Total synthesis of agelagalastatin

Article abstract of DOI:10.1021/ol061444o

The total synthesis of-agelagalastatin, an antineoplastic glycosphingolipid, has been achieved. The synthesis involved an α-selective glycosylation of the ceramide moiety with the trisaccharide fluoride. The trisaccharide component was constructed employi

Full text of DOI:10.1021/ol061444o

ORGANIC
LETTERS
2006
Vol. 8, No. 18
3971-3974
Total Synthesis of Agelagalastatin
Yong Joo Lee, Bo-Young Lee, Heung Bae Jeon, and Kwan Soo Kim*
Center for BioactiVe Molecular Hybrids and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea
kwan@yonsei.ac.kr
Received June 13, 2006
ABSTRACT
The total synthesis of agelagalastatin, an antineoplastic glycosphingolipid, has been achieved. The synthesis involved an
r-selective glycosylation
of the ceramide moiety with the trisaccharide fluoride. The trisaccharide component was constructed employing the CB glycoside method
which permitted a completely -stereoselective galactofuranosylation.
r
Agelagalastatin (1) was isolated from the Western Pacific
marine sponge Agelas sp., and its structure was elucidated
in 1991.1 This compound is a member of a family of
glycospingolipids found in Agelas sp. which include age-
lasphin-9b,2 longiside,3 triglycosylceramide,4 and KRN7000.5
These glycosphingolipids, which commonly contain R-O-
galactopyranosyl ceramide moieties as their integral parts,
have shown immunomodulating activity.5,6 Among them,
agelasphin-9b and KRN7000 exhibited immunostimulatory
activity, which was suggested to be related to the interesting
in vivo antitumoral properties of these compounds through
an activation of the immune system,2,7 and thus, KRN7000
is in clinical trials as a novel anticancer agent.8 Agelagal-
astatin (1) displayed significant in vitro inhibitory activities
against human cancer cell growth, its GI50 values ranging
from 0.77 µg/mL for lung NCI-H460 to 2.8 µg/mL for the
ovarian OVCAR-3.1 Besides its biological activity, the
unique structure of 1 has attracted our attention as it is
composed of two galactofuranosides having R- and â-gly-
cosyl linkages and a galactopyranoside having an R-glycosyl
linkage with ceramide, as shown in structure 1. The galacto-
furanoside moiety with R-configuration is quite rare in Nature
and has been found only in a few microorganisms such as
Penicillium Varians,9 Leishmania major,10 and Talaromyces
flaVus.11 In addition, agelagalastatin (1) was isolated as a
mixture of two structural isomers (1a/1b ) 4:1) in very low
yield (7.42 × 10-6%) because of the difficulty in separation.1
Herein, we describe the first total synthesis of pure 1a and
1b.
(1) Pettit, G. R.; Xu, J.-p.; Gingrich, D. E.; Williams, M. D.; Doubek,
D. L.; Chapius, J.-C.; Schmidt, J. M. Chem. Commun. 1999, 915.
(2) Natori, T.; Morita, M.; Akimoto, K.; Koezuka, Y. Tetrahedron 1994,
50, 2771.
(3) Cafieri, F.; Fattorusso, E.; Mahajnah, Y.; Mangoni, A. Leibigs Ann.
Chem. 1994, 1187.
(4) Costantino, V.; Fattorusso, E.; Mangoni, A.; Aknin, M.; Gaydon, E.
M. Leibigs Ann. Chem. 1994, 1181.
(5) Costantino, V.; Fattorusso, E.; Mangoni, A. Tetrahedron 1996, 52,
1573.
(6) D′Ambrosio, M.; Ripamonti, M.; Debitus, C.; Waikedre, J.; Pietra,
F. HelV. Chim. Acta 1996, 79, 727.
(7) Motoki, K.; Kobayashi, E.; Uchida, T.; Fukushima, H.; Koezuka, Y.
Bioorg. Med. Chem. Lett. 1995, 5, 705.
(8) Crul, M.; Mathoˆt, R. A. A.; Giaccone, G.; Punt, C. J. A.; Rosing,
H.; Hillebrand, M. J. X.; Ando, Y.; Nishi, N.; Tanaka, H.; Schellens, J. H.
M.; Beijnen, J. H. Cancer Chemother. Pharmacol. 2002, 49, 287.
Retrosynthesis of the target compound 1 leads to three
galactoside building blocks 5-7 and a ceramide moiety 3
(Scheme 1). For the successful synthesis of 1, it is essential
to choose efficient glycosylation methods. Particularly chal-
lenging is its R-galactofuranosyl moiety, which is often a
(9) Jansson, P.-E.; Lindberg, B. Carbohydr. Res. 1980, 82, 97.
(10) McConville, M. J.; Homans, S. W.; Thomas-Oates, J. E.; Dell, A.;
Bacic, A. J. Biol. Chem. 1990, 265, 7385.
(11) Parra, E.; Jime´nez-Barbero, J.; Bernabe´, M.; Leal, J. A.; Prieto, A.;
Go´mez-Miranda, B. Carbohydr. Res. 1994, 251, 315.
10.1021/ol061444o CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/05/2006

Scheme 1. Retrosynthesis of Agelagalastatin 1
Scheme 2. Preparation of Monosaccharide Building Blocks 5
and 6
problematic subunit to incorporate stereoselectively. Atten-
tion needs to be paid to coupling the four building blocks, 3
and 5-7, in the proper and correct order. The stereospecific
construction of 1,2-cis R-galactofuranosyl and R-galactopy-
ranosyl linkages has been one of the great challenges of
D-glycoside synthesis.12 In particular, there are no reliable
methods available for the stereospecific synthesis of R-ga-
lactofuranosides, although a few attempts have been made
employing galactofuranosyl trichloroacetimidates13 and thio-
galactofuranosides14 as glycosyl donors. Our original plan
was to employ 2′-carboxybenzyl (CB) glycosides as glycosyl
donors15 for the coupling of all four components of 1. We
envisioned that the construction of the R-galactopyranosyl
linkage of 1 could be carried out by coupling between CB
trisaccharide 2 and the ceramide moiety 3 in the later stages.
We also believed that the crucial R-galactofuranosylation
might be possible in the reaction between CB galactofura-
noside 5 and 2′-(benzyloxycarbonyl)benzyl (BCB) disac-
charide 4, which would be readily obtainable from 6 and 7.
Our synthesis commenced with the preparation of three
building blocks 5-7 starting from D-galactose: compounds
5 and 6 were prepared via peracetylgalactofuranose 816 as
shown in Scheme 2 and compound 7 via 2,3,4,6-tetra-O-
acetyl-R-D-galactopyranosyl bromide (see the Supporting
Information).
BCB benzylgalactoside 11 thereafter provided the building
block 5. Protection of 10 with dimethoxypropane, on the
other hand, gave the 5,6-O-isopropylidene derivative 12 and
the subsequent protection of the diol 12 with TBDMSCl
afforded the desired 2-O-silyl ether 13 along with a small
amount of 3-O-silyl ether 14 (13/14 ) 10:1) in 90% yield.
O-Benzylation of 13 followed by O-desilylation of the
resulting 15 with Bu4NF gave alcohol 16. After O-pivaloyl-
ation of 16, the resultant BCB galactoside 17 was converted
into the building block 6. Attempts to directly convert 12
into 17 by selective O-pivaloylation of the diol 12 by using
PivCl in the presence of pyridine at 0 °C resulted in the
formation of a mixture of 2-O-, 3-O- and 2,3-di-O-pivaloyl
compounds.
To evaluate the exact reaction conditions needed and
appropriate structures for glycosyl donors and acceptors in
the crucial R-galactofuranosylation, we carried out a model
study on the reaction of the donor 5 with the simpler acceptor
16 in the place of the disaccharide acceptor 4. The model
study began with dropwise addition of a diluted solution of
Tf2O in CH2Cl2 to a solution of 5, 16, and 2,6-di-tert-butyl-
4-methylpyridine (DTBMP) in CH2Cl2 at -78 °C to afford
the desired R-disaccharide 19 exclusively in 68% yield along
with self-condensed ester 20, which probably resulted from
coupling between the carboxylate anion and the oxocarbe-
nium ion generated from 5, in 24% yield (method A in
Scheme 3). To suppress formation of the ester 20, the
glycosylation was carried out with reversal of the order of
the addition of reactants such that the concentration of 5
could be kept to a minimum during the glycosylation. Thus,
slow addition of the donor 5 to a solution of acceptor 16,
DTBMP, and Tf2O in CH2Cl2 at -78 °C afforded only the
R-galactosyl disaccharide 19 in 82% yield (method B). The
stereochemistry at the newly generated anomeric center of
Anomeric chlorination of 8 followed by coupling of the
resulting crude galactosyl chloride and benzyl 2-hydroxy-
methylbenzoate (18) afforded BCB tetraacetylgalactoside 9.
After conversion of 9 to 10 by deacetylation, the tetrol 10
was subjected to O-benzylation. Subsequent selective hy-
drogenolysis of the benzyl ester functionality15 in the resultant
(12) For a review on 1,2-cis-glycosylation, see: Demchenko, A. V.
Synlett 2003, 1225.
(13) Gelin, M.; Ferrie´res, V.; Plusquellec, D. Carbohydr. Lett. 1997, 2,
381.
(14) Gelin, M.; Ferrie´res, V.; Plusquellec, D. Eur. J. Org. Chem. 2000,
1423.
(15) (a) Kim, K. S.; Kim, J. H.; Lee, Y. J.; Lee, Y. J.; Park, J. J. Am.
Chem. Soc. 2001, 123, 8477. (b) Kim, K. S.; Park, J.; Lee, Y. J.; Seo, Y.
S. Angew. Chem., Int. Ed. 2003, 42, 459. (c) Kim, K. S.; Kang, S. S.; Seo,
Y. S.; Kim, H. J.; Lee, Y. J.; Jeong, K.-S. Synlett 2003, 1311. (d) Kwon,
Y. T.; Lee, Y. J.; Lee, K.; Kim, K. S. Org. Lett. 2004, 6, 3901. (e) Lee, Y.
J.; Lee, K.; Jung, E. H.; Jeon, H. B.; Kim, K. S. Org. Lett. 2005, 7, 3263.
(16) Ferrie´res, V.; Gelin, M.; Boulch, R.; Toupet, L.; Plusquellec, D.
Carbohydr. Res. 1998, 314, 79.
1
the disaccharide 19 was determined on the basis of its H
and 13C NMR spectral data, especially the H1′-H2′ coupling
constant (JH1′-H2′ ) 4.5 Hz) and the C1′ chemical shift (δC1′
99.0).14
3972
Org. Lett., Vol. 8, No. 18, 2006

process and its wide functional group tolerance.17 Wittig
reaction of known D-lyxose derivative 2318 with ylide CH2d
PPh3 gave olefin 24. The olefin cross-metathesis of 24 with
11-methyldodec-1-ene (39) and with 12-methyltridec-1-ene
(40) in the presence of Grubbs’ catalyst 4119 in refluxing
CH2Cl2 provided the desired trans-olefins 25 and 26 in 75%
and 76% yields, respectively (Scheme 5). After hydrogena-
Scheme 3. Glycosylation of 16 with 5 for
R-Galactofuranosylation
Scheme 5. Preparation of Ceramides 3a and 3b and Their
Glycosylations with CB Trisaccharide 2
With this promising result for the R-galactofuranosylation
with 5, the stage was set for the assembly of building blocks
5-7 to make the trisaccharide 2 (Scheme 4). Glycosylation
Scheme 4. Preparation of Trisaccharide Glycosyl Donor 2
of 7 with 6 was carried out under the conditions of method
B to give the â-disaccharide 21 exclusively in 79% yield.
Removal of the O-pivaloyl group from 21 with NaOBn gave
the alcohol 4. Then, the crucial R-galactofuranosylation of
the acceptor 4 with the donor 5 was successfully executed
under the conditions of method B to afford the desired
R-galactofuranosyl trisaccharide 22 (JH1′-H2′ ) 4.6 Hz and
δC1′ 98.5) exclusively in 91% yield. No â-trisaccharide was
detected at all in the reaction mixture. The BCB trisaccharide
22 was converted into the CB trisaccharide 2 by selective
hydrogenolysis. It is noteworthy that the reaction of the CB
galactofuranoside 5 with the disaccharide 4 provided only
the R-trisaccharide 22 in excellent yield, which is the first
example of a completely stereoselective R-galactofuranosyl-
ation.
tion of 25, the hydroxy group in 27 was activated as an
O-triflate, and this subjected to an SN2 reaction with
tetramethylguanidinium azide (TMGA)20 to give azido
compound 29 with inverted configuration. Removal of the
O-isopropylidene group of 29 by trifluoroacetic acid (TFA)
followed by benzoylation of the resulting diol afforded
(17) (a) Torssell, S.; Somfai, P. Org. Biomol. Chem. 2004, 2, 1643. (b)
Rai, A. N.; Basu, A. Org. Lett. 2004, 6, 2861. (c) Rai, A. N.; Basu, A. J.
Org. Chem. 2005, 70, 8228. (d) Chaudhari, V. D.; Ajish Kumar, K. S.;
Dhavale, D. D. Org. Lett. 2005, 7, 5805.
(18) Cheng, X.; Khan, N.; Mootoo, D. R. J. Org. Chem. 2000, 65, 2544.
(19) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953.
Our approach to the synthesis of ceramides 3a and 3b
made use of the olefin cross-metathesis reaction to install
the main carbon chain. Recently, the cross-metathesis ole-
fination for the synthesis of sphingosines has become
attractive due to the high E-selectivity and good yield of the
(20) Tuch, A.; Sanie´re, M.; Merrer, Y. L.; Depezay, J.-C. Tetrahedron:
Asymmetry 1996, 7, 897.
Org. Lett., Vol. 8, No. 18, 2006
3973

compound 31 which, after hydrogenolysis, gave amine 33.
The homologue of 33, compound 34, was readily obtained
in a like fashion from 26. Coupling reaction of 33 with (R)-
2-acetoxypentacosanoic acid (35) using 2-ethoxy-1-ethoxy-
carbonyl-1,2-dihydroquinoline (EEDQ)21 afforded amide 37,
and same reaction of 34 with (R)-2-acetoxytetracosanoic acid
(36) provided amide 38. Finally, a desilylation of 37 and 38
with Bu4NF gave alcohols 3a and 3b, respectively.
Scheme 6. Synthesis of 1a and 1b
Glycosylation of the ceramide 3a with the CB trisaccharide
2 afforded the desired protected agelagalastatin 42aR (δC1
100.6) in 45% yield, along with undesired â-anomer 42aâ
(δC1 104.5) in 32% yield, and the reaction of ceramide 3b
with 2 also gave a mixture of R- and â-anomers 42b with
the same ratio (R/â ) 1.4:1) in 75% yield. Poor stereo-
selectivity in the glycosylation of 3 with the CB glycoside 2
led us to examine other glycosyl donors that could be readily
prepared from 2. We envisaged that sequential treatment of
the CB glycoside 2 with Tf2O and then with a fluoride
reagent would give the glycosyl fluroride. Reaction of 2 with
Tf2O and then with (diethylamino)sulfur trifluoride (DAST)
as a fluoride source afforded a mixture of R- and â-glycosyl
fluoride 43 in 48% yield, whereas the same reaction with
hydrogen fluoride pyridine as a fluoride source was quite
efficient giving 43 in 83% (â/R ) 20:1) yield (Scheme 6).
Glycosylations of 3a and 3b with 43 as the glycosyl donor
were carried out by dropwise addition of a solution of 43 in
THF to a solution of acceptor 3a or 3b, SnCl2, AgClO4,22
and DTBMP in THF at -10 °C to afford desired R-glyco-
sides 42aR and 42bR exclusively in 72% and 73% yield,
respectively. The deprotection of 42aR started with removal
of the O-benzylidene and O-isopropylidene groups using
TFA and was followed by hydrogenolysis to remove the
benzyl groups. Finally, the O-acetyl and O-benzoyl protecting
groups were removed using sodium methoxide in MeOH and
CH2Cl2 to give the target compound 1a. The other target
compound 1b was obtained from 42bR by the same
deprotection sequence under the same reaction conditions.
Particularly, the glycosylation of 4 with the CB galactofura-
noside 5 showed complete R-stereoselectivity, giving only
the R-galactofuranosyl trisaccharide 22. On the other hand,
for the coupling of the trisaccharide moiety and the ceramide
part, trisaccharyl fluoride 43 was utilized as the glycosyl
donor to afford R-glycosides 42aR and 42bR exclusively.
For the synthesis of ceramides 3a and 3b, we employed
olefin cross metathesis to install the main carbon chain
efficiently. The biological activity of each component of
agelagalastatin, 1a and 1b, will be reported in a separate
communication.
The H NMR and 13C NMR spectra of the authentic
1
agelagalastatin isolated by Pettit and of our synthetic
agelagalstatin (1) were identical in all respects.23 Also, the
optical rotations of synthetic compounds 1a and 1b were
measured to be [R]D +58.8 (c 0.65, CH3OH) and [R]D +59.8
(c 0.45, CH3OH), respectively, which is exactly matched with
the reported value of [R]D +59 (c 0.65, CH3OH) for the
authentic agelagalastatin, which is a mixture of 1a and 1b
with a ratio of 4:1.
Acknowledgment. This work was supported by a grant
from the Korea Science and Engineering Foundation through
Center for Bioactive Molecular Hybrids (CBMH). We also
thank to Professor G. R. Pettit (Arizona State University)
for providing copies of the NMR spectra of the natural
agelagalastatin.
We have thus achieved the first total synthesis of both
the major component 1a and the minor component 1b of
agelagalastatin (1). For the construction of the trisaccharide
moiety 2, we made use of the CB glycoside method.
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(21) Belleau, B.; Malek, G. J. Am. Chem. Soc. 1968, 90, 1651.
(22) For SnCl2/AgClO4 as the promoter for glycosyl fluoride, see:
Mukaiyama, T.; Murai, Y.; Shoda, S. Chem. Lett. 1981, 431.
(23) See the Supporting Information.
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