Synthesis of the CDE/FG ring models of prymnesins: Reassignment of the relative configuration of the E/F ring juncture

Article abstract of DOI:10.1021/ol049569l

Synthesis of two diastereomeric models (3a and 3b) corresponding to the CDE/FG ring of prymnesins, polycyclic ether toxins isolated from the red tide phytoflagellate Prymnesium parvum, is described. Comparison of the ¹H and ¹³C NMR d

Full text of DOI:10.1021/ol049569l

ORGANIC
LETTERS
2004
Vol. 6, No. 9
1501-1504
Synthesis of the CDE/FG Ring Models
of Prymnesins: Reassignment of the
Relative Configuration of the E/F Ring
Juncture
,†
Makoto Sasaki,* Makoto Ebine,† Hiroyuki Takagi,† Hiroyuki Takakura,†
Takeshi Shida,‡ Masayuki Satake,† Yasukatsu Oshima,† Tomoji Igarashi,§ and
Takeshi Yasumoto§
Graduate School of Life Sciences, Tohoku UniVersity, Tsutsumidori-amamiya,
Aoba-ku, Sendai 981-8555, Japan, Graduate School of Science, UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Tama Laboratory,
Japan Food Research Laboratories, Nagayama, Tama, Tokyo 206-0025, Japan
masasaki@bios.tohoku.ac.jp
Received March 8, 2004
ABSTRACT
Synthesis of two diastereomeric models (3a and 3b) corresponding to the CDE/FG ring of prymnesins, polycyclic ether toxins isolated from
the red tide phytoflagellate Prymnesium parvum, is described. Comparison of the 1H and 13C NMR data for each compound with those reported
for prymnesins suggests that the earlier stereochemical assignment of the E/F ring juncture needs to be revised.
Prymnesium parVum is a unicellular alga that blooms in
brackish water and causes massive fish kills worldwide. The
causative toxins, prymnesin-1 (PRM1, 1) and prymnesin-2
(PRM2, 2), were isolated from cultured cells of the
phytoflagellate. These toxins possess extremely potent
hemolytic activity, which is about 5000-fold greater than that
of Merk saponin on a molar basis, and also exhibit potent
ichthyotoxicity. Their gross structures, including partial
stereochemistry, were determined by Igarashi et al.1,2
Prymnesins possess unique structural features: an un-
branched single chain of 90 carbons except for a single
methyl group, a fused polycyclic ether ring system (A-E
ring), four distinct 1,6-dioxadecalin units (FG, HI, JK, and
LM rings), conjugated double and triple bonds, chlorine
atoms and an amino group, and glycosidic residues, including
an uncommon L-xylose. The relative stereochemistry of the
fused A-E polycyclic ether ring domain and four 1,6-
dioxadecalin units was determined by extensive NMR
analysis. Recently, the absolute configuration at C14 bearing
an amino group in PRM2 was determined to be S by using
a chiral anisotropic reagent and that at chlorinated C85 to
be S by fluorimetric chiral HPLC comparison between a
degradation product and synthetic references (Figure 1).3
On the basis of the extensive NOE data as well as coupling
constants, it was proposed that the E/F ring juncture adopted
a twisted gauche rotamer, where two pairs of diaxial protons
(33-H/37-H and 38-H/42-H) were aligned under approxi-
mately 20° of the dihedral angle.2 Although this conformation
best explained the observed NMR data, it remained some-
† Tohoku University.
‡ University of Tokyo.
§ Japan Food Research Laboratories.
(1) Igarashi, T.; Satake, M.; Yasumoto, T. J. Am. Chem. Soc. 1996, 118,
479-480.
(2) Igarashi, T.; Satake, M.; Yasumoto, T. J. Am. Chem. Soc. 1999, 121,
8499-8511.
(3) Morohashi, A.; Satake, M.; Oshima, Y.; Igarashi, T.; Yasumoto, T.
Chirality 2001, 13, 601-605.
10.1021/ol049569l CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/08/2004

Figure 1. Proposed structures of prymnesin-1 (PRM1, 1) and prymnesin-2 (PRM2, 2).
what ambiguous for the relative configuration between the
C-37 and C-38 positions.
envisioned that the six-membered E ring within 3a and 3b
could be formed by reductive cyclization of dihydroxy
ketones 4a and 4b. These intermediates, in turn, would be
obtained through convergent coupling of the acetylide anion
generated from alkyne 5 with aldehydes 6 and ent-6.
We have already demonstrated that the synthesis of model
fragments coupled with J-based configuration analysis
(JBCA)4 has successfully elucidated the relative configuration
of the acyclic portions of maitotoxin, the most toxic and
largest nonbiopolymer.5 A similar approach was applied to
prymnesins, and the relative configuration of the I/J ring
juncture was unambiguously confirmed.6 As part of our
studies toward complete stereochemical assignment of
prymnesins, we describe herein the synthesis of two dia-
stereomeric CDE/FG ring models 3a and 3b for comparison
of their NMR data with those of prymnesins, which led to
reassignment of the proposed stereochemical assignment of
the E/F ring juncture of the natural products.
The synthesis of the CD ring alkyne 5 started with bicyclic
ketone 77 (Scheme 2). Ring expansion8 of 7 with tri-
Scheme 2. Synthesis of the CD Ring Alkynea
Retrosynthetic analysis for convergent synthesis of model
compounds 3a and 3b is outlined in Scheme 1. We
Scheme 1. Retrosynthetic Analysis
a Reagents and conditions: (a) trimethylsilyldiazomethane,
BF3‚OEt2, CH2Cl2, -78 °C; (b) CSA, MeOH, rt, 67% (two steps);
(c) LiHMDS, Et3N, TMSCl, THF, -78 °C; (d) OsO4 (cat.), NMO,
THF/H2O, rt, 89% (two steps); (e) DIBAL-H, CH2Cl2, -78 °C; (f)
Me2C(OMe)2, CSA, CH2Cl2, rt, 95% (two steps); (g) H2, Pd(OH)2/
C, MeOH, rt, 97%; (h) SO3‚pyr, Et3N, DMSO, CH2Cl2, 0 °C; (i)
CBr4, PPh3, CH2Cl2, rt, 85% (two steps); (j) NaHMDS, THF, -100
°C, then n-BuLi, 90%.
methylsilyldiazomethane in the presence of BF3‚OEt2 gave,
after acidic treatment, seven-membered ketone 8 in 67% yield
for the two steps. Ketone 8 was converted to the correspond-
ing silyl enol ether, which was then oxidized with OsO4 to
afford R-hydroxy ketone 9 in 89% yield as the sole product.9
DIBAL-H reduction of 9 followed by protection of the
resultant diol gave acetonide 10 in 95% overall yield. The
stereochemistry of 10 was unambiguously established by
(4) Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.; Tachibana,
K. J. Org. Chem. 1999, 64, 866-876.
1502
Org. Lett., Vol. 6, No. 9, 2004

NOEs between 30-H/32-H and 32-H/33-H.10 Removal of
the benzyl group by hydrogenolysis provided alcohol 11
(97%), which was then converted into dibromoolefin 12 in
two steps (85% yield).11 Finally, sequential treatment with
NaHMDS and n-BuLi afforded the desired alkyne 5 in 90%
yield.12
Scheme 4. Synthesis of Model Compounds 3a and 3ba
For the construction of the FG ring aldehyde 6, we used
ketone 77 as the same starting material, which was converted
to exo-methylene 13 through Peterson olefination13 (Scheme
3). This olefin was subjected to hydrogenation/hydrogenoly-
Scheme 3. Synthesis of the FG Ring Fragmenta
a Reagents and conditions: (a) TMSCH2MgCl, THF, 0 °C; (b)
HOAc, 120 °C, 55% (two steps); (c) H2, Pd/C, EtOAc, rt, 50%;
(d) SO3‚pyr, Et3N, DMSO, CH2Cl2, 0 °C, 85%.
a Reagents and conditions: (a) 5, n-BuLi, THF, -78 °C, CeCl3,
then 6, 93%; (b) H2, Pd/C, MeOH, rt; (c) TPAP, NMO, 4 Å MS,
CH2Cl2, rt, 80% (two steps); (d) 6 M HCl, THF, 0 f 30 °C, 89%;
(e) p-TsOH‚H2O, MeOH, rt, 44% (92% based on recovered 4a);
(f) Et3SiH, BF3‚OEt2, CH2Cl2/MeCN, 0 °C, 83%; (g) Ac2O, DMAP,
CH2Cl2, rt, quant.
sis to produce an approximately 2:1 mixture of reduced
products, from which the desired alcohol 14 was separated
by column chromatography on silica gel in 50% yield.14 The
stereochemistry of the axial-oriented methyl group at C39
was confirmed by NOE between 39-Me and 41-H.10 Oxida-
tion of 14 gave aldehyde 6 in 85% yield.
alcohol 15 together with recovered 5 and 6. However, a less
basic alkynylcerium reagent15 prepared from the lithium
acetylide and cerium(III) chloride was more satisfactory and
added successfully to aldehyde 6 to give 15 as an ap-
proximately 7:3 diastereomeric mixture in high yield.
Hydrogenation of the triple bond followed by oxidation of
the secondary alcohol with TPAP/NMO provided ketone 16
in 80% yield over the two steps. The acetonide group was
removed by treatment with 6 M HCl (89%), and the obtained
keto diol 4a was then treated with p-toluenesulfonic acid in
methanol to yield methyl ketal 17 in 44% yield along with
recovered 4a (52%).16 Finally, reduction of 17 with tri-
ethylsilane in the presence of BF3‚OEt2 furnished the CDE/
FG ring model 3a in 83% yield.17 The stereochemistry of
3a was unambiguously established by NOE and coupling
constant data. Similarly, the diastereomeric model 3b was
prepared from alkyne 5 and aldehyde ent-6.18
With the desired 5 and 6 in hand, we next focused our
attention on their coupling (Scheme 4). An initial attempt to
couple the lithium acetylide generated from 5 (n-BuLi, THF,
-78 °C) with 6 gave a poor yield of the desired propargylic
(5) (a) Sasaki, M.; Nonomura, T.; Murata, M.; Tachibana, K. Tetrahedron
Lett. 1994, 35, 5023-5026. (b) Sasaki, M.; Nonomura, T.; Murata, M.;
Tachibana, K. Tetrahedron Lett. 1995, 36, 9007-9010. (c) Sasaki, M.;
Matsumori, N.; Murata, M.; Tachibana, K.; Yasumoto, T. Tetrahedron Lett.
1995, 36, 9011-9014. (d) Sasaki, M.; Matsumori, N.; Maruyama, T.;
Nonomura, T.; Murata, M.; Tachibana, K.; Yasumoto, T. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1782-1785. (e) Nonomura, T.; Sasaki, M.;
Matsumori, N.; Murata, M.; Tachibana, K.; Yasumoto, T. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1786-1789. For the work from Kishi’s
group, see: (f) Zheng, W.; DeMattei, J. A.; Wu, J.-P.; Duan, J. J.-W.;
Cook, L. R.; Oinuma, H.; Kishi, Y. J. Am. Chem. Soc. 1996, 118, 7946-
7968.
(6) Sasaki, M.; Shida, T.; Tachibana, K. Tetrahedron Lett. 2001, 42,
5725-5728.
(7) Mori, Y.; Yaegashi, K.; Furukawa, H. Tetrahedron Lett. 1999, 40,
7239-7242.
The two diastereomeric CDE/FG ring models 3a and 3b
were subjected to the NMR study, and the 1H and 13C NMR
(8) Mori, Y.; Yaegashi, K.; Furukawa, H. Tetrahedron 1997, 53, 12917-
12932.
(9) For similar stereoselective introduction of a hydroxy group into a
seven-membered ether, see: Tsukano, C.; Sasaki, M. J. Am. Chem. Soc.
2003, 125, 14294-14295.
(10) Numbering of all compounds in this paper corresponds to that of
prymnesins.
(11) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769-
3772.
(15) (a) Imamoto, T.; Kusumoto, T.; Tawarayama, Y.; Sugiura, Y.; Mita,
T.; Hatanaka, Y.; Yokoyama, M. J. Org. Chem. 1984, 49, 3904-3912. (b)
Imamoto, T.; Sugiura, Y.; Takiyama, N. Tetrahedron Lett. 1984, 25, 4233-
4236. (c) Fox, C. M. J.; Hiner, R. N.; Warrier, U.; White, J. D. Tetrahedron
Lett. 1988, 29, 2923-2926. (d) Molander, G. A. Chem. ReV. 1992, 92,
29-68.
(16) Direct treatment of 16 with excess amounts of p-toluenesulfonic
acid in methanol resulted in a low yield of methyl ketal 17.
(17) Lewis, M. D.; Cha, J. K.; Kishi, Y. J. Am. Chem. Soc. 1982, 104,
4976-4978.
(18) Synthesis of compounds ent-6 and 3b are included in Supporting
Information.
(12) Grandjean, D.; Pale, P.; Chuche, J. Tetrahedron Lett. 1994, 35,
3529-3530.
(13) Boeckman, R. K., Jr.; Silver, S. M. Tetrahedron Lett. 1973, 14,
3497-3500.
(14) For similar face-selective hydrogenation, see: (a) Wipf, P.; Uto,
Y.; Yoshimura, S. Chem. Eur. J. 2002, 8, 1670-1681. (b) Ireland, R. E.;
Daub, J. P. J. Org. Chem. 1981, 46, 479-485.
Org. Lett., Vol. 6, No. 9, 2004
1503

Figure 2. Revised structure of prymnesins.
of the E/F ring juncture needs to be revised to be represented
by structure 3b, though the NOE data of 3b did not fully
reproduce those observed for NAPRM2.19
chemical shifts for the C35-C40 portion of each compound
were compared with those of N-acetylprymnesin-2 (NAPRM2)
(Table 1). The NMR data observed for 3b, and not for 3a,
Table 2. Selected NMR Data of the C35-C40 Regions in
PAPRM2 and Model Compounds 18a and 18b (600 MHz,
CDCl3)
Table 1. Selected NMR Data of the C35-C40 Regions in
NAPRM2 and Model Compounds 3a and 3b (600 MHz, 1:1
C5D5N/CD3OD)
PAPRM2
18a
18b
NAPRM2
3a
3b
δH
δC
δH
δC
δH
δC
δH
δC
δH
δC
δH
δC
35
36
1.46
2.11
1.27
2.05
3.11
3.23
2.17
1.53
1.78
0.93
30.8
1.44
2.10
1.24
1.64
3.22
3.32
1.92
1.58
1.80
0.96
30.9
1.45
2.08
1.26
2.04
3.10
3.24
2.17
1.55
1.79
0.94
30.8
35
36
1.37
2.02
1.23
1.94
3.07
3.12
2.04
1.44
1.67
0.88
32.4
1.42
2.03
1.22
1.52
3.24
3.31
1.81
1.52
1.71
0.89
32.4a
1.38
2.03
1.24
1.96
3.09
3.16
2.09
1.45
1.69
0.91
32.3
29.1
26.5
29.1
30.9
28.4
30.6
37
38
39
40
76.4
82.1
29.5
36.4
78.1
82.3
30.4
37.3
76.2
82.1
29.5
36.5
37
38
39
40
77.9
84.4
31.6
38.5
79.7
84.4
32.5a
39.0
77.6
83.9
31.3
38.1
39-Me
13.0
13.2
13.0
39-Me
14.5
14.5
14.2
a Interchangeable.
In conclusion, we have synthesiszed two diastereomeric
CDE/FG ring models 3a and 3b of prymnesins through
alkynylcerium-aldehyde coupling and reductive ring-closure
of the E ring. Comparison of the NMR chemical shifts for
models 3a and 3b with those reported for natural products
allowed the reassignment of the relative configuration of the
E/F ring juncture of prymnesins to be that shown in Figure
2.
matched well the reported values for NAPRM2. In particular,
the δC values of C36 and C37 in 3a deviated from those of
NAPRM2 over 1 ppm, while the relevant values of 3b match
those of NAPRM2 within 0.5 ppm. The coupling constants,
J37,38 ) 9.0 Hz and J38,39 ) 2.5 Hz, of 3b also agreed well
with those reported for NAPRM2 (8.5 and 2.5 Hz, respec-
tively), while the diastereomer 3a showed somewhat different
values (7.8 and 1.9 Hz, respectively). In addition, NMR
chemical shift values for acetate 18b (600 MHz, CDCl3)
corresponded well with those for peracetyl prymnesin-2
(PAPRM2), while those for acetate 18a did not (Table 2).
These results suggest that the formerly assigned configuration
Acknowledgment. We are grateful to Professor Kazuo
Tachibana (University of Tokyo) for helpful discussion. This
work was supported by a Grant-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
Supporting Information Available: Experimental pro-
cedure and spectroscopic data for all compounds; synthetic
scheme and NOE data for compound 3b. This material is
available free of charge via the Internet at http://pubs.acs.org.
(19) NOE data for compound 3b is included in Supporting Infor-
mation.
OL049569L
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Org. Lett., Vol. 6, No. 9, 2004