Unified total synthesis of pteriatoxins and their diastereomers

Article abstract of DOI:10.1021/ja0618954

A unified total synthesis is reported to access all of the possible diastereomers of pteriatoxins A-C, with the use of an intramolecular Diels-Alder reaction as the key step to form the carbo-macrocyclic core structure. The C34/C35-diol protecting groups were found to have significant effects on both the exo/endo-selectivity and the exo-facial selectivity of the intramolecular Diels-Alder process. Copyright

Full text of DOI:10.1021/ja0618954

Published on Web 05/19/2006
Unified Total Synthesis of Pteriatoxins and Their Diastereomers
Fumiyoshi Matsuura, Rene´ Peters, Masahiro Anada, Scott S. Harried, Junliang Hao, and
Yoshito Kishi*
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138
Received March 20, 2006; E-mail: kishi@chemistry.harvard.edu
Outbreaks of human poisoning due to the ingestion of Pinna
shellfish (P. muricata and P. pectinata) were recorded in China
and Japan, with typical neurotoxin symptoms, such as paralysis,
diarrhea, and convulsion. In 1995, Uemura and co-workers isolated
pinnatoxin A (PnTX A), one of the major toxic principles
responsible for outbreaks of Pinna shellfish intoxication, from the
shellfish P. muricata. They elucidated its gross structure and relative
stereochemistry and suggested a biosynthetic pathway.1 Its unique
molecular architecture, accompanied by its pronounced biological
activity as a Ca2+ channel activator, makes PnTX A an intriguing
synthetic target.2 We previously reported the total synthesis of PnTX
A, not only confirming its gross structure and relative stereochem-
istry, but establishing its absolute configuration (Figure 1).3
In 2001, Uemura and co-workers reported two new developments
in this area: (1) isolation of PnTXs B and C from the Okinawan
bivalve P. muricata and (2) isolation of pteriatoxins A, B, and C
(PtTXs A, B, and C) from Pteria penguin.4 These new members
were isolated in very minute amounts5 but were reported to exhibit
extremely potent and acute toxicity against mice.6 Considering the
similarity in the 1H NMR characteristics of the new toxins to those
of PnTX A, Uemura and co-workers suggested the gross structures
shown in Figure 1. They proposed that these new alkaloids and
PnTX A share the same stereochemistry at the macrocyclic core.
However, the C34 stereochemistry of PnTXs B/C and the C34 and
C2′ stereochemistry of PtTXs A-C were unassigned.4
Figure 1. Structure of pinnatoxins and pteriatoxins.
Figure 2. Unified synthesis of the PnTX/PtTX class of marine natural
products.
Our research interests in this area are two-fold: (1) to establish
the complete stereochemistry of PtTXs A-C and PnTXs B/C, and
(2) to secure an access to stereochemically homogeneous PtTXs
A-C7 and PnTXs B/C, thus permitting unambiguous determination
of their individual biological profiles. The naturally occurring PtTXs
B/C, as well as PnTXs B/C, were isolated as a mixture and shown
to be chromatographically inseparable.4 Therefore, the individual
biological profile of each toxin was not characterized. To achieve
our goals, we relied on organic synthesis to access each possible
diastereomer in a stereochemically well-defined manner. To
synthesize all diastereomers of PtTXs A-C and PnTXs B/C
efficiently, we envisioned a unified synthetic plan outlined in Figure
2. This strategy will allow us to synthesize the C34 as well as the
C2′ stereoisomers independently and, consequently, secure the
complete stereochemistry for all the members of the PnTX/PtTX
family. In this paper, we report a total synthesis of all members of
the PtTX class of natural compounds.8
PtTXs are envisioned to be assembled from three building
blocks: dithiane 1, vinyl iodide 2, and alkyl iodide 3 (Figure 3).9
It is worth noting that 1 and 2 are the building blocks used for our
PnTX A synthesis, but it is necessary to develop a synthetic route
to the building block 3. The synthesis of the C33-C35 segments
7a (C34-â series) and 7b (C34-R series) is summarized in Scheme
1. Hydrolysis of the racemic vinyl bromide diacetate 410 with
Amano lipase PS800 furnished a mixture of optically active 5
(optical purity >96%) and 6 (optical purity >96%). On the basis
Figure 3. Three building blocks of PtTXs A-C.
Scheme 1 a
a Reagents: (a) Amano lipase PS800 (5, 46%; 6, 41%); (b) cyclopen-
tanone, p-TsOH (79%); (c) LiOH (97%).
of literature precedent,11 their absolute configuration was assigned
as indicated, which was further confirmed via chemical correlation
with D-(+)- and L-(-)-solketals.12 This route secured access to
antipodes 7a and 7b in multigram quantities.
The optically active vinyl bromide 7a was then elaborated to
the C26-C35 segment 3a (Scheme 2). In this synthesis, the C32-
C33 bond was formed via Ni/Cr-mediated coupling3,13,14 of 7a and
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J. AM. CHEM. SOC. 2006, 128, 7463-7465
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C O M M U N I C A T I O N S
Scheme 2 a
Table 1. Intramolecular Diels-Alder Reactionsa
a Reagents: (a) (1) 7a, NiCl2, 86%; (2) Ac2O, Py, 93%; (3) Pd(OAc)2,
CaCO3, 97%; (b) (1) TFA, H2O, CH2Cl2, 91%; (2) Ac2O, Py, 99%; (3)
TFA, H2O, CH2Cl2, 93%; (4) CH(OMe)3, PPTS; (5) K2CO3, MeOH; (6)
TBSCl, DMAP, 98% over 3 steps; (7) DIBAL, 82%; (8) Mel, DEAD, PPh3,
88%.
Scheme 3 a
a A: product ratio (exo-desired:exo-undesired:endo-1:endo-2). B: com-
bined yield. C: isolated yield of desired exo-product. D: exo/endo ratio.
E: exo-facial selectivity (desired:undesired).
correlated with the two exo-products in the previous synthesis,3,12
thereby establishing their stereochemistry. However, the stereo-
chemistry of endo-product(s) remains to be established. Upon
changing the C34/C35 protecting groups from TBS to acyl groups,
the cycloaddition took place noticeably faster.16,17 Interestingly, the
exo/endo-selectivity was found to depend sharply on the acyl
protecting group, with the benzoate giving the best ratio. The facial
selectivity of exo-addition was found also to depend on the
protecting group, with the p-methoxybenzoate giving the best ratio.
On the balance of the two selectivities, we chose the p-methoxy-
benzoate substrate for preparative purposes and obtained the desired
product 14a-iii in 51% isolated yield.
The overall trend of the intramolecular Diels-Alder reaction in
the C34-R series was similar to that in the â series, except that the
chemical yield in the R series was lower than that in the
corresponding â series. In this series, the temperature effect on the
exo/endo-selectivity and the facial selectivity of exo-products was
tested on the acetate substrate 13b-iii. Interestingly, on lowering
the reaction temperature, the exo/endo-selectivity was noticeably
improved, whereas the facial selectivity of exo-products remained
virtually unchanged.
a Reagents: (a) (1) 3a, t-BuLi, HMPA; (2) K2CO3, MeOH, 73% over 2
steps; (b) (1) PPTS; (2) K2CO3, MeOH, 73% over 2 steps; (3) PIFA, 74%;
(c) (1) SO3‚Py, DMSO, 83%; (2) 2, NiCl2, CrCl2, 84%; (3) Dess-Martin
oxidation, 79%; (d) (1) HF‚Py, Py; (2) RCOCl, NEt3.
8,3 to furnish a diastereomeric mixture of allylic alcohols, which,
upon acylation and Pd-mediated elimination, afforded diene 9a.
At this stage, it was necessary to convert the C29/C30 acetonide
to the more labile ortho ester protecting group (3a) due to our
inability to deprotect the acetonide at later stages in the synthesis.
Coupling of iodide 3a with dithiane 1 then furnished the C6-
C35 portion of the PtTXs in high yield (Scheme 3). Elaboration to
the requisite Diels-Alder precursor 12a commenced with removal
of the ortho ester and dithiane deprotection with concomitant
formation of the C25-C30 bicycloketal. Oxidation of the C6
hydroxyl then permitted installation of the C1-C5 moiety via a
Ni/Cr-mediated coupling.3,13 Alternatively, introduction of the C1-
C5 moiety, followed by the C25-C30 bicycloketal formation, also
afforded 12a, but the material throughput via the sequence of
reactions shown in Scheme 3 proved superior.15
We next planned to introduce the cysteine moiety via epoxide
15a, which was in turn prepared from the C34/C35-diester 14a-iii.
Considering the allylic nature of epoxide, also supported by
literature precedents,18 we anticipated that ring opening could be
achieved preferentially at the secondary center in a SN2 fashion,
that is, 15a f 16. Experimentally, treatment of 15a with the anion
derived from N-Boc-L-Cys(SH)-OCHPh2 yielded a 2:1 mixture of
(34R,2′R)-16 and (34S,2′R)-17 in an approximately 85% combined
yield. Critically, both 16 and 17 were found to be free from
contamination of the corresponding C34 or C2′ stereoisomers,19
thereby demonstrating that the epoxide ring opening took place in
a SN2 fashion, and therefore, their stereochemistry was assigned
as indicated. Similarly, when we employed the anion derived from
N-Boc-D-Cys(SH)-OCHPh2, 15a afforded a 2:1 mixture of two
products corresponding to (34R,2′S)-16 and (34S,2′S)-17 in com-
The next stage of synthesis was the crucial intramolecular Diels-
Alder reaction to form the carbo-macrocycle. Previously, we
successfully relied on an intramolecular Diels-Alder reaction to
construct the macrocycle of PnTX A.3 However, it is worth noting
that the diene used in that series was conjugated with a tert-butyl
ester, which exhibited a high tendency for self [4 + 2] cycloaddition,
and therefore it was necessary to handle the diene only as a dilute
solution. To the contrary, we anticipated, and indeed found, that
the diene in the current series shows no tendency of self [4 + 2]
cycloaddition.
We first studied the intramolecular Diels-Alder reaction on
substrate 12a in the C34-â series. Under thermal conditions
(dodecane, 170 °C), 12a slowly cyclized to give a 0.8:1.0:0.8
mixture of the intramolecular Diels-Alder products in 78%
combined yield (Table 1). The two exo-products were chemically
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C O M M U N I C A T I O N S
Scheme 4 a
Acknowledgment. Financial support from the National Institutes
of Health (NS12108) is gratefully acknowledged. R.P. and S.S.H.
thank the German Academic Exchange Service (DAAD) and the
National Institutes of Health (NIH NRSA GM19950), respectively,
for a postdoctoral fellowship.
Supporting Information Available: Experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(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.
(2) For the synthetic work on PnTX, see the following. Hirama group: (a)
Sakamoto, S.; Sakazaki, H.; Hagiwara, K.; Kamada, K.; Ishii, K.; Noda,
T.; Inoue, M.; Hirama, M. Angew. Chem., Int. Ed. 2004, 43, 6505. (b)
Wang, J.; Sakamoto, S.; Kamada, K.; Nitta, A.; Noda, T.; Oguri, H.;
Hirama, M. Synlett 2003, 891. (c) Ishiwata, A.; Sakamoto, S.; Noda, T.;
Hirama, M. Synlett 1999, 692. (d) Nitta, A.; Ishiwata, A.; Noda, T.;
Hirama, M. Synlett 1999, 695. (e) Noda, T.; Ishiwata, A.; Uemura, S.;
Sakamoto, S.; Hirama, M. Synlett 1998, 298. Hashimoto group: (f)
Nakamura, S.; Inagaki, J.; Kudo, M.; Sugimoto, T.; Obara, K.; Nakajima,
M.; Hashimoto, S. Tetrahedron 2002, 58, 10353. (g) Nakamura, S.;
Inagaki, J.; Sugimoto, T.; Ura, Y.; Hashimoto, S. Tetrahedron 2002, 58,
10375. (h) Nakamura, S.; Inagaki, J.; Sugimoto, T.; Kudo, M.; Nakajima,
M.; Hashimoto, S. Org. Lett. 2001, 3, 4075. Murai group: (i) Ishihara,
J.; Horie, M.; Shimada, Y.; Tojo, S.; Murai, A. Synlett 2002, 403. (j)
Ishihara, J.; Tojo, S.; Kamikawa, A.; Murai, A. Chem. Commun. 2001,
1392. (k) Sugimoto, T.; Ishihara, J.; Murai, A. Synlett 1999, 541. (l)
Ishihara, J.; Sugimoto, T.; Murai, A. Synlett 1998, 603. (m) Sugimoto,
T.; Ishihara, J.; Murai, A. Tetrahedron Lett. 1997, 38, 7379. Kitching
group: (n) Suthers, B. D.; Jacobs, M. F.; Kitching, W. Tetrahedron Lett.
1998, 39, 2621. Zakarian group: (o) Pelc, M. J.; Zakarian, A. Org. Lett.
2005, 7, 1629.
a Reagents: (a) (1) K2CO3; (2) HF‚Py, Py, 74% over 2 steps; (2) TsCl;
(3) K2CO3, 78% over 2 steps; (b) N-Boc-L-Cys(SH)-OCHPh2 (an inseparable
mixture of 16 and 17), 85%; (c) (1) Pd(PPh3)4, AcOH, 72%; (2) 1,3,5-(i-
Pr)3C6H2CO2H/Et3N salt 80 °C, xylene; (3) TFA, CH2Cl2, followed by
HPLC separation of 18 and 19.
parable yield. Once again, both products were found to be
stereochemically homogeneous.19
In the pinnatoxin A synthesis, the Diels-Alder product was
converted to the natural product in three steps: (1) Pd(PPh3)4, AcOH
(deprotection of the Alloc group), (2) 200 °C, 1 × 10-2 mmHg
(imine cyclization), and (3) TFA/CH2Cl2 (deprotection of the t-Bu
ester).3 For the present series, the Alloc deprotection smoothly took
place under the same conditions, to give the desired amino ketone.
Disappointingly, the resultant amino ketone did not survive under
the thermolysis conditions. In our earlier work, we found imine
formation under traditional, weakly acidic conditions to be unsuc-
cessful. For example, in the PnTX A synthesis, reaction in the
presence of AcOH and Et3N did not promote imine cyclization at
room temperature, whereas undesired N-acetylation was observed
at elevated temperatures. However, the instability of amino ketone
in this series under the original thermolysis conditions led us to
revisit the imine cyclization under weakly acidic conditions.
Specifically, we searched for weakly acidic conditions under which
the undesired N-acylation might be avoided or suppressed; in
particular, we focused on combinations of sterically congested
carboxylic acids and tertiary amines and eventually found that the
2,4,6-(i-Pr)3C6H2CO2H20/Et3N salt meets our needs.
(3) McCauley, J. A.; Nagasawa, K.; Lander, P. A.; Mischke, S. G.; Semones,
M. A.; Kishi, Y. J. Am. Chem. Soc. 1998, 120, 7647.
(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.
(5) A 1:1 mixture (0.3 mg) of PnTXs B/C was isolated from 21 kg of the
viscera of Pinna muricata. PtTX A (20 µg) and PtTX B/C (8 µg as a 1:1
mixture) were isolated from 82 kg of the viscera of Pteria penguin.
(6) Reported LD99 in ref 4: PnTXs B/C ) 22 µg/kg; PtTX A ) 100 µg/kg;
PtTXs B/C ) 8 µg/kg).
(7) The stereochemical homogeneity of PtTX A is discussed in a subsequent
paper, see ref 23.
(8) A total synthesis of PnTXs B and C from the Diels-Alder products 14a-
iii and 14b-iii, respectively, will be reported elsewhere.
(9) The building blocks dithiane 1 and vinyl iodide 2 were reported in ref 3.
We adopted the reported synthesis for preparation of vinyl iodide 2.
However, we have made substantial modifications on the synthesis of
dithiane 1; for details, see Supporting Information.
(10) (a) Petrow, A. A. Zh. Obshch. Khim. 1940, 10, 1013. (b) 5 is known; see
Ohgiya, T.; Nishiyama, S. Heterocycles 2004, 63, 2349.
(11) Faber, K. Biotransformations in Organic Chemistry; Springer-Verlag:
Heidelberg, Germany, 2004.
To complete the total synthesis, the only remaining task was to
remove the protecting groups of the cysteine moiety. For two
specific reasons, we chose N-Boc-Cys-OCHPh2. First, our previous
work3 demonstrated that all the functional groups present in PnTX
A, including the seven-membered imine, survive under the TFA/
CH2Cl2 (deprotection of the t-Bu ester) conditions. Second, the
combination of these two specific protecting groups ensured that
the protecting group of the carboxylic acid is cleaved prior to that
of the amine.21 Upon treatment with TFA in CH2Cl2 at room
temperature, both of the protecting groups were smoothly removed.
Finally, preparative LC allowed separation and isolation of pure
synthetic (34S,2′R)-PtTX A and (34R,2′R)-PtTX B/C.22 Overall,
the epoxide 15a furnished two out of the four possible stereoisomers
at C34 and C2′ for both the PtTX A and PtTX B/C series.
Applying the same synthetic sequence on the building block 7b,
we were able to synthesize the remaining stereoisomers at C34 and
C2′ for both the PtTX A and PtTXs B/C series. However,
comparison of NMR spectroscopic data between the synthetic and
natural samples did not lead us to the conclusion. In a subsequent
paper, we report our efforts to establish the stereochemistry of
natural PtTXs A-C, demonstrating that the availability of all pos-
sible stereoisomers is essential to rigorously address the problem.23
(12) For details, see Supporting Information.
(13) For selected reviews on Cr-mediated reactions, see: (a) Fu¨rstner, A. Chem.
ReV. 1999, 99, 991. (b) Wessjohann, L. A.; Scheid, G. Synthesis 1999, 1.
(c) Nozaki, H.; Takai, K. Proc. Jpn. Acad. 2000, 76, 123. (d) Saccomano,
N. A. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon: Oxford. 1991; Vol. 1, p 173.
(14) A Ni/Cr-mediated coupling similar to the present case is known: Taylor,
R. E.; Ciavarri, J. P. Org. Lett. 1999, 1, 467.
(15) The alternative sequence of reactions for transformation from 10a to 12a
were (a) 1, PIFA; 2, TPAP, NMO; 3, NiCl2, CrCl2, 2; 4, Dess-Martin
oxidation, 27% over 4 steps; (b) 1, PPTS; 2, K2CO3, MeOH; 3, PPTS,
63% over 3 steps.
(16) The substrates 12 and 13 did not survive in the presence of Lewis acids
or in the absence of Sumilizer.
(17) The intramolecular Diels-Alder reaction of 12a and 12b was not observed
below the indicated temperature.
(18) (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.
(19) This was evident because these stereoisomers were not detected in the
corresponding C34-R series or vice versa.
(20) Fuson, R. C.; Horning, E. C. J. Am. Chem. Soc. 1940, 62, 2962.
(21) We assumed, and indeed observed, that acid-promoted deprotection of
the t-Bu ester requires much more forcing conditions, once the amino-
protecting group is hydrolyzed.
(22) Column: YMC-Pack ODS-A, 250 × 10 mm. Solvent: H2O/CH3CN )
3/1 (v/v) containing 0.1% TFA. Flow rate:
Detection: 216 nm.
2 mL/min, isocratic.
(23) Hao, J.; Matsuura, F.; Kishi, Y.; Kita, M.; Uemura, D.; Asai, N.; Iwashita,
T. J. Am. Chem. Soc. 2006, 128, published online May 25, 2006
http://dx.doi.org/10.1021/ja061893j.
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