Synthesis and absolute configuration of an exomethylene portion of zooxanthellatoxin-A

Article abstract of DOI:10.1016/S0957-4166(98)00270-5

The absolute configuration of the exomethylene portion of zooxanthellatoxin-A (ZT-A) was established as 71R,73R,74S,75R by comparing an MTPA ester of an acyclic degradation product of ZT-A with those of the compound synthesized from methyl-α-D-glucopyranoside.

Full text of DOI:10.1016/S0957-4166(98)00270-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 2571–2574
Synthesis and absolute configuration of an exomethylene portion
of zooxanthellatoxin-A
Hideshi Nakamura,a,∗ Michiko Takahashi b and Akio Murai b
aDivision of Biomodeling, Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences,
Nagoya University, Nagoya 464-8601, Japan
bDivision of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 16 June 1998; accepted 4 July 1998
Abstract
The absolute configuration of the exomethylene portion of zooxanthellatoxin-A (ZT-A) was established as
71R,73R,74S,75R by comparing an MTPA ester of an acyclic degradation product of ZT-A with those of the
compound synthesized from methyl-α-D-glucopyranoside. © 1998 Elsevier Science Ltd. All rights reserved.
Zooxanthellatoxin-A (ZT-A) and -B were isolated from the symbiotic dinoflagellate, Synbiodinium sp.
(strain Y-6) as vasoconstrictive compounds1a and found to be activators of rabbit platelet aggregation.1b–d
The structures were determined on the basis of degradation experiments coupled with extensive spectral
analyses.2 In order to investigate their biogenesis and mechanisms of the bioactivities, we started to
determine their stereochemistry by a combination of synthetic and spectroscopic studies and have
reported the absolute configuration of the tetrahydropyran portion of the common terminal acid (L) in
zooxanthellatoxins and the spiroacetal portion of zooxanthellatoxin-A.3 Since major differences of the
structures between ZT-A and the congener ZT-B were seen around the exomethylene containing portion,
it was interesting to know the differences of the stereochemistry of their exomethylene units from a
biosynthetic point of view. Here we report the absolute configuration of an exomethylene portion of
ZT-A.
Under the selected oxidation conditions with a use of limited amounts of NaIO4, the tetraacetate 1b
was obtained after reduction with NaBH4 followed by acetylation. The relative configuration of the
tetrahydropyran structure of 1b was determined on the basis of coupling constants and NOE data. Due to
low reproducibility of the degradation experiments, it was difficult to obtain enough 1 to determine the
absolute configuration. We selected a final glycol cleavage product with NaIO4 2a as a key compound
to determine the absolute configuration by comparing NMR data of its MTPA esters. For this purpose
either 1a or its C73 and/or C74 epimers could be used as a precursor of 2a.
∗
Corresponding author. E-mail: hnak@agr.nagoya-u.ac.jp
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00270-5

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H. Nakamura et al. / Tetrahedron: Asymmetry 9 (1998) 2571–2574
Bromination of the readily available acetate 34,5 derived from α-methyl-D-glucoside with NBS gave
bromide 44,6 which was transformed to cyano compound 54 upon treatment with KCN in DMSO.
Successive reduction of 5 with DIBALH followed by NaBH4 afforded a triol which was isolated as
a triacetate 6 after acetylation.7 After several unsuccessful attempts to introduce a C4 unit to the
tetrahydropyran ring of 6 with an allylsilane AcOCH2CH2C(CH2)CH2TMS,8 we attempted the C-
glycosidation reaction after acetolysis of 6.5a Treatment of 6 with Ac2O:H2SO4 (100:1) gave a mixture of
acetates, from which a tetraacetate 79 was obtained in 40% yield. Under acetolysis conditions, inversion
of the acetoxy group at the C3 position occurred, in which a cyclic acetoxonium intermediate between
C1 and C3 carbons might be involved. C-Glycosidation of 7 with allylsilane smoothly proceeded at
1
0°C to afford 1b in a moderate yield of 33% (Scheme 1). H NMR spectra of the synthetic 1b10 and
the degradation product 1b from ZT-A were superimposable, thus the relative configuration of the
exomethylene portion of ZT-A was unambiguously established. After deacetylation of 1b, the tetraol
1
1a11 was subjected to NaIO4 oxidation to give a synthetic acyclic tetraol 2a.12 Because the H NMR
spectra of the (R)- and (S)-MTPA esters (synthetic 2c and 2d) were distinguishable,13,14 it is possibe to
establish the absolute configuration by an MTPA ester of the degradation product 2a derived from ZT-A.
Scheme 1.
A seco-acid of ZT-A was treated with NaIO4 followed by reductive work-up and acetylation to afford
a mixture of acetates containing the exomethylene portion. Since cleavage of the glycols was incomplete,
the mixture was further treated with NaIO4 after deacetylation. Reductive work-up followed by desalting
by ion exchange gave a mixture of alcohols. The mixture was acylated with (R)-MTPA acid, DCC and
1
DMAP to yield an (R)-MTPA ester of 2a. The H NMR spectrum of the MTPA ester were identical
to that of the (R)-MTPA ester of the synthetic product 2a, by which the absolute configuration of the
exomethylene portion of ZT-A was proved to be 71R,73R,74S,75R as shown in Fig. 1.
Studies on the absolute configuration of the other parts of zooxanthellatoxins such as an exomethylene
portion of zooxanthellatoxin-B are in progress.
Acknowledgements
This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports
and Culture, Japan and by a grant from the Fujisawa Foundation.

H. Nakamura et al. / Tetrahedron: Asymmetry 9 (1998) 2571–2574
2573
Fig. 1.
References
1. (a) Nakamura, H.; Asari, T.; Matsuoka, S.; Ohizumi, Y.; Kobayashi, J.; Yamasu, T.; Murai, A., Toxicon, 1993, 31, 371–376.
(b) Rho, M.-C.; Nakahata, N.; Nakamura, H.; Murai, A.; Ohizumi, Y., Br. J. Pharmacol., 1995, 115, 433–440. (c) Rho,
M.-C.; Nakahata, N.; Nakamura, H.; Murai, A.; Ohizumi, Y. Eur. J. Pharmacol., 1997, 319 (2/3), 375–378. (d) Rho, M.-C.;
Nakahata, N.; Nakamura, H.; Murai, A.; Ohizumi, Y., J. Pharmacol. Exp. Ther., 1997, 282(1), 496–504.
2. (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.
3. (a) Nakamura, H.; Fujimaki, K.; Murai, A., Tetrahedron Lett., 1996, 37, 3153–3156. (b) Nakamura, H.; Sato, K.; Murai,
A., Tetrahedron Lett., 1996, 37, 7267–7270.
4. All new compounds gave satisfactory spectral data.
5. (a) Richtmyer, N. K., Methods in Carbohydr. Chem., 1962, 1, 107–113. (b) Baer, H. H.; Hanna, H. R., Carbohydr. Res.,
1982, 110, 19–41.
6. (a) Hanessian, S.; Plessas, N. R., J. Org. Chem., 1969, 34(4), 1035–1044. (b) Bernet, B.; Vasella, A., Helv. Chim. Acta.,
1979, 62(6), 1990–2016.
7. Nakata, M.; Ishiyama, T.; Akamatsu, S.; Hirose, Y.; Maruoka, H.; Suzuki, R.; Tatsuta, K., Bull. Chem. Soc. Jpn, 1995, 68,
967–989.
8. Ochiai, M.; Fujita, E.; Arimoto, M.; Yamaguchi, H., Chem. Pharm. Bull., 1983, 31(1), 86–93.
22
9. Tetraacetate 7: [α]D +90.8 (c 1.02, CHCl3); 1H NMR (270 MHz, CDCl3) δ 6.20 (1H, brddd, 4 Hz, H1), 5.28 (1H, ddd,
5, 10, 12 Hz, H3), 4.86 (1H, t, 10 Hz), 4.16, 4.10 (each 1H, m, H7), 3.93 (1H, dt, 3, 10 Hz, H5), 2.25 (1H, brdd, 5, 14 Hz,
H2α), 2.12, 2.07 (each 3H, s, OAc), 2.03 (6H, s, OAc), 1.93 (1H, ddd, 4, 12, 14 Hz, H2β), 1.83 (1H, m, H6), 1.76 (1H,
tdd, 6, 10, 15 Hz, H6); HR-EIMS: m/z 287.1114 (M−OAc)+. Calcd for C13H19O7 287.1131.
21
10. Tetraacetate 1b: [α]D +35.6 (c 0.52, CHCl3); IR (KBr) νmax 3022, 2962, 2932, 2866, 1740, 1371, 1242, 1074, 1050,
753 cm−1; 1H NMR (400 MHz, C6D6) δ 5.32 (1H, ddd, 5, 8, 10 Hz, H73), 4.98 (1H, t, 8 Hz, H74), 4.80, 4.79 (each 1H,
brs, exomethylene), 4.22, 4.21 (each 1H, td, 7, 10 Hz, H77), 4.09 (2H, t, 7 Hz, H67), 3.91 (1H, tdd, 5, 6, 9 Hz, H71), 3.78
(1H, dt, 4, 8 Hz, H75), 2.34 (1H, dd, 9, 14 Hz, H70), 2.18 (2H, t, 7 Hz, H68), 1.88–1.94 (2H, m, H76), 1.89 (1H, dd, 6, 14
Hz, H70), 1.78 (1H, td, 5, 13 Hz, H72α), 1.75, 1.74, 1.72, 1.71 (each 3H, s, OAc), and 1.67 (1H, ddd, 5, 9, 13 Hz, H72β);
HR-EIMS m/z 354.1684 (M−AcOH)+. Calcd for C18H26O7 354.1678.
11. Tetraol 1a: [α]D21 +40.9 (c 0.12, MeOH); 1H NMR (400 MHz, CD3OD, 35°C) δ 4.89, 4.87 (each 1H, brs, exomethylene),
4.14 (1H, dtd, 2, 6, 9 Hz, H71), 3.73 (1H, ddd, 5, 9, 11 Hz, H73), 3.66 (2H, t, 7 Hz, H67), 3.66 (1H, m, H77), 3.58 (1H,
td, 7, 11 Hz, H77), 3.51 (1H, dt, 3, 9 Hz, H75), 3.00 (1H, t, 9 Hz, H74), 2.57 (1H, dd, 9, 15 Hz, H70), 2.29 (2H, t, 7 Hz,

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H. Nakamura et al. / Tetrahedron: Asymmetry 9 (1998) 2571–2574
H68), 2.23 (1H, dd, 6, 15 Hz, H70), 2.05 (1H, dtd, 3, 7, 14 Hz, H76), 1.90 (1H, ddd, 2, 5, 14 Hz, H72α), 1.69 (1H, ddd, 6,
11, 14 Hz, H72β), and 1.63 (1H, m, H76); EI-MS m/z 161 [(M−C5H9O)+, 13%], 149 (28%), 143 (6%), and 125 (100%);
HR-FDMS m/z 247.1518 (M+H)+. Calcd for C12H23O5 247.1545.
21
12. Tetraol 2a: [α]D −1.8 (c 0.11, MeOH); 1H NMR (400 MHz, D2O) δ 4.95, 4.94 (each 1H, s, exomethylene), 3.81 (1H,
ddt, 5, 6, 7 Hz, H71), 3.72 (2H, t, 6 Hz, H67), 3.68 (2H, t, 7 Hz, H73), 3.70–3.65 (1H, m, H75), 3.70–3.65 (2H, m, H77),
3.64 (1H, dd, 4, 11 Hz, H74), 3.53 (1H, dd, 5, 11 Hz, H74), 2.38 (1H, dd, 6, 14 Hz, H70), 2.31 (2H, t, 6 Hz, H68), 2.22
(1H, dd, 7, 14 Hz, H70), 1.81 (1H, ddd, 5, 7, 14 Hz, H72), 1.72 (1H, m, H72), and 1.81–1.70 (2H, m, H76); HR-FDMS
m/z 249.1702 (M+H)+. Calcd for C12H25O5 249.1702.
26
13. (R)-MTPA ester of 2c: [α]D +47.6 (c 0.087, CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.47 (8H, m), 7.34 (12H, m), 4.76,
4.73 (each 1H, brs, exomethylene), 4.37 (1H, td, 6, 11 Hz, H73), 4.36 (1H, td, 6, 11 Hz, H77), 4.31 (2H, t, 7 Hz, H67), 4.26
(1H, ddd, 5, 7, 11 Hz, H77), 4.20 (1H, dd, 4, 12 Hz, H74), 4.17 (1H, ddd, 5, 7, 11 Hz, H73), 4.07 (1H, dd, 5, 12 Hz, H74),
3.54 (1H, m, H75), 3.51, 3.48, 3.47, 3.46 (each 3H, OMe), 3.42 (1H, tt, 6, 7 Hz, H71), 2.24 (2H, t, 7 Hz, H68), 2.12 (1H,
dd, 6, 15 Hz, H70), 1.95 (1H, dd, 7, 15 Hz, H70), 1.80, 1.74 (each 1H, m, H76), 1.72 (1H, dtd, 6, 7, 15 Hz, H72), and 1.62
(1H, dtd, 5, 6, 15 Hz, H72); HR-EIMS m/z 811.2158 (M−C15H16O3F3). Calcd for C37H36O10F9, 811.2164; HR-FABMS
m/z 1113.3380 (M+H)+. Calcd for C52H53O13F12, 1113.3294.
24
14. (S)-MTPA ester of 2d: [α]D −40.2 (c 0.063, CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.47 (8H, m), 7.36 (12H, m), −7.35
(20H, m, Ph), 4.77, 4.72 (each 1H, brs, exomethylene), 4.40 (1H, td, 6, 11 Hz, H77), 4.32 (2H, t, 7 Hz, H67), 4.27 (2H,
m, H73), 4.21 (1H, dd, 4, 12 Hz, H74), 4.20 (1H, td, 6, 11 Hz, H77), 4.08 (1H, dd, 4, 12 Hz, H74), 3.51, 3.50, 3.48, 3.47
(each 3H, OMe), 3.49 (1H, m, H75), 3.47 (1H, m, H71), 2.24 (2H, t, 7 Hz, H68), 2.14 (1H, dd, 5, 14 Hz, H70), 1.89 (1H,
dd, 7, 14 Hz, H70), 1.78 (1H, m, H72), 1.77 (2H, m, H76), and 1.61 (1H, m, H72); HR-EIMS m/z 811.2126. Calcd for
C37H36O10F9, 811.2164.