Synthesis and preliminary biological evaluation of truncated zoanthenol analogues

Article abstract of DOI:10.1016/j.bmcl.2004.02.064

Zoanthamines are a family of marine alkaloids that have complex heptacyclic structures and are reported to be interleukin-6 modulators. While the structure of zoanthamines, especially the ABC-ring portion, is similar to that of steroids, the CDEFG-ring po

Full text of DOI:10.1016/j.bmcl.2004.02.064

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2647–2651
Synthesis and preliminary biological evaluation of truncated
zoanthenol analogues
Go Hirai,a Hiroki Oguri,a,* Masahiko Hayashi,b Koji Koyama,a Yuuki Koizumi,a
Sameh M. Moharrama and Masahiro Hiramaa,*
aDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
bKitasato Institute for Life Sciences, Tokyo 108-8641, Japan
Received 14 January 2004; accepted 13 February 2004
Abstract—Zoanthamines are a family of marine alkaloids that have complex heptacyclic structures and are reported to be inter-
leukin-6 modulators. While the structure of zoanthamines, especially the ABC-ring portion, is similar to that of steroids, the
CDEFG-ring portion, composed of aminoacetal and lactone core, is a unique structural element. In this report, we designed and
synthesized ABC-ring 6 and CDEFG-ring 7, which are truncated analogues of the northern and southern hemispheres of zoanthenol
5, respectively, and which incorporate all of the functionality of each hemisphere. A preliminary SAR study suggested that the
hydrochloride of the CEFG-ring portion is an active pharmacophore for suppressing the growth of interleukin-6-dependent MH60
cells.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
The structure of zoanthamines, especially the ABC-ring
portion, is similar to that of steroids. On the other hand,
the CDEFG-ring portion, composed of an aminoacetal
and a lactone, is a unique structural element. We thus
hypothesized that the CDEFG-ring portion and its
hydrochloride might be a pharmacophore of the
zoanthamines. To date, neither the total synthesis of
zoanthamines8;9 nor the structure–activity relationship
(SAR) study using synthetic analogues has been
reported, although SAR study using natural product
derivatives has been described.1e
Zoanthamines (1, 2, 5) are a family of marine alkaloids
isolated from the zoanthid Zoanthus sp. and possess a
unique array of skeletal and stereochemical complexi-
ties.1 Zoanthamines have several pharmacological
properties,1;2 and their activity as a possible osteoporosis
drug is of particular interest. Uemura and co-workers
reported that norzoanthamine hydrochloride 4 sup-
pressed the loss of bone weight and strength in ovari-
ectomized mice.3 Unlike estrogens, serious side effects,
such as an increase in uterine weight, are not observed.
Therefore, the mode of action of 4 on osteoporosis
appears to be different from that of estrogen.3c;4 In
addition, norzoanthamine and its hydrochloride are
reported to suppress interleukin-6 (IL-6) production.3
Because IL-6 is known as a mediator of bone resorption
in osteoporosis,5 it is likely that the activity as an IL-6
modulator might be associated with its effects on osteo-
porosis6;7 (Fig. 1).
In this report, we designed the ABC-ring 6 and the
CDEFG-ring 7, which are truncated analogues of the
northern and southern hemispheres of zoanthenol 5,
respectively, and which incorporate all of the function-
ality of each hemisphere. We report herein the synthesis
and the preliminary biological evaluation of these
truncated analogues.
2. Synthesis
As outlined in Scheme 1, we planned the synthesis of the
truncated analogues (6, 7) to allow the total synthesis of
zoanthenol 5. A fully functionalized ABC-ring moiety
10 was designed as a key intermediate that could be
* Corresponding authors. Tel.: +81-22-217-6563; fax: +81-22-217-6566
(M.H.); e-mail addresses: oguri@ykbsc.chem.tohoku.ac.jp; hirama@
ykbsc.chem.tohoku.ac.jp
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.02.064

2648
G. Hirai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2647–2651
O
R
cyclization of 11 leading to 5. Compound 10 would be
O
A
R
B
H
H
H
synthesized from the previously reported compound 98c
by incorporation of C19 and C9 methyl groups.10 For
the synthesis of the analogue 7, we performed a model
study to establish a synthetic route to 5 starting from 10.
In this model study, we used the C-ring moiety 12, which
possesses the double bond and ketone functionalities
found in 10.
17
O
H
20
C
O
O
H
22
12
OH
H
D
9
O
O
E
Cl-
+
N
O
N
O
1
6
F
G
To synthesize 10 and 6, the C19 methyl group was first
incorporated into the ketone 98c via regio- and stereo-
controlled alkylation (Scheme 2). Deprotonation of 9
with lithium diisopropylamide (LDA) followed by
addition of methyl iodide generated 14 in 91% yield.
Reduction of ketone 14 with lithium aluminumhydride
and subsequent protection as a methoxymethyl (MOM)
ether gave a 3:1 diastereomeric mixture of 15 and its C20
epimer (77%, two steps).11 Treatment of 15 with tetra-
butylammonium fluoride (TBAF) in N,N0-dimethyl-
propyleneurea (DMPU) at 95 ꢁC cleanly removed the
silyl group, producing the corresponding alcohol,12
which was oxidized by Dess–Martin periodinane to
furnish ketone 16 in quantitative yield (two steps). We
then conducted the critical C9 methylation installing
consecutive quaternary centers on the C-ring. Condi-
tions developed for the methylation in the previously
reported model study8a were applied to the more elab-
orated substrate 16. First, exposure of 16 with trimeth-
ylsilyl iodide (TMSI) and HN(TMS)2 produced silyl
enol ether 17 regioselectively, which was then treated
with methyl lithium to generate the corresponding lith-
ium enolate. Treatment of the lithium enolate with
samarium iodide (SmI2) and chloroiodomethane pro-
duced the desired cyclopropanol 18, which was then
exposed with p-toluenesulfonic acid (TsOH) to produce
the key intermediate 10 in 42% yield (three steps). As
expected, the SmI2-mediated Simmons–Smith reaction
proceeded exclusively at the less hindered b-face of the
enolate. We therefore accomplished the synthesis of the
fully elaborated ABC-ring system of 5 in a highly ste-
reocontrolled manner. The analogue 6 was then syn-
thesized via a four step procedure: (i) removal of the
MOM groups, (ii) protection of the phenol as a benzyl
Zoanthamine: 1 (R = Me)
Norzoanthamine: 2 (R = H)
3: R = Me
4: R = H
OH
O
H
OH
O
B
A
O
O
H
O
N
C
O
6
Zoanthenol: 5
O
OH
C
D
O
O
Cl-
+
E
N
N
F
O
O
G
7
8
Figure 1. Structures of zoanthamines (1, 2, and 5), designed analogues
(6, 7), and its hydrochlorides (3, 4, and 8).
converted to the analogue 6 and that would also install
the aminoacetal–lactone core (DEFG-ring portion) after
the oxidative cleavage of double bond followed by
PO
MOMO
MOMO
19
X
OMOM
O
C19, C9
Methylation
OH
5
H
H
H
O
9
O
H
O
11
10
OTBS
9
H2N
O
OH
6
Model study
CO2H
C
7
NH2
O
O
O
OH
12
13
Scheme 1. Synthesis plan of zoanthenol (5) and designed analogues (6, 7).

G. Hirai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2647–2651
2649
R
MOMO
O
MOMO
19
20
OMOM
O
H
a
b, c
d - f
12
10
b, c
O
20
19
OH
H
H
OH
H
OTBS
OTBPS
OTBS
9: R = H
14: R = Me
15
1
a
5
d, e
+
PhSO2
NHBoc
O
MOMO
MOMO
21: R = OMe
22: R = CN
23: R = CHO
O
g
h
24
OMOM
OMOM
R
f
H
O
H
OTBPS
H
OTMS
17
g, h
16
k
i
SO2Ph
O
MOMO
NBoc
l - o
p
O
X
OMOM
j - m
i
25: X = H, OH
26: X = O
6
j
H
O
9
HO
OTBPS
10
18
Scheme 2. Reagents and conditions: (a) LDA, THF, )78 ꢁC, then MeI,
HMPA, )78 ꢁC fi rt, 91%; (b) LiAlH4, Et2O, 0 ꢁC; (c) MOMCl,
(i-Pr)2NEt, (CH2Cl)2, 50 ꢁC, 77% (15/C20epi-15 ¼ 3:1, two steps); (d)
TBAF, DMPU, 95 ꢁC; (e) Dess–Martin periodinane, CH2Cl2, 99%
(two steps); (f) TMSI, HN(TMS)2, hexane, )20 ꢁC to rt, 75%; 16 25%
(recovery); (g) MeLi, DME, HMPA, 0 ꢁC; (h) SmI2, ClCH2I, THF,
)78 ꢁC fi rt; (i) p-TsOH, CH2Cl2, 0 ꢁC, 42% (three steps), 16 25%
(recovery); (j) TMSBr, CH2Cl2, )40 fi )20 ꢁC; (k) BnBr, Cs2CO3,
DMF, 76% (two steps); (l) Dess–Martin periodinane, CH2Cl2; (m) H2,
Pd/C, AcOEt, 40% (two steps).
NBoc
OH
O
O
27
CO2H
7
8
q
NBoc
O
O
O
28
Scheme 3. Reagents and conditions: (a) Li, NH3, Et2O/EtOH, )60 ꢁC,
87% (19/C10epi-19 ¼ 3:1); (b) OsO4, NMO, t-BuOH–H2O, 90%; (c)
NaIO4, THF–H2O, 99%; (d) DIBAL-H, CH2Cl2, )78 ꢁC; (e)
TsOHÆH2O, MeOH; (f) TBPSCl, imidazole, DMF, 60% (three steps);
(g) TMSCN, TMSOTf, CH2Cl2, 0 ꢁC, 91%; (h) DIBAL-H, CH2Cl2–
hexane, )90 ꢁC; (i) 24, n-BuLi, THF, )78 ꢁC; (j) Dess–Martin peri-
odinane, pyridine, CH2Cl2, 37% (three steps); (k) SmI2 (20 equiv),
MeOH (5 equiv), THF, )78 ꢁC fi rt, 79%; (l) Dess–Martin period-
inane, pyridine, CH2Cl2, 99%; (m) TBAF, THF, 99%; (n) Dess–Martin
periodinane, pyridine, CH2Cl2, 99%; (o) NaClO2, NaH2PO4, 2-methyl-
2-butene, t-BuOH–H2O; (p) AcOH/H2O (96:4), 100 ꢁC, 10 h, then
Na2SO4, rt, 1 h, 61% (three steps); (q) HCl, Et2O.
ether, (iii) Dess–Martin oxidation, and (iv) treatment
with Pd/C under a H2 atmosphere.
We next turned our attention to the synthesis of the
analogue 7 starting from 1213 (Scheme 3). Stereoselective
reduction of ketone 12 by treatment with lithium in
NH3–EtOH–Et2O yielded a 3:1 mixture of 19 and
C10epi-19 (87% yield). After separation, OsO4-catalyzed
dihydroxylation of 19 and subsequent oxidative cleav-
age of the resulting diol produced 20 in 89% yield (two
steps). Reduction of the aldehyde 20 followed by con-
version of the hemiacetal into the corresponding methyl
ketal and protection of alcohol as silyl ether furnished
21 (60% yield, three steps). Treatment of the methyl
ketal 21 with TMSCN and TMSOTf produced the
nitrile 22 in 91% yield,14 which was reduced with DI-
BAL-H to give aldehyde 23.15 To connect the C1–C5
unit 2416 with 23, we next conducted a Julia coupling
reaction. Deprotonation of the sulfone 24 with n-BuLi
and subsequent treatment with 23 produced the corre-
sponding adduct 25, which was oxidized to generate
ketone 26. Upon treatment of 26 with SmI2 in the
presence of methanol as a proton source, successive
desulfonation and cleavage of the furan ring furnished
27 in 79% yield, which was then converted to 28 by
standard synthetic procedures. Finally, we constructed
the aminoacetal and lactone core according to the pro-
tocol reported by Kobayashi and co-workers.9c Treat-
ment of 28 in AcOH/H2O (96:4) at 100 ꢁC followed by
the addition of anhydrous sodium sulfate produced the
pentacyclic analogue 7 in 61% yield (three steps). The
hydrochloride salt 8 was also synthesized by exposure
with HCl in Et2O.1e
3. Biological evaluation
With the synthetic analogues in hand, we performed a
preliminary SAR study to investigate the ability to
inhibit the growth of IL-6-dependent MH60 cells

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G. Hirai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2647–2651
ganic Research. Fellowships to G.H. and M.M.S. from
the Japanese Society for the Promotion of Science are
gratefully acknowledged. This research was also par-
tially supported by the Ministry of Education, Science,
Sports and Culture, a Grant-in-Aid for the COE project,
Giant Molecules and Complex Systems, 2002.
References and notes
1. (a) Rao, C. B.; Anjaneyula, A. S. R.; Sarma, N. S.;
Venkateswarlu, Y.; Rosser, R. M.; Faulkner, D. J.; Chen,
M. H. M.; Clardy, J. J. Am. Chem. Soc. 1984, 106, 7983;
(b) Rao, C. B.; Anjaneyulu, A. S. R.; Sarma, N. S.;
Venkateswarlu, Y.; Rosser, R. M.; Faulkner, D. J. J. Org.
Chem. 1985, 50, 3757; (c) Rao, C. B.; Rao, D. V.; Raju, V.
S. N.; Sullivan, B. W.; Faulkner, D. J. Heterocycles 1989,
28, 103; (d) Rahman, A.-U.; Alvi, K. A.; Abbas, S. A.;
Choudhary, M. I.; Clardy, J. Tetrahedron Lett. 1989, 30,
6825; (e) Kuramoto, M.; Hayashi, K.; Yamaguchi, K.;
Yada, M.; Tsuji, T.; Uemura, D. Bull. Chem. Soc. Jpn.
1998, 71, 771; (f) Nakamura, H.; Kawase, Y.; Maruyama,
K.; Murai, A. Bull. Chem. Soc. Jpn. 1998, 71, 781; (g)
ꢀ
Deranas, A. H.; Fernandez, J. J.; Gavın, J. A.; Norte, M.
Tetrahedron 1998, 54, 7891; (h) Deranas, A. H.; Fernan-
ꢀ
dez, J. J.; Gavın, J. A.; Norte, M. Tetrahedron 1999, 55,
ꢀ
Figure 2. Effects of hydrochlorides of zoanthamines (3, 4), synthetic
analogues (6, 7), and 8 on the growth of IL-6-dependent MH60 cells.
Madindoline A (35) was used as a positive control.
ꢀ
5539.
2. Villar, R. M.; Gil-Longo, J.; Daranas, A. H.; Souto, M.
ꢀ
ꢀ
L.; Fernandez, J. J.; Peixinho, S.; Barral, M. A.; Santafe,
ꢀ
(Fig. 2).17 Madindoline A (35) was used as a positive
control for inhibition of IL-6 effects [IC50 ¼ 8 lM].6;18
We first confirmed that the hydrochlorides of zoanth-
amines 3 and 4 suppressed IL-6-induced cell growth of
the MH60 cells in a dose-dependent manner [IC50 for 3
and 4 was 26 and 13 lM, respectively].19 We also found
that the hydrochloride of the CEFG-ring analogue 8
dose-dependently suppressed cell growth, although the
potency [IC50 ¼ ꢀ70 lM] was lower than for 3 and 4. In
contrast, the analogues, ABC-ring 6 and CDEFG-ring
7, showed very weak inhibitory activities. Thus, the
hydrochloride of CEFG-ring portion may be an active
pharmacophore for zoanthamine inhibition of IL-6
activity.
ꢀ
G.; Rodrıguez, J.; Jimenez, C. Bioorg. Med. Chem. 2003,
11, 2301.
3. (a) Fukuzawa, S.; Hayashi, Y.; Uemura, D.; Nagatsu, A.;
Yamada, K.; Ijuin, Y. Heterocycl. Commun. 1995, 1, 207;
(b) Kuramoto, M.; Hayashi, K.; Fujitani, Y.; Yamaguchi,
K.; Tsuji, T.; Yamada, K.; Ijuin, Y.; Uemura, D.
Tetrahedron Lett. 1997, 38, 5683; (c) Yamaguchi, K.;
Yada, M.; Tsuji, T.; Kuramoto, M.; Uemura, D. Biol.
Pharm. Bull. 1999, 22, 920; (d) Kuramoto, M.; Arimoto,
H.; Uemura, D. J. Synth. Org. Chem. Jpn. 2003, 61, 1099,
and Ref. 1e.
4. Our preliminary investigation based on estrogen receptor
binding assay using fluorescence polarization showed that
zoanthamines (1, 2) and its hydrochlorides (3, 4) did not
bind tightly to either ER-a or ER-b.
5. (a) Ishimi, Y.; Miyaura, C.; Jin, C. H.; Akatsu, T.; Abe,
E.; Nakamura, Y.; Yamaguchi, A.; Yoshiki, S.; Matsuda,
T. J. Immunol. 1990, 145, 3297; (b) Jilka, R. L.; Hangoc,
G.; Girasole, G.; Passeri, G.; Williams, D. C.; Abrams, J.
S.; Boyce, B.; Broxmeyer, H.; Manolagas, S. C. Science
1992, 257, 88; (c) Poli, V.; Balena, R.; Fattori, E.;
Markatos, A.; Yamamoto, M.; Tanaka, H.; Ciliberto,
G.; Rodan, G. A.; Costantini, F. EMBO J. 1994, 13, 1189.
6. (a) Omura, S.; Hayashi, M.; Tomoda, H. Pure Appl.
Chem. 1999, 71, 1673; (b) Hayashi, M.; Rho, M. C.;
Enomoto, A.; Fukami, A.; Kim, Y. P.; Kikuchi, Y.;
Sunazuka, T.; Hirose, T.; Komiyama, K.; Omura, S. Proc.
Natl. Acad. Sci. U.S.A. 2002, 99, 14728; (c) Hayashi, M.;
Rho, M.-C.; Fukami, A.; Enomoto, A.; Nonaka, S.;
Sekiguchi, Y.; Yanagisawa, T.; Yamashita, A.; Nogawa,
T.; Kamano, Y.; Komiyama, K. J. Pharm. Exp. Therap.
2002, 303, 104.
In summary, we have synthesized truncated analogues
(6, 7) of zoanthenol 5 in a stereocontrolled manner. A
preliminary SAR study suggested that the hydrochloride
of CEFG-ring part is required for suppression of growth
of IL-6-dependent MH60 cells. This study not only
provides synthetic strategies for the total synthesis of
zoanthamines but also gives insight into the active
pharmacophore. Further investigations are currently
underway in our laboratories.
Acknowledgements
We thank Prof. D. Uemura (Nagoya University) for the
generous gift of zoanthamine 1 and norzoanthamine 2.
This work was supported by the Ministry of Education,
Science, Sports and Culture (Japan) Grant-in-Aid for
Scientific Research and the Suntory Institute of Bioor-
7. Yamaguchi, K.; Yada, M.; Tsuji, T.; Hatanaka, Y.; Goda,
K.; Kobori, T. Bioorg. Med. Chem. Lett. 1999, 9, 957.
8. (a) For synthetic studies from our laboratory, see: Hirai,
G.; Oguri, H.; Hirama, M. Chem. Lett. 1999, 141; (b)
Hirai, G.; Oguri, H.; Moharram, S. M.; Koyama, K.;

G. Hirai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2647–2651
Hirama, M. Tetrahedron Lett. 2001, 42, 5783; (c) Hirai,
2651
O
G.; Koizumi, Y.; Moharram, S. M.; Oguri, H.; Hirama,
M. Org. Lett. 2002, 4, 1627.
TIPSO
OH
OH
HO
OMe
32
9. For synthetic studies of zoanthamines from other groups,
see: (a) Tanner, D.; Tedenborg, L.; Somfai, P. Acta Chem.
Scand. 1997, 51, 1217; (b) Williams, D. R.; Cortez, G. S.
Org. Lett. 2000, 2, 1023; (c) Hikage, N.; Furukawa, H.;
Takao, K.; Kobayashi, S. Chem. Pharm. Bull. 2000, 48,
1370; (d) Sakai, M.; Sasaki, M.; Tanino, K.; Miyashita,
M. Tetrahedron Lett. 2002, 43, 1705, and references cited
therein.
c - g
a, b
TIPSO
OH
33
h - j
TIPSO
24
NBoc
10. Carbon numbering corresponds to that for zoanthenol 5,
see Ref. 1h.
O
34
11. The 3:1 diasteromeric mixture was subjected to the
synthesis of 6 without separation.
12. No reaction was observed when 15 was treated with TBAF
in refluxing THF.
13. The C-ring moiety 12 was synthesized as follows. The
ketone 29 was readily available from S-(+)-Wieland-
Miescher ketone, see: Vellekoop, A. S.; Smith, R. A. J.
Tetrahedron 1998, 54, 11971,
Scheme 4. Reagents and conditions: (a) t-BuOOH, Ti(Oi-Pr)4, diethyl-
-(+)-tartrate, MS4A, )20 ꢁC, CH2Cl2, 90%; (b) DIBAL-H, toluene,
0 ꢁC, 96%; (c) TsCl, Et3N, DMAP, CH2Cl2; (d) NaN3, DMF, 73%
(two steps); (e) Pd/C, THF; (f) (Boc)2O, NaOH, 84% (two steps); (g)
2,2-dimethoxypropane, p-TsOH, benzene, 96%; (h) TBAF, THF, 95%;
(i) PhSSPh, n-Bu3P, pyridine, 91%; (j) mCPBA, NaHCO3, 0 ꢁC,
CH2Cl2, 93%.
L
16. The C1–C5 unit 24 was synthesized as shown in Scheme 4.
Allyl alcohol 32 was synthesized from methyl 2S-3-
hydroxy-2-methylpropionate according to reported pro-
tocols, see: Ishiwata, A.; Sakamoto, S.; Noda, T.; Hirama,
M. Synlett 1999, 692.
O
OH
L-Selectride (1.0 eq)
THF, -78 ˚C, 86%
O
O
30
29
17. Matsuda, T.; Hirano, T.; Kishimoto, T. Eur. J. Immunol.
1998, 18, 951.
18. (a) Hayashi, M.; Kim, Y.-P.; Takamatsu, S.; Enomoto,
A.; Shinose, M.; Takahashi, Y.; Tanaka, H.; Komiyama,
K.; Omura, S. J. Antibiot. 1996, 49, 1091; (b) Takamatsu,
S.; Kim, Y.-P.; Enomoto, A.; Hayashi, M.; Tanaka, H.;
Komiyama, K.; Omura, S. J. Antibiot. 1997, 50, 1069; (c)
Sunazuka, T.; Hirose, T.; Shirahata, T.; Harigaya, Y.;
Hayashi, M.; Komiyama, K.; Omura, S.; Smith, I. A. B.
J. Am. Chem. Soc. 2000, 122, 2122.
Tf2O, 2,6-lutidine
CH2Cl2, 64%
31
O
12
O
12:31=1:3
.
14. Kim, S.; Do, J. Y.; Kim, S. H.; Kim, D. J. Chem. Soc.,
Perkin Trans. 1 1994, 2357.
15. DIBAL-H reduction of nitrile 22 always yielded 23 and a
primary amine in a 1:1 ratio.
19. Hydrochlorides of zoanthamines (3, 4) showed negligible
activities for the suppression of the cell growth of the IL-6-
independent MH60 cells.