Total synthesis of floridoside

Article abstract of DOI:10.1055/s-2007-984499

The native floridoside [2′-O-(α-D-galactopyranosyl)-glycerol], isolated from the red marine alga Rhodymenia palmata, has demonstrated a potent immunostimulating activity. In order to carry out further biological investigations, the synthesis of this compo

Full text of DOI:10.1055/s-2007-984499

1736
LETTER
Total Synthesis of Floridoside
Total
Synthesis
o
e
f
Floridosid
r
e
trand Thollas,* Claire Boisset
Laboratoire de Biotechnologie et Molécules Marines, IFREMER, BP 70, 29280 Plouzané, France
Fax +33(298)224757; E-mail: bertrand.thollas@ifremer.fr
Received 20 February 2007
The retrosynthetic analysis of compound 1 is depicted in
Scheme 1. The a-galactoside synthon 6 was obtained
starting from a suitable protected galactosyl donor 4 and,
as alcohol acceptor, commercially available 5. The
trichloroacetimidate 6 can be readily prepared from the
4-methoxyphenyl b-D-galactopyranoside9 (2).
Abstract: The native floridoside [2¢-O-(a-D-galactopyranosyl)-
glycerol], isolated from the red marine alga Rhodymenia palmata,
has demonstrated a potent immunostimulating activity. In order to
carry out further biological investigations, the synthesis of this com-
pound appeared necessary and is reported here, for the first time.
Key words: floridoside, 2¢-O-(a-D-galactopyranosyl)glycerol, gly-
cosylation, bioactive glycosides, antitumor agent
BnO
BnO
OBn
O
1
The native floridoside [2¢-O-(a-D-galactopyranosyl)-
glycerol, Figure 1] was first isolated in 1930 by Colin1
and Gueguen from the red marine alga Rhodymenia
palmata. Its molecular structure was first established by
BnO
O
6
O
Ph
O
BnO
BnO
OBn
O
1
Putman and Hassid2 and the complete H NMR and 13C
+
O
OH
CCl3
NMR spectra were unambiguously assigned by Simon–
Colin3 et al. Floridoside is considered to be the main pho-
tosynthetic carbon-reserve product in most red algae,4–7
where it is also believed to behave as an intracellular
osmotic regulator.
BnO
O
4
NH
Ph
O
5
OH
OH
O
Some hemolytic tests were carried out using human serum
deficient in C2, C4, or C1q proteins in order to check the
activity of the natural floridoside towards the complement
system. These tests have shown that floridoside was able
to activate the classical pathway of the complement
system.
The floridoside has an a-Gal structure, and Galili8 has
already demonstrated that an a-Gal epitope was highly
immunogenic.
OPMB
HO
2
OH
Scheme 1 Retrosynthetic analysis of floridoside
The b-galactoside 2 (Scheme 2) was 2,3,4,6-tetra-O-
benzylated by treatment with benzyl bromide and sodium
hydride in DMF. It was purified by flash chromatography
(toluene–ethyl acetate), then crystallized from hexane–
ethyl acetate and finally obtained in 85% yield.
The results obtained with the floridoside have promoted
an increasing interest and to go further in biological inves-
tigations, especially to make the coupling of floridoside to
a therapeutic antibody, larger amounts of pure 1 are there-
fore needed. We report here on a facile route for the syn-
thesis of floridoside (1), which could be easily scaled up
to several grams.
Oxidative removal of the 4-methoxyphenyl group in 3
with ceric ammonium nitrate followed by imidoylation10
with trichloroacetonitrile and 1,8-diazabicyclo[5,4,0]un-
dec-7-ene afforded a mixture of a- and b-imidate 4 in 58%
overall yield. The analytical data of compound 4 were in
accordance with those published by Schmidt11 et al.
The galactosylation of cis-1,3-O-benzylideneglycerol (5,
1.5 equiv) with imidate 4 (1 equiv) in anhydrous dichloro-
methane in the presence of trimethylsilyl trifluoro-
methanesulfonate gave, after separation by flash
chromatography (hexane–ethyl acetate), 6 in 65% yield
and 7 in 20% yield (a/b = 3:1).12
OH
OH
HO
O
OH
OH
HO
O
1
The 1H NMR spectrum of 6 clearly shows a doublet at d =
5.08 ppm with a coupling constant of 2.8 Hz correspond-
ing to the anomeric proton and demonstrates an a-stereo-
chemistry of the glycosidic bond. The 1H NMR spectrum
of 7 shows a doublet at d = 4.84 ppm with a coupling
constant of 8.0 Hz that corresponds to the anomeric
proton b.13
Figure 1 Floridoside (1)
SYNLETT 2007, No. 11, pp 1736–1738
Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-984499; Art ID: G06107ST
© Georg Thieme Verlag Stuttgart · New York
0
3
.0
7
.2
0
0
7

LETTER
Total Synthesis of Floridoside
1737
BnO
BnO
References and Notes
OBn
O
a
2
(1) Colin, H.; Guéguen, E. C. R. Hebd. Seances Acad. Sci 1930,
191, 163.
(2) Putman, E. W.; Hassid, W. Z. J. Am. Chem. Soc. 1954, 76,
2221.
OPMB
85%
BnO
58%
3
b
(3) Simon-Colin C., Kervarec N., Pichon R., Deslandes E.;
Carbohydr. Res.; 2002, 337: 279.
(4) Bean, R. C.; Hassid, M. M. J. Biol. Chem. 1955, 212, 411.
(5) Kirst, G. O.; Bisson, M. A. Aust. J. Plant Physiol. 1979, 6,
539.
(6) Ben-Amote, A.; Avron, M. Ann. Rev. Microbiol. 1983, 37,
95.
(7) Reed, R. H. In Biology of the Red Algae; Cole, K. M.;
Sheath, R. G., Eds.; Cambridge University Press:
Cambridge, 1990, 147–170.
BnO
BnO
OBn
OH
O
O
O
O
+
CCl3
NH
Ph
BnO
4
5
c
BnO
OBn
O
BnO
BnO
OBn
BnO
O
BnO
O
O
BnO
O
(8) Galli, U. Immunol. Today 1993, 14, 480.
O
O
6
65%
Ph
O
(9) Ohlsson, J.; Magnusson, G. Carbohydr. Res. 2000, 329, 49.
(10) Schmidt, R. R. Angew. Chem., Int. Ed. Engl. 1986, 25, 212.
(11) Schmidt, R. R.; Michel, J.; Roos, M. Liebigs Ann. Chem.
1984, 1343.
Ph
7
20%
Scheme 2 Reagents and conditions: a) PhCH2Br, NaH, DMF, r.t.,
2 h; b) CAN, toluene–MeCN–H2O, r.t., 20 min; then CCl3CN, DBU,
CH2Cl2, r.t., 30 min; c) 5 (1.5 equiv), TMSOTf (1 M), MS 4 Å,
CH2Cl2, r.t., 30 min.
(12) Melting points were determined in capillary tubes with a
Büchi apparatus and are uncorrected. Optical rotations are
measured at 20–25 °C with Perkin-Elmer 341 polarimeter.
1H NMR and 13C NMR spectra were recorded at 25 °C with
a Bruker DPX400 spectrometer operating at 400 and 100
MHz, respectively. Assignments were based on homo- and
heteronuclear correlations using supplier’s software. Flash
column chromatography was performed on silica gel (E.
Merck, 40–63 mm). Microanalyses were performed by the
department of microanalysis of CNRS (ICSN, Gif sur
Yvette, France).
(13) General Procedure for the Glycosylation
A mixture of trichloroacetimidate (600 mg, 0.75 mmol), cis-
1,3-O-benzylideneglycerol (315 mg, 1.75 mmol) and
activated powdered MS 4 Å in anhyd CH2Cl2 (10 mL) was
stirred for 1 h at r.t. under dry N2, then cooled to 0 °C. A
solution of TMSOTf in toluene (1 M, 131 mL, 15%) was
added, and the mixture was stirred for 30 min at r.t., then was
quenched with Et3N, filtered and concentrated. Flash silica
gel chromatography (hexane–EtOAc = 3:1, containing 0.1%
of Et3N) gave 7 (125 mg, 20%) and 6 (400 mg, 65%).
cis-1¢,3¢-O-Benzylidene-2¢-O-(2,3,4,6-tetra-O-benzyl-a-D-
galactopyranosyl)glycerol (6)
Alternative condensation conditions were also tested in
toluene at –10 °C using a trimethylsilyl trifluoromethane-
sulfonate catalysis, but the yield and the ratio a/b was not
modified.
The 1¢,3¢-O-benzylidene group was removed by acidic
hydrolysis with aqueous acetic acid (60%) at 100 °C for
30 minutes. Compound 814 was obtained in 77% yield
(Scheme 3), after purification by flash chromatography
(toluene–ethyl acetate then dichloromethane–methanol).
BnO
BnO
OBn
O
BnO
BnO
OBn
a
b
O
1
BnO
77%
O
90%
OH
OH
BnO
6
O
O
8
Ph
O
[a]D +7 (c 1.0, CHCl3). 1H NMR (CDCl3): d = 7.40–7.10 (m,
25 H, arom. H), 5.57 (s, 1 H, PhCH), 5.10 (d, 1 H, J1,2 = 2.8
Hz, H-1), 5.00–4.53 (m, 6 H, PhCH2), 4.48–4,38 (m, 5 H,
PhCH2, OCH2, OCH), 4.20–4.13 (m, 1 H, H-5), 4.10–3.90
(m, 5 H, H-2, H-3, H-4, OCH2), 3.58–3.49 (m, 2 H, H-6a, H-
6b). 13C NMR (CDCl3): d = 138.9–126.3 (arom. C), 101.4
(PhCH), 96.7 (C-1), 78.8 (C-4), 76.7 (C-2), 75.1 (C-3), 74.8–
73.0 (PhCH2), 70.2 (OCH2), 69.6 (OCH), 69.5 (C-5), 69.1
(C-6), 68.5 (OCH2). Anal. Calcd for C44H46O8: C, 74.19; H,
6.60. Found: C, 74.09; H, 6.71.
Scheme 3 Reagents and conditions: a) AcOH (60%), 100 °C, 30
min; b) H2, 10% Pd/C, EtOAc–MeOH–H2O, r.t., 18 h.
Final hydrogenation of 8 in aqueous methanol afforded
the target molecule 1 in a 90% yield.15
The 1H NMR and 13C NMR data of the synthetic florido-
side were in accordance with those published for the
natural and purified compound by Simon–Colin3 et al.
(14) 2¢-O-(2,3,4,6-Tetra-O-benzyl-a-D-galacto-
pyranosyl)glycerol (8)
In conclusion, the total synthesis of compound 1 was
successfully achieved starting from 4-methoxyphenyl b-
D-galactopyranoside (2) in five steps. This sequence easi-
ly allows the scaling up of this synthesis in order to obtain
bulk quantities of 1 for further biological studies.
1H NMR (CDCl3): d = 7.40–7.10 (m, 25 H, arom. H), 4.95
(d, 1 H, J1,2 = 3.6 Hz, H-1), 4.94–4.35 (m, 8 H, PhCH2),
4.13–4.07 (m, 2 H, H-2, H-5), 3.98 (dd, 1 H, J3,2 = 10.1 Hz,
J
3,4 = 2.8 Hz, H-3), 3.87 (d, 1 H, H-4), 3.79–3.70 (m, 1 H,
OCH), 3.60–3.50 (m, 5 H, 2 OCH2, H-6a), 3.28 (dd, 1 H,
6a,6b = 4.1 Hz, J6b,5 = 9.6 Hz, H-6b). 13C NMR (CDCl3): d =
J
138.2–127.5 (arom. C), 99.4 (C-1), 85.4 (OCH), 79.4 (C-3),
76.4 (C-2), 74.6 (C-4), 74.5-73.6 (PhCH2), 70.7 (C-5), 69.7
(C-6), 62.9 (OCH2), 62.3 (OCH2). Anal. Calcd for C37H42O8:
C, 72.99; H, 6.89. Found: C, 71.13; H, 7.42.
Synlett 2007, No. 11, 1736–1738 © Thieme Stuttgart · New York

1738
B. Thollas, C. Boisset
LETTER
(15) General Procedure for the Catalytic Hydrogenation
A mixture of compound 8 (300 mg, 0.488 mmol) and Pd/C
10% in EtOAc–MeOH–H2O (4:4:2 mL) was stirred under a
H2 atmosphere at r.t. for 18 h. The mixture was filtered,
concentrated, and co-evaporated with toluene. The
compound was eluted from a column (3.0 × 30 cm) of
Sephadex LH-20 with H2O to give 1 (112 mg, 90%) as a
white powder.
2¢-O-(a-D-Galactopyranosyl)glycerol (1)
[a]D +164 (c 1.0, H2O), lit. 2: [a]D +165. 1H NMR (D2O):
d = 5.13 (d, 1 H, J1,2 = 3.9 Hz, H-1), 4.12–4.09 (m, 1 H,
H-5), 3.95 (d, 1 H, J4,3 = 2.8 Hz, H-4), 3.87 (dd, 1 H, J3,2
=
10.1 Hz, H-3), 3.82–3.78 (m, 1 H, H-2), 3.75–3.70 (m, 7 H,
2 × OCH2, OCH, H-6a, H-6b). 13C NMR (CDCl3): d = 100.8
(C-1), 81.4 (OCH), 74.0 (C-5), 72.2 (C-3), 72.1 (C-4), 71.3
(C-2), 64.2 (OCH2), 63.9 (OCH2), 63.2 (C-6); cf. lit. 3. Anal.
Calcd for C9H18O8: C, 42.52; H, 7.14. Found: C, 42.36; H,
7.25.
Synlett 2007, No. 11, 1736–1738 © Thieme Stuttgart · New York

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