polysaccharides of T. flavus
employing the latent-active glycosylation strategy with CB glyco-
sides and 20-(benzyloxycarbonyl)benzyl (BCB) glycosides.11
In order to establish the efficient and stereoselective method for
challenge for the construction of the
a-D-galactofuranosyl linkage,
we were interested in the synthesis of the repeating unit A. For the
construction of the a-D-galactofuranosyl linkages, galactofuranosyl
trichloroacetimidates7 and thiogalactofuranosides8 have been used
the construction of the a-galactofuranosyl linkage, we at first
examined galactosyl donors having different protective groups
such as 5,6-isopropylidene CB galactoside 1 tribenzoyl CB galact-
oside 2 and tetrabenzyl CB galactoside 3. Common starting material
4 was converted into galactosyl donor 3 and intermediates 5 and 6
by the known procedure.10 Then, benzylation of 5 followed by
selective hydrogenolysis of resulting BCB galactoside on Pd/C in
the presence of NH4OAc11 afforded 1 as shown in Scheme 1. Com-
pound 6 was converted into 2 by the following sequence: (i) benzo-
ylation of the C-3 hydroxy group of 6, (ii) removal of the TBS group
at O-2, (iii) benzylation of the resulting C-2 hydroxyl group, (iv)
hydrolysis of the 5,6-isopropylidene group, (v) benzoylation of the
resulting 5,6-diol, and (vi) selective hydrogenolysis of the benzyl
ester functionality of the BCB moiety. Galactosylations of primary
alcohol acceptors 7 and 8 with donors 1–3 were carried out
Figure 1. A repeating unit of cell wall polysaccharides of T. flavus.
* Corresponding author. Tel.: +82 2 2123 2640; fax: +82 2 365 7608.
E-mail address: kwan@yonsei.ac.kr (K. S. Kim).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.118

J. Y. Baek et al. / Tetrahedron Letters 49 (2008) 4734–4737
4735
Scheme 1. Reagents and conditions: (i) (a) BnBr, NaH, DMF, RT, 30 min, 84%; (b) H2, Pd/C, NH4OAc, CH3OH/EtOAc, RT, 1 h, 95%; (ii) (a) BzCl, DMAP, pyridine, CH2Cl2, RT,
30 min, 98%; (b) n-Bu4NF, THF, RT, 1 h, 96%; (c) BnBr, NaH, DMF, RT, 30 min, 84%; (d) TFA, THF/H2O, RT, 95%; (e) BzCl, DMAP, pyridine, CH2Cl2, RT, 30 min, 98%; (f) H2, Pd/C,
NH4OAc, CH3OH/EtOAc, RT, 1 h, 95%.
Table 1
Galactosylation of primary alcohol acceptors 7 and 8 with CB galactosides
Entry
Donor
Acceptor
Product
Yielda (%)
Ratio (a
/b)a
1
2
3
4
5
6
1
1
2
2
3
3
7
8
7
8
7
8
9
10
11
12
13
14
83
81
86
83
88
84
a
a
a
a
8:1
7:1
only
only
only
only
a
Determined after isolation.
employing Tf2O as an activator in the presence of 2,6-di-t-
butyl-4-methylpyridine (DTBMP) as shown in Table 1. Galactosyl-
ations of 7 and 8 with the 5,6-O-isopropylidene galactosyl donor 1
donor 3 were less stereoselctive to provide a mixture of
a- and
b-anomers of 13 ( /b = 8:1) and 14 ( /b = 7:1), respectively (entries
a
a
5 and 6). The result indicates that the galactosylation of the primary
alcohol acceptor with the donor 1 or 2 is more stereoselective than
that with the donor 3.
gave exclusively
a-disaccharides 9 and 10, respectively, in high
yield (entries 1 and 2 in Table 1). The galactosylations of 7 and 8
with the tribenzoyl galactosyl donor 2 were also very stereoselective
On the other hand, in the galactosylation of secondary alcohol
to afford exclusively
a-products 11 and 12 (entries 3 and 4). The
acceptor 15,12 the donor 3 turned out to be more
a-selective than
same galactosylations of 7 and 8 with the tetrabenzyl galactosyl
the donor 1 or 2 as shown in Table 2. Thus, galactosylations of 15
Table 2
Galactosylation of secondary alcohol acceptors 15 with various donors
Entry
Donor
Promoters and conditions
Product
Yielda (%)
Ratio (a
/b)a
1
2
3
4
5
6
1
2
3
19
20
21
Tf2O, DTBMP, 4 Å MS, CH2Cl2, ꢀ78 °C to RT
Same as above
Same as above
SnCl2, AgClO4, THF, ꢀ10 °C to RT
Sn(OTf)2, NIS, 4 Å MS, CH2Cl2, ꢀ78 °C to 0 °C
Tf2O, CH2Cl2, ꢀ78 °C to RT
16
17
18
18
18
18
85
80
85
82
84
93
3:1
5:1
a only
1.2:1
3:1
8:1
a
Determined after isolation.

4736
J. Y. Baek et al. / Tetrahedron Letters 49 (2008) 4734–4737
with 1 and 2 afforded a mixture of
a
- and b-anomers of 16 (
a
/
of the bulky furanose ring in the b-side at C-2 of 23 in addition
to the anomeric effect.
b = 3:1) and 17 (a/b = 5:1), respectively (entries 1 and 2 in Table
2), whereas, the reaction of 3 and 15 was completely stereoselec-
Synthesis commenced with galactosylations of 15 and 25 with
tive to provide
a-disaccharide 18 exclusively in 85% yield (entry
the tetrabenzyl galactosyl donor 3 to provide exclusively a-disac-
charides 18 and 26, respectively, both in 85% yield as shown in
3).13 We also examined donors having different anomeric leaving
groups such as tetrabenzyl galactosyl fluoride 19,14 tetrabenzyl
thiogalactoside 20,8 and tetrabenzyl galactosyl sulfoxide 21.15
Donors 19 and 20 exhibited poor stereoselectivities (entries 4
and 5 in Table 2) while galactosyl sulfoxide 21 showed reasonable
Scheme 2. Anomeric carbon chemical shifts at d 100.3 of 18 and
d 100.2 of 26 and the H10–H20 coupling constant, JH1 ꢀH2 ¼ 4:4 Hz
for both 18 and 26 clearly indicated that the newly generated
0
0
galactosyl linkage of the disaccharides is in the a
-configuration.16
stereoselectivity (
results indicated that tetrabenzyl CB galactoside 3 was superior
donor for the construction of -galactofuranosyl linkage in the
glycosylation of secondary alcohol acceptor 15. The origin of the
present protective group effect on the outcome of the stereo-
chemistry in the galactofuranosylation is not yet clear.
Based on the above study, the retrosynthesis of target tetrasac-
charide 22 and disaccharides 23 and 24 in a suitably protected
form of the repeating unit A was performed to lead to monosaccha-
ride building blocks 3, 15, and 25 as shown in Figure 2. The protec-
tive groups in the target tetrasaccharide 22 were chosen after
consideration of the future synthesis of a hexasaccharide or an
octasaccharide from 22. Thus, the naphthylmethyl (NAP) group at
C-6 of the mannose moiety in the reducing side of the tetrasaccha-
ride 22 would be selectively cleaved to provide the tetrasaccharide
acceptor. We also envisioned that coupling of 23 and 24 would
a
/b = 8:1) in its reaction with 15 (entry 6). The
Then, the reductive cleavage of the benzylidene group of 26 with
BH3ꢁTHF/Bu2BOTf17 afforded disaccharide acceptor 24 in 78% yield.
Coupling of acceptor 24 and disaccharide donor 27, which was
obtained from 18 by selective hydrogenolysis, by glycosylation
reaction employing Tf2O, however, failed to provide a desired
tetrasaccharide. We, therefore, cleaved the conformationally dis-
arming benzylidene group18 of the compound 18 with BH3ꢁTHF/
Bu2BOTf to obtain compound 28. Then, we treated compound 28
with naphthylmethyl (NAP) bromide in the presence of NaH in
order to make NAP-protected BCB disaccharide 29. Unexpectedly,
however, not only 29 (20%) but also NAP-protected CB disaccharide
23 (40%) was generated. It is quite unusual that the BCB group was
converted into the CB group under the present reaction condition
since the BCB group was very stable during the protection of
hydroxy groups with benzyl, NAP, and PMB groups in other
sugar.10–12 This unexpected result, however, led us to save one step
in the synthetic sequence by obtaining 23 directly from 28 without
preparation of the intermediate 29. Thus, when the reaction was
a
provide
a-mannosyl tetrasaccharide 22 owing to the steric effect
conducted by addition of
a small amount of H2O (26 lL,
0.02 mmol) to a mixture of 28, NAP bromide (24 mg, 0.109 mmol),
and NaH (5.8 mg, 0.146 mmol) in DMF at 50 °C. The desired NAP-
protected CB disaccharide 23 was obtained 70% yield as shown in
Scheme 2. Under this reaction condition, the more nucleophilic
alkoxide of 28 would attack NAP bromide while the hydroxide
would hydrolyze the benzyl ester of the BCB moiety to give the
NAP-protected CB glycoside 23 (Scheme 3).
Crucial coupling of 23 and 24 was carried out by slow addition
of Tf2O to a solution of 23, 24, and DTBMP in CH2Cl2 at ꢀ40 °C to
afford exclusively desired
a
-tetrasaccharide 2219 in 65% yield.
Deprotection of all benzyl groups and a NAP group of 22 by
hydrogenolysis employing Pd(OH)2 catalyst gave fully deprotected
tetrasaccharide subunit 3020 as a methyl glycoside. The stereo-
chemistry at newly generated anomeric center of the tetrasaccha-
ride 30 was determined unequivocally on the basis of the one bond
C1–H1 coupling constant: JC1 ꢀH1 ¼ 170:2 Hz.21
Figure 2. Retrosynthesis of tetrasaccharide 22.
0
0
Scheme 2. Reagents and conditions: (i) DTBMP, 4 Å MS, Tf2O, CH2Cl2, ꢀ78 °C to 0 °C, 1 h, 85% for both 18 and 26; (ii) BH3 ꢁ THF, Bu2BOTf, THF, 0 °C to RT, 30 min, 78% for 24
and 81% for 28; (iii) NAPBr, NaH, n-Bu4NI, H2O (trace), DMF, 50 °C, 1 h, 70%.

J. Y. Baek et al. / Tetrahedron Letters 49 (2008) 4734–4737
4737
8. Gelin, M.; Ferrieres, V.; Plusquellec, D. Eur. J. Org. Chem. 2000, 1423–
1431.
9. Bai, Y.; Lowary, T. L. J. Org. Chem. 2006, 71, 9658–9671.
10. Lee, Y. J.; Lee, B.-Y.; Jeon, H. B.; Kim, K. S. Org. Lett. 2006, 8, 3971–
3974.
11. Kim, K. S.; Kim, J. H.; Lee, Y. Joo; Lee, Y. Jun; Park, J. J. Am. Chem. Soc. 2001, 123,
8477–8481.
12. For preparation of 15, see: Kim, K. S.; Kang, S. S.; Seo, Y. S.; Kim, H. J.; Lee, Y. J.;
Jeong, K.-S. Synlett 2003, 1311–1314.
13. A typical procedure for a-galactosylation: A solution of the acceptor 15 (521 mg,
0.89 mmol) and DTBMP (456 mg, 2.22 mmol) in CH2Cl2 (10 mL) in the presence
of 4 Å molecular sieves was stirred for 10 min at room temperature and cooled
to ꢀ78 °C. To the resulting solution was added Tf2O (187
lL, 1.11 mmol) and
subsequently was added dropwise a solution of donor 3 (500 mg, 0.74 mmol)
in CH2Cl2 (10 mL). After stirring at ꢀ78 °C for further 1 h, the reaction mixture
was allowed to warm to 0 °C over 1 h, and quenched with saturated aqueous
NaHCO3 (1 mL).
Scheme 3. Reagents and conditions: (i) DTBMP, 4 Å MS, Tf2O, CH2Cl2, ꢀ78 °C to
0 °C, 1 h, 65%; (ii) Pd(OH)2, H2, CH3OH/CH2Cl2, RT, 95%.
14. Galactosyl fluoride 19 was prepared by fluorination of 2,3,5,6-tetra-O-benzyl-
-galactofuranose7a with diethylaminosulfur trifluoride (DAST).
Acknowledgements
D
15. Galactosy sulfoxide 21 was prepared by the oxidation of thiogalactoside 20
with mCPBA.
16. Gelin, M.; Ferriéres, V.; Plusquellec, D.; Lefeuvre, M. Eur. J. Org. Chem. 2003,
1285–1293.
17. Jiang, L.; Chan, T. H. Tetrahedron Lett. 1998, 39, 355–358.
18. Fraser-Reid, B.; Wu, Z.; Webster, A.; Skowronski, E. J. Am. Chem. Soc. 1991, 113,
1434–1435.
This work was supported by Grant from the Korea Science and
Engineering Foundation through the Center for Bioactive Molecular
Hybrids (CBMH). J.Y.B. and J.Y.J. thank the fellowship of the BK 21
program from the Ministry of Education and Human Resources
Development.
19. The procedure for the coupling of 23 and 24: A solution of the donor 23 (170 mg,
0.147 mmol), the acceptor 24 (174 mg, 0.191 mmol), and DTBMP (73 mg,
0.353 mmol) in CH2Cl2 (5 mL) in the presence of 4 Å molecular sieves was
stirred for 10 min at room temperature and cooled down to ꢀ40 °C. Then, to
References and notes
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a diluted solution of Tf2O (30 lL,
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0.176 mmol) in CH2Cl2 (1 mL) at ꢀ40 °C. After being stirred at ꢀ40 °C for
further 30 min, the reaction mixture was allowed to warm up to 0 °C over 1 h,
and quenched with saturated aqueous NaHCO3 (1 mL).
20. Compound 30: Colorless oil; Rf = 0.48 (MeOH); ½a D20
ꢂ
þ 3:0 (c 0.8, MeOH); 1H
NMR (400 MHz, MeOD) d 3.26–3.28 (m, 3H), 3.35 (s, 3H), 3.51–3.57 (m, 5H),
3.58–3.64 (m, 5H), 3.65–3.69 (m, 3H), 3.70–3.76 (m, 4H), 3.79 (dd, J = 9.6,
3.2 Hz, 1H), 4.14-4.20 (m, 2H), 4.97 (d, J = 4.4 Hz, 1H), 5.02 (d, J = 4.8 Hz, 1H),
5.04 (d, J = 1.2 Hz, 1H), 5.21 (d, J = 1.2 Hz, 1H);13C NMR (100 MHz, CD3OD) d
49.9, 55.6, 62.9, 64.7, 64.8, 67.8, 68.7, 69.1, 69.3, 72.1, 72.6, 72.8, 73.0, 74.4,
74.86, 74.93, 75.1, 78.7, 78.8, 81.7, 81.9, 83.1, 83.2, 99.9, 101.1, 104.6, 104.8;
HRMS: [M+Na]+ Calcd for C25H44O21Na: 703.2273. Found: 703.2266.
21. Bock, K.; Pedersen, C. J. Chem. Soc. Perkin Trans. 2 1974, 293–297.