Synthesis of a proposed biosynthetic intermediate of a marine cyclic ether brevisamide for study on biosynthesis of marine ladder-frame polyethers

Article abstract of DOI:10.1016/j.tetlet.2010.01.013

A monocyclic ether alkaloid, brevisamide (1) was isolated from the dinoflagellate Karenia brevis that produces a variety of ladder-frame polyethers. Its proposed biosynthetic intermediate (2) comprising a linear backbone with an E-olefin functionality was

Full text of DOI:10.1016/j.tetlet.2010.01.013

Tetrahedron Letters 51 (2010) 1394–1396
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Tetrahedron Letters
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Synthesis of a proposed biosynthetic intermediate of a marine cyclic ether
brevisamide for study on biosynthesis of marine ladder-frame polyethers
Tomohiro Shirai a, Takefumi Kuranaga a, Jeffrey L. C. Wright b, Daniel G. Baden b, Masayuki Satake a,
,
*
Kazuo Tachibana a,
*
a Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
b Center for Marine Science, University of North Carolina, Wilmington, Marvin K. Moss Lane, Wilmington, NC 28409, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A monocyclic ether alkaloid, brevisamide (1) was isolated from the dinoflagellate Karenia brevis that pro-
duces a variety of ladder-frame polyethers. Its proposed biosynthetic intermediate (2) comprising a linear
backbone with an E-olefin functionality was synthesized for biosynthetic studies on the marine ladder-
frame polyethers.
Received 16 November 2009
Revised 25 December 2009
Accepted 7 January 2010
Available online 11 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
The red tide dinoflagellate Karenia brevis, is known to produce
ladder-frame polyethers, brevetoxins1–3 and brevenal.4 Recently a
monocyclic ether amide, brevisamide (1)5 and a polycyclic ether,
brevisin6 were isolated from K. brevis. The brevetoxins and breve-
nal are characterized by ladder-frame polyether skeletons, while
1 consists of a single tetrahydropyran ring with a 3,4-dimethylhep-
ta-2,4-dienal side chain and an acetylated terminal amine. Our re-
cent success of chemical synthesis of 1 confirmed its unique
structure.7 Recently, synthetic works were accomplished by inde-
pendent groups.8–10 This cyclic ether amide is the first nitrogen-
containing cyclic ether from K. brevis and can be regarded as a
truncated analog of brevenal and brevisin containing the A-ring
portion and the dienal side chain (Fig. 1). In comparison to progress
in the structural elucidation and chemical synthesis of ladder-
frame polyethers,11,12 much less is understood about their biosyn-
thesis specifically the formation of the fused or ladder-frame
polyether ring systems. One particularly appealing hypothesis on
the formation of these ladder-frame structures was put forward
by Nakanishi which involves formation of ether rings by a stepwise
or cascading series of condensations starting from a putative poly-
epoxide intermediate.13 An intriguing new development in the
assembly of ladder-frame polyethers was heralded by the recent
report that polyether ladders form spontaneously from a suitable
polyepoxide intermediate in polar solvent, but only if a template
or nucleating hydroxytetrahydropyranyl functionality is built in
to the intermediate.14 This suggests the feasibility of a model in
which only the first ether ring formed requires catalysis by an en-
zyme and that the remaining cyclizations proceed spontaneously
governed by the spacing and configuration of the epoxide function-
ality. Consequently, brevisamide is an important molecule for our
general understanding of the biosynthesis of fused polyether ring
Me
OH
Me
H
O
N
Me
O
H
H
Me
Me
O
H
Brevisamide (1)
O
Me
Me
OH
H
H
H
Me
O
O
E
A
B
O
C
O
D
H
O
H
Me
H
H
Me
OH
Brevenal
OH
Me
Me
O
B
Me
OH
OH
A
O
C
O
O
O
E
OH
Me
D
O
Me
Me
F
O
Brevisin
Me
Figure 1. Structures of brevisamide (1), brevenal, and brevisin.
systems by epoxide-opening pathways.5 Although the E-olefin 2
and epoxide 3 are plausible biosynthetic intermediates in the bio-
synthesis of 1 (Scheme 1), neither of these proposed linear inter-
mediates have as yet been isolated from the dinoflagellate. While
remarkable progress has been made in our understanding of the
biosynthesis polyether antibiotics from actinomycetes at the
molecular level, the biosynthetic genes of dinoflagellate polycyclic
ethers have not been identified fully.15,16 In order to obtain clues
about this intriguing biosynthetic route, we undertook the synthe-
sis of 2 with two goals in mind. First, it would serve as an HPLC
* Corresponding author. Tel: +81 3 5841 4357; fax: +81 3+5841 8380.
E-mail address: msatake@chem.s.u-tokyo.ac.jp (M. Satake).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.013

T. Shirai et al. / Tetrahedron Letters 51 (2010) 1394–1396
1395
reaction of 6 with (EtO)2P(O)CH2CO2Et in the presence of NaHMDS
gave (E,E)-bromodienoate 7 and undesired (Z,E)-bromodienoate 8
in a highly stereoselective fashion (6:1). Reduction of the desired
(E,E)-dienoate 7 with DIBAL afforded the dienol side chain frag-
ment 4 in 98% yield.
Me
OTBS
Me
H
N
HO
Br
Me
+
I
Me
O
5
4
Me
Optically active homoallylic alcohol 11 was prepared stereose-
lectively following published procedures.21 First aldehyde 10 was
prepared in two steps from propane-1,3-diol 9. Reagent-controlled
enantioselective crotylation of aldehyde 10 using (Z)-2-butene and
(+)-b-methoxydiisopinocampheylborane gave the desired (3S,4S)-
alcohol 11 (46%). The absolute configuration of the hydroxy-bearing
carbon of 11 was determined by a modified Mosher method.22 Pro-
tection of the secondary alcohol with TBSCl gave TBS ether 12. Hyd-
roboration with 9-BBN–H followed by oxidative work-up, oxidation
with TEMPO and PhI(OAc)2, followed one-pot Wittig reaction with
Ph3PCHCO2Et afforded enoate 13 in 69% yield for three steps from
11. The enoate 13 was reduced with DIBAL to give allylic alcohol
14 in 98% yield. The alcohol 14 was converted to the iodide using
imidazole, PPh3, and I2 which was converted to the azide with
NaN3 and then reduced to afford the amine. This amine was acety-
lated with acetic anhydride to give amide 15 in 83% yield over four
steps. Removal of the MPM group with DDQ in phosphate buffer/
CH2Cl2 generated alcohol 16 which was converted to iodide 5 with
imidazole, PPh3, and I2 in 74% yield for two steps (Scheme 3).
10
Me
H
N
O
Me
Me
OH
1
13
O
Me
O
2
Me
Me
H
O
N
OH
O
Me
O
3
Me
OH
Me
H
O
N
Me
H
H
Me
O
1
proposed biosynthetic route
retrosynthetic analysis
Scheme 1. Retrosynthetic analysis of 2 and proposed biosynthetic route of 1.
c
a,b
HO
OH
OH
MPMO
d
O
standard material to assist in the search for biosynthetic precursors
produced by cultures of K. brevis, and second it could serve as a
substrate for epoxidation and subsequent cyclization enzymes
present in the dinoflagellates. In this Letter we report chemical
synthesis of the E-olefin intermediate 2 using a Suzuki–Miyaura
cross coupling reaction.17–19
9
10
OTBS
Me
MPMO
MPMO
Our synthetic strategy using Suzuki–Miyaura cross coupling as
a key reaction is shown in Scheme 1. The olefin intermediate 2 was
synthesized by coupling between a dienol side chain fragment 4
and an iodoenamide fragment 5. In our first total synthetic ap-
proach to 1 which was reported previously,7 the iododienol side
chain fragment and an ether ring exo-vinyl were joined via a Suzu-
ki–Miyaura cross coupling.7 However, the yield and reproducibility
of the iododienol fragment which was prepared from cis-but-2-
ene-1,4-diol were low, and therefore we synthesized a bromodie-
nol fragment as a coupling material.
Me
11
12
OTBS
e,f
g
CO2Et
MPMO
Me
13
OTBS
h,i,j,k
OH
MPMO
In this approach, the dienol fragment 4 was prepared from a re-
Me
ported bromoenone 6 (Scheme 2).20 Horner–Wadsworth–Emmons
14
OTBS
H
N
l
Me
Me
Me
MPMO
HO
a
O
Br
Br
O
Me
O
15
OEt Me
Me
OTBS
H
N
7
6
m
Me
+
EtO
O
Me
O
16
5
Me
Br
OTBS
H
N
Me
Me
I
8
Me
O
Me
b
Scheme 3. Reagents and conditions: (a) MPMCl, NaH, TBAI, THF, 0 °C to rt, 64%; (b)
SO3ꢁpyridine, Et3N, DMSO, CH2Cl2, 0 °C, 94%; (c) (+)-(Z)-crotyldiisopinocampheylbo-
rane, Et2O, THF, ꢀ78 °C; then 3 N NaOH, H2O2, reflux, 76%, 88% ee; (d) TBSCl,
imidazole, DMF, 78%; (e) 9-BBN–H, THF; then NaHCO3 aq, H2O2; (f) TEMPO,
PhI(OAc)2, CH2Cl2; then Ph3P@CHCO2Et, 88% (two steps); (g) DIBAL, CH2Cl2, ꢀ78 °C,
98%; (h) I2, PPh3, imidazole, toluene; (i) NaN3, DMF; (j) PPh3, THF; then H2O, 45 °C;
(k) Ac2O, pyridine, 83% (four steps); (l) DDQ, CH2Cl2, pH 7 phosphate buffer, 0 °C,
91%; (m) I2, PPh3, imidazole, toluene, 82%.
HO
Br
Me
4
Scheme 2. Reagents and conditions: (a) (EtO)2P(O)CH2CO2Et, NaHMDS, toluene,
0 °C to rt, 60% for 7, 10% for 8; (b) DIBAL, CH2Cl2, ꢀ78 °C, 98%.

1396
T. Shirai et al. / Tetrahedron Letters 51 (2010) 1394–1396
4 + 5
a,b
References and notes
1. Lin, Y.-Y.; Risk, M.; Ray, S. M.; Van Engen, D.; Clardy, J.; Golik, J.; James, J. C.;
Nakanishi, K. J. Am. Chem. Soc. 1981, 103, 6773.
2. Shimizu, Y.; Chou, H. N.; Bando, H.; Van Duyne, G.; Clardy, J. J. Am. Chem. Soc.
1986, 108, 514.
3. Baden, D. G.; Bourdelais, A. J.; Jacocks, H.; Michelliza, S.; Naar, J. J. Environ.
Health Perspect. 2005, 113, 621.
4. Bourdelais, A. J.; Jacocks, H. M.; Wright, J. L. C.; Bigwarfe, P. M., Jr.; Baden, D. G. J.
Nat. Prod. 2005, 68, 2.
5. Satake, M.; Bourdelais, A. J.; Van Wagoner, R. M.; Baden, D. G.; Wright, J. L. C.
Org. Lett. 2008, 10, 3465.
6. Satake, M.; Campbell, A.; Van Wagoner, R. M.; Bourdelais, A. J.; Jacocks, H.;
Baden, D. G.; Wright, J. L. C. J. Org. Chem. 2009, 74, 989.
7. Kuranaga, T.; Shirai, T.; Baden, D. G.; Wright, J. L. C.; Satake, M.; Tachibana, K.
Org. Lett. 2009, 11, 217.
8. Fadeyi, O. O.; Lindsley, C. W. Org. Lett. 2009, 11, 3950.
9. Ghosh, A. K.; Li, J. Org. Lett. 2009, 11, 4164.
OH
Me
H
N
Me
Me
HO
Me
Me
17
O
O
c
OH
Me
H
N
O
Me
Me
10. Lee, J.; Panek, J. S. Org. Lett. 2009, 11, 4390.
11. Satake, M.. In Topics in Heterocyclic Chemistry; Gupta, R. R., Kiyota, H., Eds.;
Springer: Berlin, Heidelberg, 2006; Vol. 5, pp 21–51.
2
12. Recent reviews for synthesis of polycyclic ethers: (a) Inoue, M. Chem. Rev. 2005,
105, 4379; (b) Nakata, T. Chem. Rev. 2005, 105, 4314; (c) Fuwa, H.; Sasaki, M.
Curr. Opin. Drug Discovery Dev. 2007, 10, 784; (d) Nicolaou, K. C.; Frederick, M.
O.; Aversa, R. J. Angew. Chem., Int. Ed. 2008, 47, 7182.
Scheme 4. Reagents and conditions: (a) 5, B-OMe-9-BBN, t-BuLi, Et2O, THF, ꢀ78 °C
to rt; then 4, 3 M Cs2CO3, Pd(PPh3)4, DMF; (b) TBAF, THF, reflux, 31% (two steps); (c)
MnO2, CH2Cl2, 0 °C, 98%.
13. Nakanishi, K. Toxicon 1985, 23, 473.
14. (a) Vilotijevic, I.; Jamison, T. F. Science 2007, 317, 1189; (b) Morten, C. J.; Byers,
J. A.; Van Dyke, A. R.; Vilotijevic, I.; Jamison, T. F. Chem. Soc. Rev. 2009, 38, 3175.
15. Gallimore, A. R.; Stark, C. B.; Bhatt, A.; Harvey, B. M.; Demydhuk, Y.; Bolanos-
Garcia, V.; Fowler, D. J.; Staunton, J.; Leadlay, P. F.; Spencer, J. B. Chem. Biol.
2006, 13, 453.
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Watanabe, M.; Tokiwano, T.; Watanabe, K.; Oikawa, H. J. Am. Chem. Soc.
2008, 130, 12230.
17. Suzuki, A.; Miyaura, N. Chem. Rev. 1995, 95, 2457.
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4544.
19. Sasaki, M.; Fuwa, H. Nat. Prod. Rep. 2008, 25, 401.
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Chem. 2005, 3, 1372.
Connection of the dienol side chain fragment 4 and the amide
fragment 5 was accomplished by Suzuki–Miyaura cross coupling
(Scheme 4). Treatment of 5 with t-BuLi and B-OMe-9-BBN pro-
duced a borate intermediate which was reacted in situ with
bromodienol 4 in the presence of aqueous Cs2CO3 and a catalytic
amount of Pd(PPh3)4 to give rise the cross-coupled product.23 The
crude product was treated with TBAF under a reflux condition to
give dienol 17. Finally, chemoselective oxidation of the allylic alco-
hol at C-1 with MnO2 led to the putative biosynthetic olefin precur-
sor 2 in 30% yield for three steps.24,25
The revised synthesis of the dienol fragment for the coupling
reaction led to a more efficient synthesis of the proposed biosyn-
thetic intermediate 2. Biosynthetic precursors such as the E-olefin
and the epoxide intermediate have not been isolated from the
dinoflagellate K. brevis, perhaps suggesting that conversion of such
putative precursors as the E-olefin or epoxide to brevisamide pro-
ceeds spontaneously. Future studies to explore the biosynthesis of
marine ladder-frame polyethers will be directed toward the epox-
idation step and subsequent ring-opening enzymes in dinoflagel-
lates using the intermediate 2.
21. (a) Brown, C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108, 5919; (b) Racherla, U. S.;
Brown, H. C. J. Org. Chem. 1991, 56, 401.
22. Otani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092.
23. To a solution of the iodide 5 (260 mg, 0.592 mmol) in anhydrous Et2O (5 mL) at
room temperature was added B-OMe-9-BBN (1.0 M solution in hexane, 1.8 mL,
1.8 mmol). After cooling to ꢀ78 °C, t-BuLi (1.58 M solution in pentane, 0.94 mL,
1.48 mmol) was added rapidly, followed by THF (5 mL). Then the reaction
mixture was stirred at ꢀ78 °C for 10 min and allowed to warm to room
temperature for 2 h. To the mixture were added 3 M aqueous Cs2CO3 (1.5 mL),
the solution of the bromodienol 4 (206 mg, 1.08 mmol) in DMF (5 mL), and
Pd(PPh3)4 (34.2 mg, 0.0296 mmol). After stirring for 16 h, usual work-up gave
the crude cross-coupled product.
24. Data for 2: ½a D27
ꢂ
ꢀ14.3 (c 0.12, CH3OH); IR (film) 3299, 2929, 1657, 1442, 1375,
1283, 1252, 1153, 1046, 970, 843, 750 cmꢀ1
;
1HNMR (500 MHz, CD3OD) d
Acknowledgments
10.10 (d, J = 8.0 Hz, 1H), 6.26 (dd, J = 7.0, 7.0 Hz, 1H), 6.05 (d, J = 8.0 Hz, 1H),
5.61 (ddd, J = 15.1, 7.5, 7.5 Hz, 1H), 5.47 (dt, J = 15.1, 5.9 Hz, 1H), 3.72 (d,
J = 5.9 Hz, 2H), 3.48 (m, 1H), 2.41 (m, 1H), 2.34 (s, 3H), 2.30 (m, 1H), 2.21 (m,
1H), 1.93 (s, 3H), 1.93–1.87 (m, 1H), 1.88 (s, 3H), 1.56 (m, 3H), 0.89 (d,
J = 6.7 Hz, 3H); 13CNMR (100 MHz, CD3OD) d 194.4, 172.9, 161.0, 137.0, 132.7,
128.3, 126.2, 74.7, 42.3, 40.0, 37.4, 34.7, 27.2, 22.5, 14.5, 14.1, 14.0; HRMS (FAB)
calcd for C18H29O3NNa [(M+Na)+] 330.2045, found 330.2059.
We thank to JEOL Ltd for measurements of mass spectra under
DART conditions. This work was financially supported by KAKENHI
(No. 2061102) and the Global COE Program for Chemistry Innova-
tion, the University of Tokyo. T.K. is grateful for a SUNBOR Scholar-
ship. The natural brevisamide was isolated from the cultures
maintained under grants P01 ES 10594 (NIEHS, DHHS) and MARBI-
ONC, Marine Biotechnology in North Carolina. J.L.C.W. acknowl-
25.
A negative referee pointed out that 2 had not been isolated from the
dinoflagellates. However, we think that 2 is a key compound to verify the
proposed biosynthetic route of marine ladder-frame polyethers by using as a
substrate for exploration of epoxidation and epoxide opening enzymes in the
dinoflagellates. Therefore the synthesis of 2 is important and will accelerate
biosynthetic studies on marine ladder-frame polyethers.
edges funding from NOAA-ECOHAB
NCDHHS (01515-04).
(MML-106390A) and