Synthesis of PI-88 analogue using novel O-glycosidation of exo-methylenesugars

Article abstract of DOI:10.1016/j.tetlet.2005.03.016

Acid-promoted α-stereoselective O-glycosidation of 1-exo-methylenesugars was successfully applied to the synthesis of a PI-88 analogue. By using methanesulfonic acid as a promoter, 1′-C-methyl- α-disaccharides with p-methoxybenzyl protection were obtained in high yield. The sequence of selective deprotection and glycosidation provided 1-C-methyl-pentasaccharide efficiently.

Full text of DOI:10.1016/j.tetlet.2005.03.016

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3033–3036
Synthesis of PI-88 analogue using novel O-glycosidation
of exo-methylenesugars
Rie Namme, Takashi Mitsugi, Hideyo Takahashi and Shiro Ikegami*
School of Pharmaceutical Sciences, Teikyo University, Sagamiko, Kanagawa 199-0195, Japan
Received 17 January 2005; revised 1 March 2005; accepted 3 March 2005
Available online 18 March 2005
Abstract—Acid-promoted a-stereoselective O-glycosidation of 1-exo-methylenesugars was successfully applied to the synthesis of a
PI-88 analogue. By using methanesulfonic acid as a promoter, 10-C-methyl-a-disaccharides with p-methoxybenzyl protection were
obtained in high yield. The sequence of selective deprotection and glycosidation provided 1-C-methyl-pentasaccharide efficiently.
Ó 2005 Elsevier Ltd. All rights reserved.
In this decade, the development of new drug candidates
involving tumor metastasis and angiogenesis inhibitory
properties has been an attractive approach to cancer
therapy. Heparanase is one of the key enzymes impli-
cated in this issue. It has been known that sulfated poly-
saccharides such as heparin, dextran sulfate and xylan
sulfate are effective inhibitors of heparanase and thus
suppress tumor metastasis.1 In 1999, Parish et al. devel-
oped phosphomannopentaose sulfate (PI-88) (1) from
the yeast P. holstii, which consists of five mannose units
linked with a-glycosidic bonds (Fig. 1). It shows potent
heparanase inhibitory activity and antiangiogenesis
properties, and is undergoing a Phase II clinical pro-
gram in metastatic melanoma.2
We have previously demonstrated an efficient O-gly-
cosidation of 1-exo-methylenesugars promoted by
trifluoromethanesulfonic acid (TfOH).3 Using glucose-
derived glycosyl acceptors, a-ketodisaccharides were
obtained in excellent yield. It was the first example of
acid-promoted O-glycosidation of exo-glycals.4–6 It should
be noted that the glycosidic linkages are formed in a com-
pletely a-stereoselective manner. Taking advantage of
this a-selective glycosidation, we explored the synthesis
of a ketoside type of oligosaccharides. For our attempt
to synthesize new biologically active compounds, the
synthesis of 1-C-methyl-substituted mannosyl pentasac-
charide 2 which mimics PI-88, was undertaken utilizing
our a-stereoselective O-glycosidation of exo-glycals.
OPO3Na2
OR
OPO3Na2
OR
O
O
OR
OR
O
OR
OR
O
RO
RO
RO
RO
OR
OR
O
OR
OR
O
RO
RO
O
O
OR
OR
O
OR
OR
O
RO
RO
O
O
RO
RO
O
O
O
O
RO
RO
RO
RO
RO
RO
O
O
OR
OR
2: Methyl-substituted analogue of PI-88
(R = SO3Na or H)
1: Phosphomannopentaose sulfate (PI-88)
(R = SO3Na or H)
Figure 1.
Keywords: 1-exo-Methylenesugar; O-Glycosidation; PI-88; Heparanase inhibitor.
*
Corresponding author. Tel.: +81 426 85 3728; fax: +81 426 85 1870; e-mail: shi-ike@pharm.teikyo-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.016

3034
R. Namme et al. / Tetrahedron Letters 46 (2005) 3033–3036
Table 1. The glycosidation of 3 and 4 using various acids as a promoter
OBn
O
PMBO
BnO
promoter
MS4A
OBn
OBn
O
OBn
O
OBn
O
PMBO
BnO
BnO
BnO
BnO
+
BnO
BnO
HO
O
CH2Cl2
O
O
3 (1.0 eq)
4 (1.5 eq)
5
Entry
Promoter
(Mol %)
Temp (°C)
Time
5
6
7
8 Yield (%)
1
2
3
4
5
6
7
TfOH
10
10
10
20
20
50
20
ꢀ78
30 min
20 min
1 h
49
—
45
38
77
15
34
14
67
—
—
—
—
—
4
—
—
—
17
—
46
29
TMSOTf
TMSOTf
MsOH
MsOH
TFA
0
40
—
—
—
—
—
ꢀ78
0
20 min
90 min
7 h
ꢀ78
0
CSA
ꢀ78 ! ꢀ5
8 h
O
OBn
O
OBn
OBn
O
PMBO
BnO
PMBO
BnO
PMBO
BnO
OBn
OBn
O
OBn
OBn
OBn
O
BnO
O
O
BnO
a
OBn
BnO
BnO
BnO
OBn
PMBO
O
BnO
O
BnO
OH
O
O
OBn
8
6
7
O
b
5
9
Figure 2.
OBn
O
PMBO
BnO
OBn
OBn
O
BnO
OBn
OBn
O
BnO
O
+
9
4
For the construction of 1-C-methyl-substituted oligosac-
charides, the p-methoxybenzyl (PMB) group was used to
differentiate hydroxyl groups in 1-exo-methylenesugars.
After glycosidation with a hydroxylactone acceptor,
the obtained disaccharides can be converted into the
corresponding glycosyl donors by the methylenation
and also into the acceptors by the selective removal of
PMB. Although TfOH gave the best result in the case
of O-glycosidation of O-benzyl protected methylenesu-
gars,3a we found that O-glycosidation of 6-O-(p-meth-
oxybenzyl)-1-exo-methylenesugar 3 with the hydroxy-
lactone 4 under strongly acidic conditions was not
promising (Table 1).7 When strong acids such as TfOH
and trimethylsilyl trifluoromethanesulfonate (TMSOTf)
were used as a promoter, the 10-C-methyl-a-disaccharide
58 was obtained in low yield. Probably, cleavage of the
PMB ether took place and cyclized product 6 was
formed (Fig. 2, entries 1 and 2). Interestingly, the inter-
molecular migration of the PMB group occurred at the
same time to afford 3-O-(p-methoxybenzyl)-mannono-
lactone 7. Trifluoroacetic acid (TFA) did not promote
O-glycosidation sufficiently and the hydrated product 8
was formed instead (entry 6). After a survey of various
acids, it was found that methanesulfonic acid (MsOH)
was the best promoter, which did not cause the removal
of PMB and promoted the desired O-glycosidation
efficiently with a-stereoselectivity (entry 5).
BnO
O
10
O
Scheme 1. Reagents and conditions: (a) Cp2TiMe2, toluene, 80 °C,
67%; (b) MsOH, MS4A, CH2Cl2, ꢀ78 °C, 36%.
reactivity, and the corresponding trisaccharide 10 was
obtained in 36% yield (Scheme 1). Therefore, we gave
up this strategy.
Next, we tried glycosidation of monosaccharidic exo-
glycals with oligosaccharidic acceptors. The 30-O-(p-
methoxybenzyl)-disaccharide 138 was similarly prepared
by the glycosidation of 12 with 11, then selectively
deprotected with DDQ to provide the disaccharidic
glycosyl acceptor 14. In contrast to the glycosidation
of the disaccharidic exo-glycal 9, the glycosidation of
12 with the disaccharide 14 proceeded smoothly in
the same manner and the trisaccharide 15 was obtained
as a single isomer. Furthermore, the chain-elongation
sequence, involving removal of PMB and subsequent
glycosidation, furnished the pentasaccharide 19 in
25% overall yield from 11 in seven steps (Scheme 2).
In conclusion, we have designed a 1-C-methyl-substi-
tuted analogue of PI-88, and a synthetic study was
explored based on the novel O-glycosidation of exo-
glycals. It was found that MsOH was the best promoter
in this glycosidation for the substrate containing the
PMB ether, and thus mannosyl pentasaccharide 19
was synthesized. To study the biological activities,
further conversion to the PI-88 analogue is under
investigation.
Then our attention was focused on sugar chain-elong-
ation. At first, methylenation of a disaccharide lactone
and the subsequent glycosidation were examined. The
methylenation of the disaccharide 5 with Cp2TiMe2
afforded disaccharide donor 9,9,3b then the second
glycosidation was tried. To our disappointment, disaccha-
ridic exo-glycals were not suitable for the MsOH-
promoted glycosidation with alcohols. The O-glycosid-
ation of the exo-glycal 9 resulted in drastic decrease of
Typical experimental procedure: To a mixture of 1-exo-
methylenesugar 12 (104 mg, 0.18 mmol), hydroxylactone
11 (68 mg, 0.15 mmol) and molecular sieves 4A (90 mg)

R. Namme et al. / Tetrahedron Letters 46 (2005) 3033–3036
3035
OBn
BnO
O
OBn
BnO
OBn
OBn
BnO
OBn
RO
BnO
O
BnO
O
O
BnO
RO
BnO
PMBO
O
12
a 97%
OH
O
BnO
O
O
a 97%
BnO
BnO
BnO
BnO
BnO
O
O
BnO
BnO
O
BnO
O
O
11
13: R = PMB
14: R = H
15: R = PMB
16: R = H
b 87%
b 82%
OBn
O
PMBO
BnO
OBn
O
OBn
BnO
BnO
RO
OBn
OBn
BnO
OBn
O
OBn
OBn
O
BnO
OBn
PMBO
BnO
BnO
3
O
O
OBn
BnO
O
O
OBn
OBn
OBn
O
BnO
O
O
BnO
O
BnO
a 80%
c 85%
O
O
BnO
BnO
BnO
O
O
BnO
BnO
BnO
O
O
17: R = PMB
18: R = H
19
b 76%
O
Scheme 2. Synthesis of pentasaccharide. Reagents and conditions: (a) 12 (1.5 equiv), MsOH, MS4A, CH2Cl2, ꢀ78 °C; (b) DDQ, CH2Cl2, H2O, 0 °C;
(c) 3 (1.5 equiv), MsOH, MS4A, CH2Cl2, ꢀ78 °C.
(b) Noort, D.; Veeneman, G. H.; Boons, G.-J. P. H.; van
in CH2Cl2 (1.8 mL), were added MsOH (2.0 lL,
der Marel, G. A.; Mulder, G. J.; van Boom, J. H. Synlett
0.03 mmol) at ꢀ78 °C. The reaction mixture was stirred
7. Cleavage of a PMB ether under acidic conditions has been
1990, 205.
for 90 min at ꢀ78 °C, then quenched with triethylamine.
After removal of the solvent, the residue was purified
reported: Greene, T. W.; Wuts, P. G. M. Protective Groups
by silica gel column chromatography (ethyl acetate/n-
in Organic Synthesis, 3rd ed.; John Wiley & Sons: New
hexane 1:3) to give disaccharide 13 (149 mg, 97%).
York, 1999.
8. Characterization data: Compound 5: 1H NMR (CDCl3,
600 MHz): d 7.36–7.09 (m, 32H, ArH), 6.73–6.71 (m, 2H,
Acknowledgements
ArH), 5 (d, 1H, J = 11.3 Hz, ArCH2–), 4.84 (d, 1H,
J = 11.5 Hz, ArCH2–), 4.73 (d, 1H, J = 11.3 Hz, ArCH2–),
4.73 (d, 1H, J = 11.5 Hz, ArCH2–), 4.62 (d, 1H,
J = 11.3 Hz, ArCH2–), 4.60 (d, 1H, J = 11.3 Hz, ArCH2–),
4.52–4.30 (m, 11H), 4.23 (d, 1H, ArCH2–), 4.03 (m, 1H,
50-H), 4.94 (m, 1H, 4-H), 3.88 (dd, 1H, J = 9.62, 9.62 Hz,
40-H), 3.73 (s, 3H, ArOCH3), 3.62 (dd, 1H, J = 10.7, 4.9 Hz,
6-H), 3.54 (dd, 1H, J = 10.7, 4.9 Hz, 6-H), 3.36 (m, 3H),
1.29 (s, 3H, –CH3); 13C NMR (CDCl3, 150 MHz): d 169.2,
158.9, 1309, 138.8, 138.7, 137.6, 137.0, 136.9, 130.8, 129.3,
128.5, 128.4, 128.3, 128.3, 128.2, 128.2, 128.1, 128.0, 128.0,
127.9, 127.8, 127.7, 127.7, 127.3, 127.2, 113.5, 102.4, 80.7,
79.8, 79.5, 74.9, 74.7, 74.4, 74.2, 74.0, 73.4, 73.2, 73.0, 72.7,
72.6, 72.0, 70.1, 69.5, 68.7, 55.2, 22.4. Compound 13: 1H
NMR (CDCl3, 600 MHz): d 7.35–7.16 (m, 30H, ArH),
7.13–7.10 (m, 2H, ArH), 6.84–6.81 (m, 2H, ArH), 4.95 (d,
1H, J = 11.6 Hz, ArCH2–), 4.85–4.82 (m, 2H, ArCH2–),
4.78 (d, 1H, J = 11.3 Hz, ArCH2–), 4.70 (d, 1H,
J = 11.3 Hz, ArCH2–), 4.62 (d, 1H, J = 12.6 Hz, ArCH2–),
4.61 (d, 1H, J = 11.6 Hz, ArCH2–), 4.55–4.51 (m, 4H,
ArCH2–), 4.47 (d, 1H, J = 12.1 Hz, ArCH2–), 4.43–4.41 (m,
1H, 30-H), 4.40 (d, 1H, J = 12.1 Hz, ArCH2–), 4.33 (d, 1H,
J = 11.6 Hz, ArCH2–), 4.29 (d, 1H, J = 11.6 Hz, ArCH2–),
4.13 (ddd, 1H, J = 4.1, 4.1, 7.4 Hz, 5-H), 4.04 (ddd, 1H,
J = 3.9, 3.9, 9.6 Hz, 50-H), 3.88 (dd, 1H, J = 9.6, 9.4 Hz, 40-
H), 3.82 (dd, 1H, J = 1.1, 2.5 Hz, 3-H), 3.80 (dd, 1H,
J = 1.1, 7.4 Hz, 4-H), 3.71 (s, 3H, ArOCH3), 3.63 (m, 1H, 2-
H), 3.64–3.63 (m, 2H, 60-H), 3.57–3.56 (m, 2H, 6-H),
1.15 (s, 3H, –CH3); 13C NMR (CDCl3, 150 MHz): d 169.0
(1-C), 159.1, 138.7, 138.5, 137.7, 137.5, 136.9, 131.0, 129.5,
128.5, 128.4, 128.3, 128.2, 128.1, 128.1, 127.9, 127.9, 127.8,
127.7, 127.6, 127.5, 127.3, 127.2, 113.7, 102.7 (10-C), 80.5
(30-C), 79.2 (20-C), 78.1 (5-C), 77.5 (3-C), 75.8 (4-C), 74.7
(ArCH2–), 74.5 (40-C), 73.9 (ArCH2–), 73.4 (ArCH2–), 73.1
(50-C), 73.0 (ArCH2–), 72.9 (ArCH2–), 72.1 (ArCH2–), 71.9
We are grateful to Miss J. Shimode, Miss M. Kitsukawa
and Miss A. Tonoki for the spectroscopic measurements.
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan.
References and notes
1. Coombe, D. R.; Parish, C. R.; Ramshaw, I. A.; Anowden,
J. M. Int. J. Cancer 1987, 39, 82.
2. (a) Parish, C. R.; Freeman, C.; Brown, K. J.; Francis, D. J.;
Cowden, W. B. Cancer Res. 1999, 59, 3433; For a review,
see: (b) Khachigian, L. M.; Parish, C. R. Cardiovasc. Drug
Rev. 2004, 22, 1, PI-88 has received orphan drug designa-
tion from FDA.
3. (a) Li, X. L.; Ohtake, H.; Takahashi, H.; Ikegami, S.
Tetrahedron 2001, 57, 4283; (b) Li, X. L.; Ohtake, H.;
Takahashi, H.; Ikegami, S. Synlett 2001, 1885.
4. (a) Chang, C.-F.; Yang, W.-B.; Chang, C.-C.; Lin, C.-H.
Tetrahedron Lett. 2002, 43, 6515; (b) Lin, H.-C.; Yang,
W.-B.; Gu, Y.-F.; Chen, C.-Y.; Wu, C.-Y.; Lin, C.-H. Org.
Lett. 2003, 5, 1087.
5. (a) Colinas, P. A.; Lieberknecht, A.; Bravo, R. D. Tetra-
hedron Lett. 2002, 43, 9065; (b) Colinas, P. A.; Ponzinibbio,
A.; Lieberknecht, A.; Bravo, R. D. Tetrahedron Lett. 2003,
44, 7985.
6. Before our report, there were two papers on the O-
glycosidation of 1-exo-methylenesugars. Enzymatic reac-
tion, see: (a) Schlesselmann, P.; Fritz, H.; Lehmann, J.;
Uchiyama, T.; Brewer, C. F.; Hehre, E. J. Biochemistry
1982, 21, 6606; the reaction promoted by iodonium ion, see:

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R. Namme et al. / Tetrahedron Letters 46 (2005) 3033–3036
(ArCH2–), 69.4 (60-C), 68.7 (6-C), 67.8 (2-C), 55.1
(ArOCH3), 21.9 (–CH3).
9. (a) Petasis, N. A.; Bzowej, E. I. J. Am. Chem. Soc. 1990,
112, 6392; (b) Petasis, N. A.; Lu, S.-P. Tetrahedron Lett.
1995, 36, 2393; For the preparation procedure of
Cp2TiMe2, see: (c) Payack, J. F.; Hughes, D. L.; Cai, D.;
Cottrell, I. F.; Verhoeven, T. R. Org. Prep. Proced. Int.
1995, 27, 707.