Absolute configuration of the common terminal acid portion of zooxanthellatoxins from a symbiotic dinoflagellate Symbiodinium sp. established by the synthesis of its ozonolysis product

Article abstract of DOI:10.1016/S0040-4039(00)00062-9

To determine the absolute configuration of the common C25 long chain acid part (L) of zooxanthellatoxins, the degradation product derived from the terminal carboxylic acid portion of L was synthesized from glucose and used to establish that the total absolute configuration of L was 3R,4R,5S,7S, 11S, 12S, 13R, 14S, 15R, 17R, 18R. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00062-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1927–1930
Absolute configuration of the common terminal acid portion of
zooxanthellatoxins from a symbiotic dinoflagellate Symbiodinium
sp. established by the synthesis of its ozonolysis product
Hideshi Nakamura,a,∗ Kyoko Maruyama,b Kazuhiro Fujimaki b and Akio Murai b
aDivision of Biomodeling, Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences,
Nagoya University, Chikusa, Nagoya 464-8601, Japan
bDivision of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 4 October 1999; revised 3 December 1999; accepted 28 December 1999
Abstract
To determine the absolute configuration of the common C25 long chain acid part (L) of zooxanthellatoxins, the
degradation product derived from the terminal carboxylic acid portion of L was synthesized from glucose and used
to establish that the total absolute configuration of L was 3R,4R,5S,7S,11S,12S,13R,14S,15R,17R,18R. © 2000
Elsevier Science Ltd. All rights reserved.
Keywords: marine metabolite; polyketides; configuration; lactones.
Marine micro-algae produce various types of compounds including nitrogenous neurotoxins, polyether
seafood toxins, sulfonium compounds as precursors of dimethylsulfide, and antineoplastic macrolides.1–3
However, there are only a few examples of large polyoxygenated molecules with a known absolute
configuration. Zooxanthellatoxin-A (ZT-A) and ZT-B have been isolated from a symbiotic marine
dinoflagellate Symbiodinium sp. (strain No. Y-6) as vasoconstrictive components.4 Their structures
contain several common features such as a 62-membered lactone, a sulfate ester, an exomethylene and
an amide linkage.5 While the major long-chain amino acid portions including the 62-membered lactone
vary with regard to the length of the carbon chain and the number of hydroxyl groups, the terminal acid
portions (compound L, 1) prepared from ZT-A and -B by basic hydrolysis are identical, including their
absolute configuration.5
To elucidate the absolute structure of zooxanthellatoxins, we started by investigating the absolute
configuration of the fragments obtained by degradation experiments.6 Ozonolysis of L followed by
treatment with HCl–MeOH and then acetic anhydride and pyridine gave two fragments which included
the tetrahydrofuran ring portion (C9–C21) and the terminal carboxylic acid part (C1–C8), respectively.6a
∗
Corresponding author. Tel: 81-52-789-4284; fax: 81-52-789-4284; e-mail: hnak@agr.nagoya-u.ac.jp (H. Nakamura)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00062-9
tetl 16342

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We have previously reported the synthesis and absolute configuration of the tetrahydrofuran fragment.6a
We report here the synthesis of the rest of the molecule, which fully characterizes compound L.
The relative configuration of the two tetrahydrofurans of 2 was difficult to elucidate by their NMR data
because of the unreliability of the coupling constants and NOE in a five-membered ring system. The large
1
influence of the side chain structure on the H NMR spectral pattern of a γ-lactone prevented us from
deducing the relative configuration at C3–C5 on the basis of comparison of the 1H NMR data of either γ-
lactone derivatives of L (1a) such as its heptaacetate 1b or a mixture of acetals of the ozonolysis product
(2) with those of the possible four diastereomeric γ-lactone models prepared from hexoses. Finally, we
applied a newly developed J-based configuration analysis by considering a population of the possible
rotamers and elucidated the relative configuration as shown in Fig. 1; i.e. an all-syn configuration of the
triol and an anti configuration of the methyl group.7 To confirm the reliability of the new method and
to determine the absolute configuration, we started the synthesis of γ-lactones 2 from readily available
glucose derivative 3 by a synthetic route applicable to the syntheses of other isomers, especially at C3
and C7 (Fig. 2).
Fig. 1. Compound L (1) and related compounds
Benzylidene acetal 3 was brominated under conditions described in the literature and reductive opening
of the pyranose ring gave an aldehyde8 which was converted to α,β-unsaturated ester 4 by treating it with
methyl (triphenylphoranylidene)acetate. 1,4-Reduction with CuH9 followed by debenzoylation yielded
an alcohol (5). Introduction of a methyl group was achieved by Evans’ procedure10,11 after protection
of the hydroxyl group with benzyloxymethyl chloride (BOMCl) and diisopropylethylamine (DIPEA).
The absolute configuration of the introduced methyl was confirmed by derivatizing to the amides of
(S)- and (R)-phenylglycine methyl esters (PGME). The proton chemical shift of the methyl group of the
(R)-PGME amide was smaller than that of (S)-PGME.12 Reduction of the ester with lithium aluminum
hydride afforded a primary alcohol (7). Treatment of the alcohol with benzyl bromide and t-BuOK gave a
benzyl ether, which was transformed to 8 by five steps: (1) deprotection with HCl–MeOH; (2) protection
with MOMCl; (3) hydroboration; (4) Swern oxidation; and (5) oxidation with NaClO2.
1
A 1:1 mixture of the acetals 2 was obtained in four steps. The H NMR spectra of the synthetic 2
in two different solvents were superimposable to those of the acetals derived from L, respectively [400
MHz in C6D6: two pairs of unassignable methoxy (δ 3.25, 3.19) and acetyl signals (δ 1.48, 1.50) and
the signals for the isomer A; δ 0.88 (3H, d, J=7 Hz), 1.09 (ddd, J=12, 8, 6 Hz, H6), 1.81 (m, H60), 2.14
(m, H7), 2.21 (dd, J=18, 7 Hz, H2), 2.49 (dd, J=18, 5 Hz, H20), 3.92 (dd, J=6, 4 Hz, H4), 4.18 (ddd,
J=8, 8, 4 Hz, H5), 4.51 (d, J=2 Hz, H8), 4.87 (ddd, J=7, 6, 5 Hz, H3): for isomer B; δ 0.98 (3H, d, J=7
Hz, Me), 1.37 (td, J=12, 10 Hz, H6), 1.53 (td, J=12, 7 Hz, H60), 1.81 (m, H7), 2.09 (dd, J=18, 7 Hz,
H2), 2.34 (dd, J=18, 3 Hz, H20), 3.95 (dd, J=6, 5 Hz, H4), 4.22 (ddd, J=10, 7, 6 Hz, H5), 4.57 (d, J=5

1929
Fig. 2. Synthesis of a γ-lactone (2). Reagents and conditions: (a) (1) NBS, AIBN, BaCO3, CCl4, reflux, 1 h; (2) Zn, EtOH–H2O,
reflux, 3 h; (3) Ph3P_CHCOOCH3, benzene, 23°C, 23 h, 77% for three steps; (b) (1) NaBH4, CuCl2, MeOH, 23°C, 3 h; (2)
NaOMe, MeOH, 23°C, 7 h, 46% for two steps; (c) (1) BOMCl, DIPEA, CH2Cl2, 23°C, 2 days, 88%; (2) NaOH, MeOH–H2O,
0°C, 18 h, 92%; (3) PivCl, Et3N, THF, LiCl, (4S)-4-isopropyl-2-oxazolidinone, −20–23°C, 6.5 h, 97%; (d) (1) LDA, MeI,
THF, −78–−40°C, 22 h, 77%; (2) LAH, THF, 0°C, 35 min; (e) (1) BnBr, t-BuOK, THF, 0°C, 5.5 h, 92%; (2) 4% HCl–MeOH,
23°C, 3 days, 78%; (3) MOMCl, DIPEA, 23°C, 2 days, 100%; (4) BH3–THF, THF, 0°C, 10 h, then H2O2, NaOH, MeOH–H2O,
0–23°C, 2.5 h, 64%; (5) (COCl)2, DMSO, CH2Cl2, −78°C for 1 h, −45°C for 1 h, then Et3N, 0°C, 35 min, 89%; (6) NaClO2,
NaHPO4, 2-methyl-2-butene, t-BuOH–H2O, 23°C, 1.5 h, 71%; (f) (1) MOMCl, DIPEA, THF, 23°C, 3.5 h, 100%; (2) 5% Pd/C,
H2, MeOH, 23°C, 2 days, 78%; (3) (COCl)2, DMSO, CH2Cl2, −78°C, 1 h, then Et3N, 0°C, 40 min, 81%; (g) 1% HCl–MeOH,
23°C, 3 days; (2) Ac2O, pyridine, 23°C, 4 h, 35% for two steps
Hz, H8), 4.86 (ddd, J=7, 5, 3 Hz, H3)], which confirmed the relative configuration deduced by J-based
configuration analysis. The [α]D data of the degradation product 2 [+34 (MeOH, c 0.02) at 20°C] and the
synthetic product [+32 (MeOH, c 0.03) at 17°C] suggested that the absolute configuration of 2 from L is
the same as that of the synthetic product. The above results, together with those of our previous paper,
indicate that the absolute configuration of L is 3R,4R,5S,7S,11S,12S,13R,14S,15R,17R,18R. There are
few examples of large polyols from marine organisms; however, several common structural features are
found, such as 1,2-syn-diol structures in a plausible acetate unit, which may be one of the characteristics
in the biogenesis of these polyoxygenated marine metabolites including palytoxins.
Zooxanthella (strain Y-6) was shown to produce another type of nitrogen-containing compound,
zooxanthellamine.13 Zooxanthellamine and zooxanthellatoxins are polyketide-derived molecules whose
chain-elongation reactions are believed to start from glycine. Similar polyketide metabolites are found
in other Symbiodinium species.14 Studies on the details of biogenesis and the absolute configuration of
these polyketide amino acids are in progress at our laboratory.
Acknowledgements
The authors acknowledge Dr. Matsumori and Prof. Murata at Tokyo University for their help in
determining the relative configuration of L, and Prof. Kusumi at Tokushima University for his generous
gift of (S)- and (R)-phenylglycine methyl esters. This work was supported in part by a Grant-in-Aid from
the Ministry of Education, Science, Sports and Culture, Japan. We are also thankful for financial support
from the Naito Foundation, Fujisawa Foundation, and Takeda Foundation.

1930
References
1. Faulkner, D. J. Nat. Prod. Rep. 1999, 16, 155–198, and references cited therein.
2. Shimizu, Y. Chem. Rev. 1993, 93, 1685–1698.
3. Yasumoto, T.; Murata, M. Chem. Rev. 1993, 93, 1897–1909.
4. Nakamura, H.; Asari, T.; Matsuoka, S.; Ohizumi, Y.; Kobayashi, J.; Yamasu, T.; Murai, A. Toxicon 1993, 31, 371–376.
5. (a) Nakamura, H.; Asari, T.; Fujimaki, K.; Maruyama, K.; Murai, A.; Ohizumi, Y.; Kan, Y. Tetrahedron Lett. 1995, 36,
7255–7258. (b) Nakamura, H.; Asari, T.; Murai, A.; Kan, Y.; Kondo, T.; Yoshida, K.; Ohizumi, Y. J. Am. Chem. Soc. 1995,
117, 550–551. (c) Nakamura, H.; Asari, T.; Murai, A.; Kondo, T.; Yoshida, K.; Ohizumi, Y. J. Org. Chem. 1993, 58, 313–314.
(d) Asari, T.; Nakamura, H.; Murai, A.; Kan, Y. Tetrahedron Lett. 1993, 34, 4059–4062.
6. (a) Nakamura, H.; Fujimaki, K.; Murai, A. Tetrahedron Lett. 1996, 37, 3153–3156. The absolute configuration at C12–C14
was incorrectly described as 12R,13S,14R. (b) Nakamura, H.; Sato, K.; Murai, A. Tetrahedron Lett. 1996, 37, 7267–7270.
(c) Nakamura, H.; Takahashi, H.; Murai, A. Tetrahedron: Asymmetry 1998, 9, 2571–2574.
7. Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.; Tachibana, K. J. Org. Chem. 1999, 64, 866–876.
8. Bernet, B.; Vasella, A. Helv. Chim. Acta 1979, 51, 12229–12238.
9. Narisada, M.; Horibe, I.; Watanabe, F.; Takeda, K. J. Org. Chem. 1989, 54, 5308–5313.
10. Evans, D. A.; Bender, S. L.; Morris, J. J. Am. Chem. Soc. 1988, 110, 2506–2526.
11. Ho, G. -J.; Mathre, D. J. J. Org. Chem. 1995, 60, 2271–2273.
12. Nagai, Y.; Kusumi, T. Tetrahedron Lett. 1995, 36, 1853–1856.
13. Nakamura, H.; Kawase, Y.; Maruyama, K.; Murai, A. Bull. Chem. Soc. Jpn. 1998, 71, 781–787.
14. Shimizu, Y. 1998 Annual Meeting of the Japanese Society of Bioscience, Biotechnology and Agrochemistry, Nagoya, April,
1998; S-5-4; Abstract, p. 466.