Synthesis of (2S,3R,11S,12R,2?R,11?S,12?R)-plakoside A, a prenylated and immunosuppressive marine galactosphingolipid with cyclopropane-containing alkyl chains

Article abstract of DOI:10.1016/S0040-4039(01)00175-7

Plakoside A (1) is a prenylated galactosphingolipid isolated from the marine sponge Plakortis simplex and is strongly immunosuppressive without cytotoxicity. (2S,3R,11S,12R,2?R,11?S,12?R)-plakoside A (1) was synthesized by combining sphingosine part 16, α-hydroxy acid part 24 and prenylated sugar part 29.

Full text of DOI:10.1016/S0040-4039(01)00175-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2357–2360
Synthesis of (2S,3R,11S,12R,2§R,11§S,12§R)-plakoside A, a
prenylated and immunosuppressive marine galactosphingolipid
with cyclopropane-containing alkyl chains
Masanori Seki, Akihiro Kayo and Kenji Mori*
Department of Chemistry, Science University of Tokyo, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan
Received 27 December 2000; accepted 26 January 2001
Abstract—Plakoside A (1) is a prenylated galactosphingolipid isolated from the marine sponge Plakortis simplex and is strongly
immunosuppressive without cytotoxicity. (2S,3R,11S,12R,2§R,11§S,12§R)-plakoside A (1) was synthesized by combining sphin-
gosine part 16, a-hydroxy acid part 24 and prenylated sugar part 29. © 2001 Elsevier Science Ltd. All rights reserved.
Plakosides
thyl-2¦-butenyl)-b-
11§R*,12§S*}-2§-hydroxy-11§,12§-methylene-5§-doco-
senamido}-11,12-methylene-1,3-docosanediol, 1] and B
were isolated in 1997 by Fattorusso and coworkers as
the metabolites of the Caribbean sponge Plakortis sim-
plex.1 Their unique structures (Fig. 1) as glycosphin-
A
[(2S,3R,11R*,12S*)-1-O-{2%-O-(3¦-me-
-galactopyranosyl}-2-{(2§R,5§Z,
ters of the two cyclopropane moieties was unknown
except that they are cis-disubstituted cyclopropanes, we
took the liberty of synthesizing (2S,3R,11S,12R,
2§R,11§S,12§R)-plakoside A (1), because of the avail-
ability of (1S,2R)-1-acetoxymethyl-2-hydroxymethyl-
cyclopropane (3) using an enzymatic method.4–6
Obviously, plakoside A (1) can be synthesized by con-
necting three building blocks, the sphingosine part, the
a-hydroxy acid part and the sugar part. In late 2000,
D
golipid with a prenylated
D-galactose and cyclo-
propane-containing alkyl chains, together with its
strong immunosuppressive activity without cytotoxicity,
have made their synthesis very attractive. Indeed, in
addition to the further isolation work by Fatturusso to
identify plakosides C and D,2 an attempt was made by
Li et al. to synthesize analogs of 1.3 The fact that only
5 mg of 1 could be secured from 57 g (dry weight) of
the sponge1 also encouraged us to explore a synthetic
route leading to 1.
Nicolaou et al. reported
a synthesis of (2S,3R,
11R,12S,2§R,11§R,12§S)-plakoside A, a diastereomer
of our target 1, by constructing the cyclopropane moie-
ties via a Charette reaction.7
Scheme 1 summarizes the synthesis of the sphingosine
part 16. Enzymatic acetylation of meso-diol 2 with
vinyl acetate in the presence of lipase AK (Amano)
gave monoacetate (1S,2R)-3, [h]2D1 −19.9 (c=1.65,
CHCl3), whose enantiomeric purity was >99.9% ee as
Since the absolute configuration of the stereogenic cen-
OH
OH
O
H
H
O
cis
(CH2)9CH3
(CH2)9CH3
(CH2)9CH3
OH
OH
HO
HO
HO
HO
NH
NH
O
O
O
(CH2)9CH3
O
cis
O
O
H
H
OH
OH
Plakoside B
(2S,3R,11S,12R,11"'S,12"'R)-Plakoside A (1)
Figure 1. Structures of plakosides A and B.
Keywords: immunosuppressive compounds; marine metabolites; sphingolipids; sponges.
* Corresponding author. Tel.: +81 3 3260-4271; fax: +81 3 3235 2214.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00175-7

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M. Seki et al. / Tetrahedron Letters 42 (2001) 2357–2360
H
H
H
H
c
e
RO
OR'
X
2 R= R'= H
a
b
5 X= CH2OH
6 X= CHO
d
g
3 R= Ac, R'= H
4 R= Ac, R'= Ts
H
H
H
H
i
RO
X
(CH2)9CH3
(CH2)9CH3
9 X= OH
7 R= TBDPS
8 R= H
b
h
f
10 X= OTs
11 X= I
CHO
OH
O
H
H
NHBoc
H
H
13
(CH2)9CH3
O
(CH2)9CH3
NHBoc
j,g
14
12
OR
H
H
k
RO
(CH2)9CH3
NH2
15 R= H
l
16 R= TBS
Scheme 1. Synthesis of the sphingosine part 16. Reagents: (a) vinyl acetate, lipase AK, THF (86%); (b) TsCl, C5H5N, CH2Cl2;
(c) Me(CH2)8MgBr, Li2CuCl4, THF (85%, two steps); (d) (COCl)2, DMSO, Et3N, CH2Cl2; (e) TBDPSO(CH2)4PPh3Br, n-BuLi,
THF (98%, two steps); (f) TBAF, THF (98%); (g) N2H4, H2O2, EtOH, H2O (94% for 9; 85% for 14); (h) NaI, DMF (90%, two
steps); (i) LiCꢀCH·H2N(CH2)2NH2, DMSO (88%); (j) 13, n-BuLi, THF, HMPA (75%); (k) dil. HCl, MeOH (quant.); (l) TBSOTf,
2,6-lutidine, CH2Cl2 (91%).
determined by HPLC analysis (Chiralcel® OD-H).4–6,8
Tosylation of 3 afforded 4, which was treated with
nonylmagnesium bromide under Schlosser conditions9 to
furnish alcohol 5, [h]D19 −20.7 (c=1.04, CHCl3). Swern
oxidation of 5 to give aldehyde 6 was followed by Wittig
reaction, yielding 7. Removal of the TBDPS protective
group of 7 afforded olefinic alcohol 8, whose diimide
reduction provided alcohol 9, [h]2D0 −5.5 (c=1.81,
CHCl3). Iodide 11 was derived from 9 via the correspond-
ing tosylate 10, which was treated with a lithium
acetylide–ethylenediamine complex to give alkyne 12,
[h]2D1 +0.65 (c=1.33, CHCl3). Coupling of 12 with Garner
aldehyde 13 derived from (S)-serine10 was executed under
standard conditions11 to afford 14, [h]2D5 −34.2 (c=1.11,
CHCl3), as the sole product after diimide reduction and
chromatographic purification.
(Z)-alkene 20, [h]2D3 −8.9 (c=1.44, CHCl3), as the only
product as determined by 13C NMR analysis. Removal
of the acetonide protective group of 20 was followed by
silylation of the resulting diol 21 to give 22, [h]2D2 +10.7
(c=0.86, CHCl3).
Treatment of 22 with trifluoroacetic acid afforded a
mixture of 21–23 from which 23 could be separated by
silica gel chromatography. Two-step oxidation of 23 with
Dess–Martin periodinane and sodium chlorite yielded
acid 24. Acylation of 16 with 24 proceeded in the presence
of DCC and DMAP to furnish tris-TBS protected
ceramide 25, [h]2D3 +7.5 (c=0.17, CHCl3). Mono-depro-
tection of the TBS protective group of 25 under acidic
conditions afforded 26, the protected ceramide part.
Scheme 3 summarizes the preparation of the galactosyl
donor 29 and completion of the synthesis of
(2S,3R,11S,12R,2§R,11§S,12§R)-plakoside A (1). Pen-
Treatment of 14 with dilute hydrochloric acid yielded
sphingosine 15 as its hydrochloride, whose hydroxy
groups were protected as TBS ethers to furnish 16, one
of the three building blocks for 1.
taacetyl b- -galactopyranose (27) was converted to the
D
known C-2% monochloroacetyl-protected bromide 29 via
28.13 Glycosidation of ceramide 26 with 29 under conven-
tional Ko¨nigs–Knorr conditions was followed by selec-
tive removal of the chloroacetyl group at C-2% of 30 to
give 31. Prenylation of 31 with 1-(1-imino-2,2,2-
trichloroethoxy)-3-methyl-2-butene in the presence of
boron trifluoride etherate3 gave bis-TBS and triacetyl-
protected compound 32.
Synthesis of the a-hydroxy acid part 24 and its coupling
with the sphingosine part 16 to give the ceramide part
26 are summarized in Scheme 2. Alcohol 9 was converted
to phosphonium salt 18 via bromide 17. The Wittig
reagent generated from 18 reacted with aldehyde 19
(prepared from
D
-glutamic acid in four steps)12 to give

M. Seki et al. / Tetrahedron Letters 42 (2001) 2357–2360
2359
O
O
CHO
O
H
H
H
H
19
d
O
X
(CH2)9CH3
c
(CH2)9CH3
20
9 X= OH
17 X= Br
a
b
18 X= PPh3·Br
OTBS
H
H
OR'
H
H
h
g
RO
HO2C
(CH2)9CH3
(CH2)9CH3
24
21 R= R'= H
e
22 R= R'= TBS
f
23 R= H, R'= TBS
OTBS
OTBS
H
H
H
H
O
O
(CH2)9CH3
(CH2)9CH3
(CH2)9CH3
f
NH
NH
HO
TBSO
(CH2)9CH3
H
H
H
H
OTBS
OTBS
26
25
Scheme 2. Synthesis of the ceramide part 26. Reagents: (a) CBr4, PPh3, CH2Cl2 (quant.); PPh3, MeCN (99%); (c) n-BuLi, THF
(57%); (d) dil. HCl, THF (quant.); (e) TBSCl, imidazole, DMF (95%); (f) 10% TFA, THF (46% for 22 with 43% of 23 and 10%
recovery of 21; 45% for 26 and 33% recovery of 25); (g) (i) Dess–Martin periodinane; (ii) NaClO2, NaH2PO4, 2-methyl-2-butene,
t-BuOH, H2O (69%, two steps); (h) 16, DCC, DMAP, CH2Cl2 (55%).
OTBS
H
H
OAc
O
OAc
O
AcO
AcO
AcO
AcO
O
a
e
c
(CH2)9CH3
OAc
O
AcO
AcO
OAc
NH
ClAcO
OAc
27
O
(CH2)9CH3
X
28 X= OAc
29 X= Br
RO
H
H
OTBS
d
b
30 R= ClAc
31 R= H
OR
H
H
OH
H
H
O
O
(CH2)9CH3
OAc
(CH2)9CH3
g
OH
AcO
AcO
HO
HO
NH
NH
O
O
O
O
O
(CH2)9CH3
(CH2)9CH3
O
H
H
OR
H
H
OH
32 R= TBS
33 R= H
(2S,3R,11S,12R,11"'S,12"'R)-1
f
Scheme 3. Synthesis of (2S,3R,11S,12R,11§S,12§R)-plakoside A (1). Reagents: (a) (i) TFA, H2O (70%); (ii) (ClCH2CO)2O,
C5H5N, CH2Cl2 (86%); (b) HBr, AcOH, CH2Cl2 (98%); (c) 26, Hg(CN)2, MeNO2, C6H6 (59%); (d) N2H4, AcOH, AcOEt, MeOH
(73%); (e) Me2CꢁCHCH2OC(ꢁNH)CCl3, BF3·OEt2, CH2Cl2 (87%); (f) TBAF, THF (87%); (g) NaOMe, MeOH (81%).
Two-step removal of the protective groups of 32 under
conventional conditions gave 1 via 33. Synthetic
(2S,3R,11S,12R,2§R,11§S,12§R)-plakoside A (1), [h]D22
+8.9 (c=0.065, MeOH), showed spectral properties14 in
agreement with those reported for the natural
product.1,15 The overall yield of 1 was 3.0% (2“16“1;
21 steps) or 2.4% (2“9“24“1; 20 steps) based on 2.
In conclusion, (2S,3R,11S,12R,2§R,11§S,12§R)-plako-
side A (1) was synthesized from (1S,2R)-1-acetoxy-
methyl-2-hydroxymethylcyclopropane (3),
L-serine, D-
glutamic acid and -galactose. We are currently synthe-
D
sizing (2S,3R,11R,12S,2§R,11§R,12§S)-plakoside A in
order to determine both the absolute configuration of
the cyclopropane moieties and the influence of stereo-

2360
M. Seki et al. / Tetrahedron Letters 42 (2001) 2357–2360
chemistry on the immunosuppressive activity of
plakoside A.20
14%-,15%-,16%-,17%-,18%-,19%-,20%-,21%-H), 1.58 (6H, s, 4¦-,5¦-
Me), 1.57–1.65 (1H, m, 5-H), 1.85–1.97 (3H, m, 4-,5-H),
2.13–2.13 (3H, m, 3§-,7§-H), 2.30–2.38 (1H, m, 3§-H),
2.55–2.65 (2H, m, 4§-H), 3.94 (1H, t, J=6.0, 5%-H),
4.02–4.10 (3H, m, 1-,2%-,3%-H), 4.18–4.23 (1H, m, 3-H),
4.35–4.43 (2H, m, 6%-H), 4.48 (1H, d, J=3.0, 4%-H), 4.58
(1H, dd, J=11.9, 7.3, 1¦-H), 4.60–4.64 (1H, m, 2§-H),
4.70–4.76 (2H, m, 1¦-,2-H), 4.74 (1H, J=7.4, 1%-H), 4.81
(1H, dd, J=10.1, 4.9, 1-H), 5.48–5.55 (1H, m, 6§-H),
5.58–5.63 (1H, m, 5§-H), 5.67–5.71 (1H, m, 2¦-H), 6.50–
6.70 (3H, m, 4×OH), 7.80 (1H, m, OH), 8.27 (1H, d,
J=9.5, NH); 13C NMR (126 MHz, C5D5N): l=11.4,
14.4, 16.2, 18.2, 23.0, 23.8, 25.8, 26.6, 27.7, 29.1, 29.9–
30.1, 32.2, 34.8, 35.7, 54.3, 62.2, 69.5, 69.8, 70.2, 71.1,
71.8, 74.3, 76.8, 79.6, 105.4, 123.1, 129.6, 130.8, 135.2,
175.0.
Acknowledgements
The authors would like to thank Dr. Y. Hirose (Amano
Pharmaceutical Co.) for the gift of lipases.
References
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15. Nicolaou’s synthetic 1 also exhibited spectroscopic data
identical to those reported for natural plakoside A.7
There are a number of reported examples wherein two
diastereomers with separated stereogenic centers show
identical spectroscopic data such as in the cases of
penazetidine A,16,17 penaresidin A,17,18 and sphingofungin
D19 (Fig. 2). In these cases, derivatization or degradation
of the natural or synthetic products was necessary to
completely solve the stereochemical problems.18,19 In the
present case of plakoside A, our endeavor in this direc-
tion will be reported later in a full paper.
6. Grandjean, D.; Pale, P.; Chuche, J. Tetrahedron 1991, 47,
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*
HO
N
H
Penazetidine A
OH
OH
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*
HO
*
N
H
Penaresidin A
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AcHN
HO2C
OH
OH
*
OH OH
Sphingofungin D
Figure 2. Examples of related natural products with aster-
14. Properties of synthetic (+)-1: [h]2D2=+8.9 (c=0.065,
MeOH) {natural product:1 [h]D25=+7 (c=0.5, MeOH);
Nicolaou’s isomer:7 [h]D=+10.4 (c=1.6, MeOH)};
FABMS (negative-ion mode): m/z 946, 878, 716; HR-
FABMS (negative-ion mode) obsd: 946.7692, calcd for
C57H104NO9: 946.7684; 1H NMR (500 MHz, C5D5N):
l=−0.25 to −0.20 (2H, m, 23-,23§-H), 0.62–0.68 (2H, m,
23-,23§-H), 0.68–0.75 (4H, m, 11-,12-,11§-,12§-H), 0.86
(6H, t, J=7.0, 22-,22§-Me), 1.15–1.50 (56H, m, 5-,6-,7-,8-,
9-,10-,13-,14-,15-,16-,17-,18-,19-,20-,21-,8%-,9%-,10%-,13%-,
isked remote stereogenic center(s).
16. Yajima, A.; Takikawa, H.; Mori, K. Liebigs Ann. 1996,
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17. Mori, K. J. Heterocyclic Chem. 1996, 33, 1497.
18. Takikawa, H.; Maeda, T.; Seki, M.; Koshino, H.; Mori,
K. J. Chem. Soc., Perkin Trans. 1 1997, 97.
19. Otaka, K.; Mori, K. Eur. J. Org. Chem. 1999, 1795.
20. Nicolaou reported their synthetic plakoside A to be only
a modest immunosuppressive agent.7
.
.