Synthesis of (+)-testudinariol A, a triterpene metabolite of the marine mollusc Pleurobrancus testudinarius

Article abstract of DOI:10.1016/S0040-4039(00)02179-1

Testudinariol A (1) is an ichthyotoxic and structurally unusual triterpene alcohol isolated from the skin and the mucus of the marine mollusc Pleurobrancus testudinarius. A stereoselective synthesis of (+)-1 was achieved by starting from (R)-glycidol.

Full text of DOI:10.1016/S0040-4039(00)02179-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1527–1530
Synthesis of (+)-testudinariol A, a triterpene metabolite of the
marine mollusc Pleurobrancus testudinarius
Hirosato Takikawa, Masao Yoshida and Kenji Mori*
Department of Chemistry, Science University of Tokyo, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan
Received 9 November 2000; revised 21 November 2000; accepted 24 November 2000
Abstract—Testudinariol A (1) is an ichthyotoxic and structurally unusual triterpene alcohol isolated from the skin and the mucus
of the marine mollusc Pleurobrancus testudinarius. A stereoselective synthesis of (+)-1 was achieved by starting from (R)-glycidol.
© 2001 Elsevier Science Ltd. All rights reserved.
In 1997, Spinella et al. isolated testudinariol A (1) and
its C-10% epimer (testudinariol B, 2) as metabolites of
the marine mollusc Pleurobrancus testudinarius.1 These
compounds are structurally unique triterpene alcohols
and thought to be defensive allomones of P. testudinar-
ius, because 1 was ichthyotoxic against Gambusia
affinis. The partially cyclized squalene skeleton present
in 1 and 2 is biosynthetically unusual and has only been
reported in the cases of limatulones [3 and 3%, defensive
metabolites of the limpet Achmeia (Collisella) limatula]2
and naurols (4 and 4%, metabolites of the marine
sponges).3 In continuation of our synthesis of marine
triterpenoid such as limatulones (3 and 3%)4 and naurol
A (4),5 we initiated the synthesis of testudinariol A (1).
Herein we report the first synthesis of (+)-1 (Fig. 1).6
Scheme 1 shows our synthetic plan for 1. The target
compound 1 can be obtained by dimerization or its
equivalent operation of A. The intermediate A may be
prepared from B, which is derived from C by means of
Figure 1. Structures of testudinariol A (1), B (2) and related metabolites.
Keywords: marine natural products; stereoselective synthesis; triterpenoids.
* 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(00)02179-1

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H. Takikawa et al. / Tetrahedron Letters 42 (2001) 1527–1530
Scheme 1. Synthetic plan for 1.
ene reaction. The Michael-type cyclization of D fol-
lowed by several steps may afford C. The intermediate
D can be synthesized from F [(R)-glycidol] via the
known diol E.
terminal double bond to give 6 (70%, four steps). The
aldehyde 6 was subjected to the Horner–Wadsworth–
Emmons reaction with 78 to afford 8a (93%, E:Z=
5:1).9 After removal of the TBS protective group (99%),
the resulting 8b was exposed to intramolecular Michael-
type cyclization. In spite of all our efforts, un-
fortunately, any appropriate conditions to furnish 9a as
the predominant isomer under kinetic control could
The starting (R)-glycidol was converted to the known
diol 5.7 Selective and stepwise protections of hydroxy
groups of 5 were followed by oxidative cleavage of the
Scheme 2. Synthesis of testudinariol A (1). Reagents: (a) TBSCl, Et3N, DMAP, CH2Cl2 (85%); (b) 4-methoxybenzyl trichloroace-
timidate, TfOH, Et2O (90%); (c) OsO4, NMO, t-BuOH, acetone, H2O (93%); (d) aq. NaIO4, SiO2, CH2Cl2 (98%); (e) 7, t-BuOK,
toluene, −20°C (93%, E:Z=5:1); (f) PPTS, MeOH (99%); (g) t-BuOK (0.1 equiv.), THF, −10 to −4°C (93% for a mixture of 9a–d);
(h) SiO2 chromatog. (overall 68% for 9a); (i) DIBAL, toluene, −78°C (98%); (j) 1.0 equiv. Me2AlCl, CH2Cl2, 0°C (59%); (k)
TBSOTf, 2,6-lutidine, CH2Cl2 (99%); (l) DDQ, aq. CH2Cl2 (97%); (m) Dess–Martin periodinane, pyridine, CH2Cl2 (94%); (n) 13,
NaHMDS, THF, −78 to −20°C (91%, E:Z=1:4); (o) DIBAL, CH2Cl2 (75%); (p) Ms2O, LiBr, DMAP, collidine, DMF (80%); (q)
PhSO2Na, DMF (ca. 90%); (r) KHMDS, 18-crown-6, THF then 15a (84%); (s) Na-Hg, Na2HPO4, MeOH; (t) TBAF, THF (48%,
two steps).

H. Takikawa et al. / Tetrahedron Letters 42 (2001) 1527–1530
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not be found.10 We therefore tried to obtain a mixture
References
of all of the possible diastereomers under thermody-
namic control. Thus, 8b was treated with t-BuOK (0.1
equiv.) in THF at −10 to 4°C to give in 93% yield a
mixture of 9a–d (9a:9b:9c:9d=5:5:2:2, as determined by
1H NMR analysis11). Under these conditions, the
geometry did not play any role in determining
diastereoselectivity, and the ratio was due to the ther-
modynamic equilibrium. Although the desired 9a was
not predominant, chromatographic separation of
diastereomers was possible, and the undesired three
isomers (9b–d) could be recycled to the initial mixture
by treatment with t-BuOK. By repeating this process
three times, 9a could be obtained in 68% yield.
1. Spinella, A.; Mollo, E.; Trivellone, E.; Cimino, G. Tetra-
hedron 1997, 53, 16891.
2. Albizati, K. F.; Pawlik, J. R.; Faulkner, D. J. J. Org.
Chem. 1985, 50, 3428.
3. De Guzman, F. S.; Schmitz, F. J. J. Org. Chem. 1991, 56,
55. The proposed structure for naurol A was disproved
by our synthesis.5
4. (a) Mori, K.; Takikawa, H.; Kido, M.; Albizati, K. F.;
Faulkner, D. J. Nat. Prod. Lett. 1992, 1, 59; (b) Mori, K.;
Takikawa, H.; Kido, M. J. Chem. Soc., Perkin Trans. 1
1993, 169.
5. Nozawa, D.; Takikawa, H.; Mori, K. J. Chem. Soc.,
Perkin Trans. 1 2000, 2043.
After reduction of 9a with DIBAL (98%), the aldehyde
10 was submitted to ene reaction by treatment with
6. Professor M. Kodama and his co-workers (Tokushima
Bunri University, Japan) recently announced their syn-
thesis of testudinariol A as an E/Z mixture: Kodama,
M.; Hioki, H.; Yoshio, S.; Matsushita, M.; Hamano, M.;
Kanehata, C.; Ohnishi, Y.; Umemori, Y.; Kubo, M.;
Sakai, H. Abstracts of Papers, 42nd Symposium on the
Chemistry of Natural Products, Okinawa, Nov. 6–8,
2000; pp. 235–240.
7. McNeill, A. H.; Thomas, E. J. Tetrahedron Lett. 1993,
34, 1669.
8. Kodama, M.; Shiobara, Y.; Sumitomo, H.; Fukuzumi,
K.; Minami, H.; Miyamoto, Y. J. Org. Chem. 1988, 53,
1437.
12
Me2AlCl in CH2Cl2 to afford the cyclized product
11a13 (59%, 11a:other isomers=59:38). The hydroxy
group of 11a was protected as TBS ether (99%), and the
resulting 11b was converted to 12 in two steps (91%).
The ketone 12 was treated with a chiral phosphonoac-
etate 13 developed by Fuji and his co-workers14 in the
presence of NaHMDS to give a mixture of 14a and its
E-isomer (91%; E:Z=ca. 1:4).15 Reduction of 14a with
DIBAL was followed by SiO2 chromatography to yield
geometrically pure 14b in 75% yield. The alcohol 14b
was then converted to the corresponding bromide 15a
(80%) and sulfone 15b (ca. 90%) in the conventional
manner. Initial attempts to dimerize 15a by metal medi-
ated homo-coupling of allylic halide16,17 met with fail-
ure under several different conditions. However, the
methodology employed for the synthesis of limatu-
lones,4 ‘sulfone coupling’, could be applied successfully
to overcome the difficulty. Accordingly, 15b was cou-
pled with 15a by treatment with KHMDS and 18-
crown-6 in THF at −78°C to give the coupling product
(84%),18 which was subsequently employed for reduc-
tive desulfonylization to afford the dimerized product
16. Final deprotection and careful chromatographic
purification gave (+)-testudinariol A (1) (47% yield, two
steps), [h]D26=+13.0 (c=0.17, CHCl3) {Ref.1 [h]D25=+
15.2 (c=0.3, CHCl3)}. Other physical and spectral data
of synthetic 119 were in good accord with those
reported for the naturally occurring 1. The overall yield
was 4.4% based on 5 in 19 steps (Scheme 2).
9. The described conditions achieved the best (E)-selectivity.
However, it turned out that this geometry was not impor-
tant for the diastereoselectivity in the later Michael-type
cyclization.
10. A wide range of bases, NaH, LiH, K2CO3, Cs2CO3,
TBAF, etc, and Lewis acids, ZnCl2, Sc(OTf)3, TiCl4, etc.
were examined under various conditions.
11. Structures of 9a and 9b were elucidated by 1H NMR
analyses of the corresponding alcohols after reduction of
the ester portions. Those of 9c and 9d were also con-
firmed by a similar procedure (Fig. 2).
In conclusion, the first synthesis of (+)-testudinariol A
(1) was accomplished by starting from (R)-glycidol. The
optimizations of each step and synthesis of testudinar-
iol B (2) are in progress.
Figure 2.
Acknowledgements
12. (a) Horiguchi, Y.; Nakamura, E.; Kuwajima, I. J. Am.
Chem. Soc. 1989, 111, 6257; (b) Snider, B. B.; Rodini, D.
J.; Karras, M.; Kirk, T. C.; Deutsch, E. A.; Cordova, R.;
Price, R. T. Tetrahedron 1981, 37, 3927.
We thank Professor G. Cimino (Istituto per la Chimica
di Molecole di Interesse Biologico, Napoli, Italy) for
sending to us the 1H NMR spectrum of 1 prior to
publication. We acknowledge the financial support by
Ministry of Education, Science, Sports and Culture,
Japan.
1
13. The H NMR data and the results of NOE experiments
of 11a were in good accord with those of natural 1
concerning the cyclopentane portion.
14. Tanaka, K.; Ohta, Y.; Fuji, K. Tetrahedron Lett. 1993,
34, 4071.

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H. Takikawa et al. / Tetrahedron Letters 42 (2001) 1527–1530
15. The NOE experiments strongly suggested that the major
isomer possessed (Z)-geometry.
1640 (w, CꢁC), 1075 (s, CꢀO) cm−1; EIMS (m/z) 470,
452, 442, 434, 424, 370, 343, 327, 300, 234, 219;
HREIMS obsd 470.3400 calcd for C30H46O4 470.3396;
1H NMR (500 MHz, CDCl3) l=1.15–1.28 (2H, m, 8/
8%-Ha), 1.41 (2H, m, 5/5%-Hax), 1.65–1.92 (8H, m, 5/5%-
Heq, 8/8%-Hb, 9/9%-H2), 1.83 (6H, s, 12/12%-H3), 1.91–2.03
(4H, m, 1/1%-Ha, 7/7%-H), 2.03–2.13 (2H, m, 1/1%-Hb),
2.17–2.27 (2H, m, 4/4%-Ha), 2.27–2.33 (2H, m, 4/4%-Hb),
2.40 (2H, ddd, J=11.5, 5.8, 5.5, 10/10%-H), 3.18 (2H,
ddd, J=10.4, 8.5, 1.9, 6/6%-H), 3.73 (2H, d, J=12.5,
15/15%-Hax), 4.15–4.20 (2H, m, 14/14%-H), 4.59 (2H, br
d, J=12.5, 15/15%-Heq), 4.81 (2H, s, 13/13%-Ha), 4.97
(2H, s, 13/13%-Hb), 5.16 (2H, br t, J=5.8, 2/2%-H); 13C
NMR (126 MHz, CDCl3) l=23.3, 26.7, 27.1, 27.3,
32.0, 33.0, 52.0, 53.1, 66.7, 74.8, 80.7, 112.2, 123.4,
134.2, 144.4.
16. The following are the tested examples: K-naphthalenide
in THF at −95°C; BaI2, Li-biphenylide in THF at −78°C,
etc. See: Yanagisawa, A.; Hibino, H.; Habaue, S.;
Hisada, Y.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1995,
68, 1263 and references cited therein.
17. Professor Kodama’s group dimerized 15a by treatment
with Ni(cod)2 and obtained a mixture of 16 and its
isomer.6
18. It was noteworthy that the conventional conditions, n-
BuLi in THF–HMPA, did not work at all, and the
corresponding chloride did not react with 15b com-
pletely even under our successful conditions.
19. Properties of synthetic (+)-1: colorless oil; [h]2D6=+13
(c=0.17, CHCl3); IR wmax (CHCl3) 3680 (w, OꢀH),
.
.