Synthesis of the polyketide (E)-olefin of the jamaicamides

Article abstract of DOI:10.1080/00397911.2011.635394

The jamaicamides, isolated in Jamaica from the cyanobacterium Lyngbya majuscula, are new mixed polyketide-peptides that are known to be sodium channel blockers. The polyketide moiety contains an (E)-vinyl chloride, an undetermined methyl stereocenter (C9), and an (E)-olefin. Herein, we report the synthesis of the (E)-olefin moiety of the polyketide of the jamaicamides utilizing a Kocienski-Julia coupling. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.635394

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Synthesis of the Polyketide (E)-Olefin of
the Jamaicamides
Satoshi Watanabe a , Sho Watanabe a , Naoto Aoki a & Toyonobu
Usuki a
a Department of Materials and Life Sciences, Faculty of Science and
Technology, Sophia University, Tokyo, Japan
Accepted author version posted online: 14 Mar 2012.Version of
record first published: 05 Mar 2013.
To cite this article: Satoshi Watanabe , Sho Watanabe , Naoto Aoki & Toyonobu Usuki (2013):
Synthesis of the Polyketide (E)-Olefin of the Jamaicamides, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:10, 1397-1403
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Synthetic Communications1, 43: 1397–1403, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.635394
SYNTHESIS OF THE POLYKETIDE (E)-OLEFIN OF THE
JAMAICAMIDES
Satoshi Watanabe, Sho Watanabe, Naoto Aoki, and
Toyonobu Usuki
Department of Materials and Life Sciences, Faculty of Science and
Technology, Sophia University, Tokyo, Japan
GRAPHICAL ABSTRACT
Abstract The jamaicamides, isolated in Jamaica from the cyanobacterium Lyngbya
majuscula, are new mixed polyketide-peptides that are known to be sodium channel
blockers. The polyketide moiety contains an (E)-vinyl chloride, an undetermined methyl
stereocenter (C9), and an (E)-olefin. Herein, we report the synthesis of the (E)-olefin
moiety of the polyketide of the jamaicamides utilizing a Kocienski–Julia coupling.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Jamaicamides; Kocienski–Julia coupling; marine natural products; polyketide
INTRODUCTION
The jamaicamides A, B, and C (Fig. 1) were isolated by Edwards and
coworkers from the marine cyanobacterium Lyngbya majuscula in Hector’s Bay,
Jamaica.[1] The jamaicamide family has unusual mixed polyketide-peptides contain-
ing a pyrrolinone ring, a b-methoxy enone, an (E)-olefin, an (E)-vinyl chloride, and
an alkynyl bromide (only in jamaicamide A). Meanwhile, the absolute configuration
of the C9 methyl group has not been determined yet. The marine natural products
display sodium channel–blocking activity and show cytotoxicity for both H-460
human lung and neuro-2a mouse neuroblastoma cell lines. Interestingly, only jamai-
camide B exhibits antimalarial activity and cytotoxicity to Vero cells.[2]
In 2009, Graf and coworkers reported the synthesis of (S)-jamaicamide C
carboxylic acid by employing a Negishi cross-coupling and a Johnson–Claisen
rearrangement as key steps.[3] Synthesis of the peptide moiety of the jamaicamides
was recently accomplished.[4] Despite their attractive biological activity and the
Received August 18, 2011.
Address correspondence to Toyonobu Usuki, Department of Materials and Life Sciences, Faculty
of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan. E-mail:
t-usuki@sophia.ac.jp
1397

1398
S. WATANABE ET AL.
Figure 1. Structures of the jamaicamides.
unique biosynthesis of those secondary metabolites,[1,5] however, the total synthesis
and structure–activity relationship (SAR) studies of the jamaicamides have not been
achieved. Herein, we describe the synthesis of the (E)-olefin, a part of the polyketide
moiety of the jamaicamides, utilizing a Kocienski–Julia coupling[6,7] as the key
reaction.
RESULTS AND DISCUSSION
As illustrated in Scheme 1, the polyketide (E)-olefin moiety 1 of the jamaica-
mides could be retrosynthetically divided into two parts. The (E)-olefin (C10-C11
bond) would be formed by a Kocienski–Julia coupling, which is useful for construct-
ing an (E)-olefin stereoselectively between phenyl tetrazol sulfone 2 and aldehyde
Scheme 1. Retrosynthesis of the polyketide (E)-olefin 1 of the jamaicamides.

JAMAICAMIDES (E)-OLEFIN SYNTHESIS
1399
Scheme 2. Synthesis of phenyl tetrazol sulfone 2. Reagents and conditions: (a) BnOH, Et3N, DMAP,
CH2Cl2, 22 h, 98%; (b) BH3 ꢀ SMe2, THF, ꢁ18 ꢂC to rt, 6.5 h, 92%; (c) PTSH, PPh3, DEAD, THF,
0 ꢂC, 2 h, 95%; and (d) mCPBA, NaHCO3, CH2Cl2, rt, 16 h, 75%.
3.[7,8] The fragments 2 and 3 would be obtained from commercially available succinic
anhydride 4 and pentane-1,5-diol 5, respectively.
The synthesis of sulfone 2 started with a ring-opening reaction of 4 (Scheme 2).
Treatment of succinic anhydride 4 with benzyl alcohol (BnOH) and triethylamine
(Et3N) in the presence of a catalytic amount of 4-dimethylaminopyridine (DMAP)
in methylene chloride (CH2Cl2) gave carboxylic acid 6 in 98% yield.[9] Subsequent
reduction of 6 using 1.3 equivalents of borane dimethylsulfide complex (BH3 ꢀ SMe2)
in tetrahydrofuran (THF) afforded the corresponding alcohol 7 in 92% yield.[10]
The Mitsunobu reaction[11] of 7 with 1-phenyl-1H-tetrazole-5-thiol (PTSH), triphenyl-
phosphine (PPh3), and diethyl azodicarboxylate (DEAD) then provided sulfide 8 in
95% yield. Surprisingly, the oxidation of sulfide 8 to phenyl tetrazol sulfone 2 did
not go to completion when the Mo(VI) complex [(NH4)6Mo7O24 ꢀ 4H2O] and hydro-
gen peroxide were used as oxidants.[12] As a result, 2 was obtained with a trace amount
of 8, which could not be separated on silica-gel column chromatography. We did find,
Scheme 3. Synthesis of aldehyde 3. Reagents and conditions: (a) BnBr, NaH, TBAI, THF, 0 ꢂC to rt, 22 h,
67%; (b) Jones’ reagent, acetone, 0 ꢂC to rt, 3 h, 86%; (c) PivCl, Et3N, THF, ꢁ78 ꢂC to rt, 4.5 h, then (R)-4-
-benzyl-2-oxazolidinone, nBuLi, ꢁ78 ꢂC to rt, 4 h, 83%; (d) NaHMDS, MeI, THF, ꢁ78 ꢂC, 2.5 h, 75%; (e)
LiBH4, H2O, Et2O, 0 ꢂC to rt, 3 h, 89%; (f) DMP, NaHCO3, CH2Cl2, rt, 10 h, 83%.

1400
S. WATANABE ET AL.
Scheme 4. Synthesis of the (E)-olefin 1 by Kocienski–Julia coupling.
however, that 2 could be prepared without contamination by 9 in 75% yield by utiliz-
ing m-chloroperbenzoic acid (mCPBA). Thus, phenyl tetrazol sulfone 2, the precursor
of the Kocienski–Julia coupling, was successfully prepared from 5 in 64% overall yield
in four steps.
Pentane-1,5-diol 5 (Scheme 3) served as the starting material for the synthesis
of aldehyde 3. Treatment of 5 with benzyl bromide (BnBr) and sodium hydride
(NaH) in the presence of tetra-n-butyl ammonium iodide (TBAI) selectively gave
the monoprotected alcohol 9 in 67% yield. Oxidation of 9 by Jones’s reagent then
provided carboxylic acid 10 in 86% yield. After acylation of 10 with pivaloyl chloride
(PivCl), the resultant ester was converted into the imide 11 in 83% yield using (R)-4-
benzyl-2-oxazolidinone and n-butyllithium (nBuLi).[13–15] Utilizing Evans reaction,
stereoselective methylation with 1.2 equivalents of sodium hexamethyldisilazide
(NaHMDS), and 5 equivalents of iodomethane (MeI) at ꢁ78 ꢂC in THF gave 12
in 75% yield.[16] Removal of the chiral auxiliary using lithium borohydride (LiBH4)
in diethylether (Et2O) led to alcohol 13. Finally, 13 was oxidized with the Dess–Mar-
tin periodinane (DMP) reagent to give the aldehyde 3 in 83% yield. Thus 5 was
obtained in 26% overall yield in six steps starting from 5.
With the two segments 2 and 3 in hand, we focused our attention on the con-
struction of the (E)-olefin via a Kocienski–Julia coupling (Scheme 4). Sulfone 2 (1.5
equivalents) in THF was deprotonated with lithium hexamethyldisilazide (LiHMDS)
at ꢁ78 ꢂC for 30 min to give the anion, which was then reacted with aldehyde 3 at
ꢁ78 ꢂC to room temperature for 21 h to give the desired olefin 1 in 66% yield.[17]
1
The E=Z selectivity, which was determined by H NMR analysis, was 3.3:1.
EXPERIMENTAL
Melting points were measured on an AS ONE ATM-01 apparatus. Optical
rotations were measured on a Jasco P-2200 digital polarimeter at the sodium lamp
T
(k ¼ 589 nm) D line and were recorded as follows: ½aꢃD (c g=100 mL, solvent).
Infrared (IR) spectra were recorded on a Jasco Fourier transform(FT)–IR 4100
1
spectrometer. H and 13C NMR spectra were recorded on a Jeol JNM-EXC 300
1
spectrometer (300 MHz) in deuterated solvents. H NMR data are reported as fol-
lows: chemical shift (d, ppm), integration, multiplicity (s, singlet; d, doublet; t, triplet;
m, multiplet), coupling constants (J, Hz), assignments. 13C NMR data are reported
in terms of chemical shift (d, ppm). Electron-impact–mass spectrometric (EI-MS)
spectra were recorded on a Shimadzu GCMS QP-5050 instrument or a Jeol JMS-
700 instrument. Elemental analysis was performed using a Perkin–Elmer PE2400
II apparatus.
All non-aqueous reactions were conducted under an atmosphere of nitrogen
with magnetic stirring using freshly distilled solvents, unless otherwise indicated.

JAMAICAMIDES (E)-OLEFIN SYNTHESIS
1401
THF, CH2Cl2 and Et2O were dried by distillation and stored over molecular
sieves. All reagents were obtained from commercial suppliers unless otherwise sta-
ted. Analytical thin-layer chromatography (TLC) was performed on silica-gel
60 F254 plates produced by Merck. Visualization was accomplished with ultraviolet
light and phosphomolybdic acid followed by heating. Column chromatography
was performed with acidic or neutral silica gel 60 (spherical) produced by Kanto
chemicals.
4-(1-Phenyl-1H-tetrazol-5-ylsulfanyl)butyric Acid Benzyl Ester (8)
Triphenylphosphine (1.56 g, 5.95 mmol), 1-phenyl-1H-tetrazole-5-thiol (1.32 g,
7.44 mmol), and diethyl azodicarboxylate (2.2 M solution in toluene, 2.92 mL,
6.42 mmol) were added to a solution of 4-hydroxybutyric acid benzyl ester 7
(961.8 mg, 4.95 mmol) in THF (4.9 mL) at 0 ꢂC. After stirring for 2 h at the same tem-
perature, the mixture was diluted with EtOAc and quenched with saturated aqueous
NaHCO3 solution. The aqueous layer was then extracted with EtOAc. The com-
bined organic layers were washed with brine, dried over Na2SO4, filtered, and con-
centrated under reduced pressure. Purification on silica-gel column chromatography
(hexane=EtOAc ¼ 5:1) provided 4-(1-phenyl-1H-tetrazol-5-ylsulfanyl) butyric acid
benzyl ester 8 in 95% yield as a colorless oil (1.67 g, 4.72 mmol); IR (neat, cmꢁ1
)
1
3064, 2952, 1731, 1597, 1499, 1455, 1140, 1014, 915, 876, 757, 699, 552, 457; H
NMR (300 MHz, CDCl3) d 7.58–7.53 (5H, m, Ph), 7.37–7.32 (5H, m, Bn), 5.12
(2H, s, CH2Ph), 3.46 (2H, t, J ¼ 8.0 Hz, H11), 2.55 (2H, t, J ¼ 8.0 Hz, H13),
2.25–2.16 (2H, m, H12); 13C NMR (75 MHz, CDCl3) d 172.4, 154.1, 135.8, 133.8,
130.3, 130.0, 128.7, 128.5, 128.4, 66.6, 32.8, 32.5, 24.5; EI-HRMS (m=z) calcd. for
C18H18N4O2S [M]þ 354.1150; found 354.1145.
4-(1-Phenyl-1H-tetrazol-5-sulfonyl)butyric Acid Benzyl Ester (2)
mCPBA (65% purity, 742.4 mg, 2.80 mmol) was added in portions to a suspen-
sion of 4-(1-phenyl-1H-tetrazol-5-ylsulfanyl) butyric acid benzyl ester 8 (205.5 mg,
0.58 mmol) and sodium hydrogen carbonate (470.5 mg, 5.60 mmol) in CH2Cl2
(10 mL). After stirring for 16 h at room temperature, the mixture was cooled to
0 ꢂC and diluted with CH2Cl2. The reaction was then quenched with saturated aque-
ous NaHCO3 and aqueous Na2S2O3 solutions and extracted with CH2Cl2. The com-
bined organic extracts were washed with brine, dried over Na2SO4, filtered, and
concentrated in vacuo. Purification on silica-gel column chromatography (hexane=
EtOAc ¼ 3:1) gave 4-(1-phenyl-1H-tetrazol-5-sulfonyl) butyric acid benzyl ester 2
in 75% yield as a colorless solid (167.7 mg, 0.43 mmol); mp 90–91 ꢂC; IR (KBr,
cmꢁ1) 2958, 1715, 1497, 1397, 1341, 1250, 1193, 1053, 968, 750, 570, 550, 513,
1
470; H NMR (300 MHz, CDCl3) d 7.71–7.57 (5H, m, Ph), 7.38–7.33 (5H, m, Bn),
5.14 (2H, s, CH2Ph), 3.86 (2H, t, J ¼ 8.7 Hz, H11), 2.62 (2H, t, J ¼ 8.0 Hz, H13),
2.36–2.27 (2H, m, H12); 13C NMR (75 MHz, CDCl3) d 171.7, 153.4, 135.6, 133.1,
131.6, 129.9, 128.8, 128.6, 128.5, 125.2, 66.9, 55.1, 32.0, 18.0; Anal. calcd for
C18H18N4O4S: C, 55.95; H, 4.70; N, 14.50; S, 8.30. Found: C, 55.66; H, 4.80; N,
14.12; S, 7.87.

1402
S. WATANABE ET AL.
(2R)-5-Benzyloxy-2-methylpentanal (3)[18]
Dess-Martin periodinane reagent (387.0 mg, 0.91 mmol) and sodium hydrogen
carbonate (574.6 mg, 6.84 mmol) were added to a solution of (2R)-5-benzyloxy-2-
methylpentan-1-ol 13 (145.9 mg, 0.70 mmol) in CH2Cl2 (7.0 mL). More Dess–
Martin periodinane (148.3 mg and 88.9 mg) was added after stirring for 6 h and
9 h, respectively. After stirring for 10 h from the initial addition of reagents, the
resulting mixture was cooled to 0 ꢂC, diluted with Et2O, and quenched with satu-
rated aqueous Na2S2O3 solution. The aqueous phase was extracted with Et2O.
The combined organic extracts were dried over Na2SO4, filtered, and concentrated
under reduced pressure. Purification on silica-gel column chromatography (hexane=
EtOAc ¼ 9:1) gave (2R)-5-benzyloxy-2-methylpentanal 3 in 83% yield as a colorless
20
20
oil (120.6 mg, 0.58 mmol); ½aꢃD ¼ ꢁ13:9 (c 1.1, CHCl3) (lit. value[18]: ½aꢃD ¼ ꢁ9:1 (c
1.0, CHCl3)); IR (neat, cmꢁ1) 3030, 2858, 2712, 1721, 1496, 1455, 1362, 1205, 1099,
1
1028, 913, 737, 699, 612; H NMR (CDCl3, 300 MHz) d 9.62 (1H, d, J ¼ 2.3 Hz,
CHO), 7.38–7.28 (5H, m, Ph), 4.45 (2H, s, CH2Ph), 3.49 (2H, t, J ¼ 7.0 Hz,
H6), 2.40–2.32 (1H, m, H9), 1.86–1.44 (4H, m, H7,8), 1.11 (3H, d, J ¼
8.0 Hz, CH3); 13C NMR (75 MHz, CDCl3) d 205.1, 138.6, 128.5, 127.8, 127.7,
73.1, 70.1, 46.2, 27.3, 13.5; EI-MS (m=z) calcd. for C13H18O2 [M]þ 206.13; found
206.15.
(6R)-9-Benzyloxy-6-methylnon-4-enoic Acid Benzyl Ester (1)
Lithium hexamethyldisilazide (1.6 M solution in THF, 0.28 mL, 0.45 mmol),
was added to a chilled solution (ꢁ78 ꢂC for 35 min) of 4-(1-phenyl-1H-tetrazol-5-
sulfonyl) butyric acid benzyl ester 2 (173.9 mg, 0.45 mmol) in THF (3.0 mL), and
the resulting mixture was stirred for 30 min at ꢁ78 ꢂC. A precooled solution (ꢁ
78 ꢂC for 40 min) of (2R)-5-benzyloxy-2-methylpentanal 3 (62.6 mg, 0.30 mmol) in
THF (6.0 mL) was then added via a cannula and stirred for 1.5 h at ꢁ78 ꢂC. The mix-
ture was allowed to warm up to room temperature slowly, and stirring was contin-
ued for 19.5 h. This solution was cooled to 0 ꢂC, diluted with Et2O, and quenched
with saturated aqueous NH4Cl. The aqueous phase was then extracted with Et2O.
The organic layer was washed with brine, dried over Na2SO4, filtered, and concen-
trated under reduced pressure. Purification on silica-gel column chromatography
(hexane=EtOAc ¼ 12:1) provided (6R)-9-benzyloxy-6-methylnon-4-enoic acid benzyl
ester 1 in 66% yield as a colorless oil (73.6 mg, 0.20 mmol, E=Z ¼ 3.3:1 determined by
20
300-MHz 1H NMR); ½aꢃD ¼ ꢁ3:6 (c 0.78, CHCl3); IR (neat, cmꢁ1) 2851, 1952, 1736,
1606, 1496, 1457, 1381, 1098, 970, 909, 736, 697, 610, 460; 1H NMR (CDCl3,
300 MHz) d 7.36–7.28 (10H, m, Ph ꢄ 2), 5.35–5.17 (2H, m, H10, 11), 5.11 (2H, s,
CH2Ph), 4.49 (2H, s, CH2Ph), 3.43 (2H, t, J ¼ 7.3 Hz, H6), 2.44–2.29 (4H, m,
H12,13), 2.08–2.00 (1H, m, H9), 1.62–1.21 (4H, m, H7, 8), 0.94 (3H, d, J ¼ 7.3 Hz,
CH3); 13C NMR (75 MHz, CDCl3) d 173.2, 173.1, 138.8, 137.7, 136.2, 128.7,
128.5, 128.3, 127.8, 127.6, 126.5, 126.3, 73.0, 70.7, 66.3, 66.2, 36.7, 34.7, 34.6, 34.0,
33.5, 31.8, 28.0, 27.9, 27.7, 23.2, 21.4, 20.9; EI-HRMS (m=z) calcd. for C24H30O3
[M]þ 366.2195; found 366.2199.
Complete experimental details can be found online in the Supplementary
Information.

JAMAICAMIDES (E)-OLEFIN SYNTHESIS
1403
CONCLUSION
In summary, we have achieved the syntheses of sulfone 2 and aldehyde 3 in
good yields starting from succinic anhydride 4 and pentane-1,5-diol 5, respectively.
Meanwhile, the stereoselective Kocienski–Julia coupling of 2 and 3 was successfully
accomplished to obtain the (E)-olefin 1. Efforts toward completion of the synthesis
of polyketide moiety and the total synthesis of the jamaicamides are currently under
way in our laboratory.
ACKNOWLEDGMENTS
This work was supported in part by a Grant-in-Aid for Young Scientists (B)
from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT)
of Japan. A fellowship from Yoshida Scholarship Foundation (to Satoshi
Watanabe) is gratefully acknowledged.
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