Total synthesis and stereochemistry of pinnatoxins B and C

Article abstract of DOI:10.1021/ol0611548

Pinnatoxins B and C were synthesized from diols (34R)-3b and (34S)-3a, respectively, in a stereochemically controlled manner. Through extensive analysis of the ¹H NMR spectra of synthetic PnTXs B and C, the diagnostic NMR signals were first ide

Full text of DOI:10.1021/ol0611548

ORGANIC
LETTERS
2006
Vol. 8, No. 15
3327-3330
Total Synthesis and Stereochemistry of
Pinnatoxins B and C
Fumiyoshi Matsuura, Junliang Hao, Reinhard Reents, and Yoshito Kishi*
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford
Street, Cambridge, Massachusetts 02138
kishi@chemistry.harVard.edu
Received May 11, 2006
ABSTRACT
Pinnatoxins B and C were synthesized from diols (34R)-3b and (34S)-3a, respectively, in a stereochemically controlled manner. Through
extensive analysis of the 1H NMR spectra of synthetic PnTXs B and C, the diagnostic NMR signals were first identified to differentiate (34S)-
and (34R)-diastereomers and then used to establish the C34 configuration of natural PnTXs B and C as 34S and 34R, respectively.
Food poisoning due to the ingestion of edible Pinna shellfish
occurs frequently in the coastal regions of China and Japan.
In 1995, Uemura and co-workers isolated pinnatoxin A
(PnTX A), one of the major toxic principles responsible for
the Pinna shellfish poisoning. They elucidated its gross
structure and relative stereochemistry.1 Its unique molecular
architecture and pronounced biological activity as a Ca2+-
channel activator make PnTX A an intriguing synthetic
target.2 We previously reported the total synthesis of PnTX
A, thereby not only confirming its gross structure and relative
stereochemistry but also establishing its absolute configu-
ration (Figure 1).3
In 2001, Uemura and co-workers reported further progress
in this area: (1) isolation of PnTXs B and C from Pinna
muricata and (2) isolation of pteriatoxins A-C (PtTXs A-C)
from Pteria penguin.4 Based on the similarity in the 1H NMR
characteristics of these new alkaloids with that of PnTX A,
they suggested that PnTXs B/C and PtTXs A-C possess
the gross structures shown in Figure 1, with the same
stereochemistry at the macrocyclic core as that of PnTX A.
In addition, through NOE studies, they proposed the absolute
configuration at C34 of natural PnTXs B and C being 34S
and 34R, respectively. However, the C34 and C2′ stereo-
chemistry of PtTXs A-C were left unassigned.4
Our research interests in this area are 2-fold: (1) to
establish the complete stereochemistry of PnTXs B/C and
(2) For the synthetic work on PnTX, see: (a) Hirama group: Sakamoto,
S.; Sakazaki, H.; Hagiwara, K.; Kamada, K.; Ishii, K.; Noda, T.; Inoue,
M.; Hirama, M. Angew. Chem., Int. Ed. 2004, 43, 6505. Wang, J.; Sakamoto,
S.; Kamada, K.; Nitta, A.; Noda, T.; Oguri, H.; Hirama, M. Synlett 2003,
891. Ishiwata, A.; Sakamoto, S.; Noda, T.; Hirama, M. Synlett 1999, 692.
Nitta, A.; Ishiwata, A.; Noda, T.; Hirama, M. Synlett 1999, 695. Noda, T.;
Ishiwata, A.; Uemura, S.; Sakamoto, S.; Hirama, M. Synlett 1998, 298. (b)
Hashimoto group: Nakamura, S.; Inagaki, J.; Kudo, M.; Sugimoto, T.;
Obara, K.; Nakajima, M.; Hashimoto, S. Tetrahedron 2002, 58, 10353.
Nakamura, S.; Inagaki, J.; Sugimoto, T.; Ura, Y.; Hashimoto, S. Tetrahedron
2002, 58, 10375. Nakamura, S.; Inagaki, J.; Sugimoto, T.; Kudo, M.;
Nakajima, M.; Hashimoto, S. Org. Lett. 2001, 3, 4075. (c) Murai group:
Ishihara, J.; Horie, M.; Shimada, Y.; Tojo, S.; Murai, A. Synlett 2002, 403.
Ishihara, J.; Tojo, S.; Kamikawa, A.; Murai, A. Chem. Commun. 2001, 1392.
Sugimoto, T.; Ishihara, J.; Murai, A. Synlett 1999, 541. Ishihara, J.;
Sugimoto, T.; Murai, A. Synlett 1998, 603. Sugimoto, T.; Ishihara, J.; Murai,
A. Tetrahedron Lett. 1997, 38, 7379. (d) Kitching group: Suthers, B. D.;
Jacobs, M. F.; Kitching, W. Tetrahedron Lett. 1998, 39, 2621. (e) Zakarian
group: Pelc, M. J.; Zakarian, A. Org. Lett. 2005, 7, 1629.
(1) (a) Uemura, D.; Chuo, T.; Haino, T.; Nagatsu, A.; Fukuzawa, S.;
Zheng, S.; Chen, H. J. Am. Chem. Soc. 1995, 117, 1155. (b) Chuo, T.;
Kamo, O.; Uemura, D. Tetrahedron Lett. 1996, 37, 4023. (c) Chou, T.;
Haino, T.; Kuramoto, M.; Uemura, D. Tetrahedron Lett. 1996, 37, 4027.
(3) McCauley, J. A.; Nagasawa, K.; Lander, P. A.; Mischke, S. G.;
Semones, M. A.; Kishi, Y. J. Am. Chem. Soc. 1998, 120, 7647.
10.1021/ol0611548 CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/24/2006

reported a total synthesis of PtTXs A-C and disclosed the
experimental evidence to establish unambiguously their
stereochemistry (Figure 1).5 In this paper, we report a total
synthesis of PnTXs B and C from the intermediates used
for the synthesis of PtTXs A-C and present the evidence to
confirm the stereochemistry proposed for PnTXs B/C by
Uemura and co-workers.
Our synthesis of PnTXs and PtTXs diverges after the
intramolecular Diels-Alder reaction to form the carbo-
macrocycle (Scheme 1).5 To establish the stereochemistry
Scheme 1
Figure 1. Structures of PnTXs and PtTXs.
PtTXs A-C and (2) to secure access to stereochemically
homogeneous PnTXs and PtTXs, thus permitting unambigu-
ous determination of their individual biological profiles. The
naturally occurring PnTXs B/C and PtTXs B/C were isolated
as an “HPLC-inseparable” mixture. Therefore, the individual
biological profile of each toxin was not characterized.4 To
achieve our goals, we relied on organic synthesis and
envisioned a unified synthetic strategy (Figure 2) to access
of the Diels-Alder products, the major diastereomer was
chemically correlated with 4 which is an advanced interme-
diate used in the previous PnTX A synthesis.3 Thus, the
current work also constitutes a formal total synthesis of PnTX
A.
We next focused on transformation of the C34/C35-diols
3a and 3b to the amino acid (Scheme 2). Through a two-
step sequence, 3a was first converted to epoxide 5a. Based
on the literature precedents,6 we anticipated that ring-opening
can be achieved preferentially at the secondary allylic
position in an SN2 fashion. Indeed, treatment of 5a with NaN3
yielded azide 6a as a sole product. This result is a sharp
contrast to the ring-opening behavior observed in the PtTX
synthesis, where the epoxide-ring opening with N-Boc-Cys-
(SH)-OCHPh2 took place at both the secondary and primary
centers to furnish a 2:1 mixture of regioisomers.5 Protected
Figure 2. Unified total synthesis of the PnTX/PtTX class of marine
natural products.
each of the possible diastereomers in a stereochemically well-
defined manner. This strategy will allow us to address the
C34 and C2′ configuration independently for all members
of the PnTX/PtTX-class of marine toxins. Recently, we have
(5) (a) Matsuura, F.; Peters, R.; Anada, M.; Harried, S. S.; Hao, J.; Kishi,
Y. J. Am. Chem. Soc. 2006, 128, 7463. (b) Hao, J.; Matsuura, F.; Kishi,
Y.; Kita, M.; Uemura, D.; Asai, N.; Iwashita, T. J. Am. Chem. Soc. 2006,
128, 7742.
(6) (a) Brittain, J.; Gareau, Y. Tetrahedron Lett. 1993, 34, 3363. (b)
Yadav, J. S.; Reddy, B. V. S.; Baishya, G. Chem. Lett. 2002, 906. (c) Fan,
R.-H.; Hou, X.-L. J. Org. Chem. 2003, 68, 726.
(4) (a) Takada, N.; Umemura, N.; Suenaga, K.; Chou, T.; Nagatsu, A.;
Haino, T.; Yamada, K.; Uemura, D. Tetrahedron Lett. 2001, 42, 3491. (b)
Takada, N.; Umemura, N.; Suenaga, K.; Uemura, D. Tetrahedron Lett. 2001,
42, 3495.
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Org. Lett., Vol. 8, No. 15, 2006

the corresponding C34 diastereomer in an extent of 5-10%
(see colored arrows in Figure 3). Since 8a and 8b were
Scheme 2
1
Figure 3. Partial H NMR spectra (500 MHz, CD3OD) of the
synthetic (34S)-stereoisomer (panel A) and (34R)-stereoisomer
(panel B).
diastereomerically homogeneous (1H NMR), the observed
partial epimerization likely took place at the final deprotec-
tion.
Extensive 1H NMR analysis revealed that the C29-H, C28-
H, and C36-H are diagnostic to differentiate the (34S)-PnTX
B/C diastereomer from the (34R)-PnTX B/C diastereomer
(Figure 3). As observed in the PtTXs A-C series,5b the
proton signals nearby the ionic centers were found to shift
significantly depending on the sample preparation and
concentration. However, the above-mentioned three charac-
teristics were found to be inert, or at least to be not affected
significantly.7 Focusing on these diagnostic signals, we then
analyzed the NMR data reported for a ca. 1:1 mixture of the
natural PnTXs B/C. This comparison allowed us to conclude
that the C34 stereochemistry of PnTXs B and C corresponds
to 34S- and 34R-diastereomers, respectively.8 This conclusion
coincides with the stereochemistry assigned through NOE
studies by Uemura and co-workers.9
amino alcohol 7a, available in one step from 6a, was
transformed to 8a through oxidation and protection of the
resultant carboxylic acid.
The particular combination of the two protecting groups
at the amino acid moiety was chosen to ensure the carboxylic
acid protecting group (CHPh2) is cleaved prior to the amine
protecting group (Boc).5 Prior to the formation of the seven-
membered imine, both silyl protecting groups at C15 and
C28 were removed with HF‚pyr. The final stage of the
synthesis was completed in three steps, i.e., (1) Alloc group
deprotection with Pd(PPh3)4, AcOH to give the corresponding
amino ketone, (2) imine cyclization with 2,4,6-(i-Pr)3C6H2-
CO2H-Et3N salt,5 and (3) removal of all the remaining
protecting groups with TFA in CH2Cl2. Preparative HPLC
allowed separation and isolation of synthetic (34R)-PnTX
B/C (9a). Applying the same synthetic sequence on diol 3b,
synthetic (34S)-PnTX B/C (9b) was obtained after HPLC
purification.
In conclusion, we have completed the first total synthesis
of PnTXs B and C in a stereochemically controlled manner
(7) Although C32-H and C34-H initially appeared to be useful diagnostic
signals for comparison purposes, we found that their chemical shifts vary
significantly depending on the sample preparation and concentration. Thus,
unless these spectra were recorded under identical conditions, they cannot
be used to differentiate PnTXs B and C. However, we did observe that the
relative chemical shifts of C32-H for PnTXs B and C remain unchanged
under various conditions. For details, see the Supporting Information.
(8) We recorded a 1H NMR spectrum of the synthetic PnTX C
(contaminated with 5∼10% of PnTX B) doped with a small amount of
natural PnTX B/C (ca. 1:1 mixture) and observed no signals doubled, except
that the intensity of C32-H, C29-H, C28-H, and C37-H peaks, corresponding
to the contaminated PnTX B, increased noticeably. For details, see the
Supporting Information.
(9) Upon comparison of 1H NMR spectra between the synthetic and
natural samples, we found that the chemical shifts for the C37 methyl groups
were different from the originally reported figures and should be δ ) 1.04
ppm for PnTX B and δ ) 1.05 ppm for PnTX C. For details, see the
Supporting Information.
With both synthetic PnTX C34-diastereomers in hand, we
then analyzed their 1H NMR profiles. 1H NMR analysis first
showed that the synthetic (34R)- and (34S)-PnTX B/C
diastereomers thus obtained were cross-contaminated with
Org. Lett., Vol. 8, No. 15, 2006
3329

and presented the experimental evidence to address the C34
stereochemistry.10 Combined with our recently reported
synthesis of PtTXs A-C,5 we have completed a unified
synthesis of the PnTX/PtTX-class of marine natural products,
and unambiguously established their stereochemistry.
natural PnTXs B/C. Financial support from the National
Institutes of Health (NS12108) is gratefully acknowledged.
R.R. thanks the Alexander von Humboldt Foundation for a
Feodor Lynen postdoctoral fellowship.
Supporting Information Available: Experimental details
and spectroscopic analysis of synthetic PnTXs B/C. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank Professor D. Uemura,
Nagoya University, Nagoya, Japan, for a generous gift of
(10) Preliminary mice acute toxicity assays showed that both synthetic
PnTXs B and C exhibit comparable activity at the dose of 0.1 mg/kg. We
thank Professor D. Uemura and Dr. M. Kita at Nagoya University, Nagoya,
Japan, for the assays.
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