Total synthesis of splenocin B, a potent inhibitor of the pro-inflammatory cytokine from marine-derived Streptomyces sp.

Article abstract of DOI:10.1016/j.tet.2015.10.075

The first total synthesis of splenocin B (1), a new potent anti-inflammatory antimycin-class antibiotic, has been described. The synthesis of 1 has been accomplished in 8 linear steps, starting from commercially available N-Boc-L-threonine benzyl ester 4

Full text of DOI:10.1016/j.tet.2015.10.075

Tetrahedron 71 (2015) 9626e9629
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Total synthesis of splenocin B, a potent inhibitor of the
pro-inflammatory cytokine from marine-derived Streptomyces sp.
*
Ken-ichi Yoshida, Minako Ijiri, Hideo Iio, Yoshinosuke Usuki
Division of Molecular Materials Science, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The first total synthesis of splenocin B (1), a new potent anti-inflammatory antimycin-class antibiotic, has
been described. The synthesis of 1 has been accomplished in 8 linear steps, starting from commercially
available N-Boc-L-threonine benzyl ester 4 and 3,4-dihydroxypentanoic acid derivative 2. KitaeTrost
lactonization via an ethoxyvinyl ester intermediate was utilized for the construction of the 9-membered
dilactone core.
Received 29 September 2015
Received in revised form 26 October 2015
Accepted 27 October 2015
Available online 3 November 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Splenocin B
9-Membered dilactone
Anti-inflammatory
KitaeTrost lactonization
Antimycin-class antibiotics
1. Introduction
3-formamidosalicylic moieties, while UK-2A possesses
hydroxy-4-methoxy-picolinic moiety.
a 3-
Splenocins were isolated from an organic extract of marine-
derived Streptomyces strain CNQ431 as potent anti-inflammatory
antibiotics in 2009, which displayed low nanomolar activity in
the suppression of cytokine production by OVA-stimulated sple-
nocytes.1a Splenocins exhibit inhibitory activities toward not only
the production of TH2 cytokines IL-5 and IL-13 but also the pro-
Splenocin B (1) is reported to be as effective as dexamethasone
in inhibiting TH2 cytokine production and is a hybrid molecule
combining some structural features of both UK-2A and AA; the
benzyl group at the C2 position in 1 had not been reported in AAs.
In our continuing studies on UK-2A and AAs,5 we have been very
interested in the structure and biological activities of splenocin B.
Herein we report the first total synthesis of splenocin B (1).
duction of the dendritic cell-associated cytokins IL-1 and TNF-a,
which provide great benefits in the treatment of asthma. The
structures of splenocins are similar to those of antimycin A3 (AA)2,3
and UK-2A, another antibiotic in the antimycin class, which was
first isolated in 1996 from a soil sample collected at our campus.4
These consist of 9-membered dilactone rings linked via an amide
bond to an aromatic acid moiety (Fig. 1); splenocins and AAs have
2. Results and discussion
Our synthesis of 1 commences with the formation of the 3,4-
dihydroxypentanoic acid derivative 2, which was achieved
through the Evans aldol reaction between aldehyde 3 and N-
hydrocinnamoyloxazolidine, as previously reported (Scheme 1).6
Condensation of 2 with commercially available N-Boc-L-threonine
benzyl ester 4 was conducted in the presence of EDCI$HCl
(3.0 equiv)eDMAP (0.30 equiv) to afford 5 in 93% yield. Removal of
the two benzyl groups by hydrogenolysis with Pd(OH)2 in EtOH
afforded the cyclization precursor seco acid 6 in 98% yield.
Next, we focused on the construction of a nine-membered
dilactone moiety, which appear to be the more difficult due to
both enthalpic and entropic factors. The use of 2-methyl-6-
nitrobenzoic anhydride (MNBA) with a DMAP or DMAPO catalyst,7
which was found to be optimal in the previous reports on the syn-
thesis of AAs,3e,3j provided only dimeric 18-membered tetralactone.
After several attempts, including the use of a 2-pyridinethiol ester-
Fig. 1. Structures of antimycin A3, splenocin B (1) and UK-2A.
* Corresponding author. Tel.: þ81 6 6605 2563; fax: þ81 6 6605 2522; e-mail
address: usuki@sci.osaka-cu.ac.jp (Y. Usuki).
http://dx.doi.org/10.1016/j.tet.2015.10.075
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

K.-ichi Yoshida et al. / Tetrahedron 71 (2015) 9626e9629
9627
3. Conclusions
The total synthesis of splenocin B (1) has been achieved via
macrolactonization using ethoxyvinyl ester as a key reagent for
furnishing the 9-membered dilactone ring skeleton. Investigations
into the application of the developed protocol to the structur-
eeactivity relationship are currently underway in our laboratory.
4. Experimental section
4.1. General remarks
Unless mentioned otherwise, 1H and 13C NMR spectra were
recorded on a JEOL JNM-LA 300 (300 and 75 MHz), a JEOL JNM-LA
400 (400 and 100 MHz), a Bruker AVANCE 300 (300 and 75 MHz),
or a Bruker AVANCE III 600 (600 and 150 MHz). Data are reported as
follows: chemical shift, multiplicity (s¼singlet, d¼doublet,
t¼triplet, q¼quartet, m¼multiplet, and br¼broad), coupling con-
stant in Hz, integration. Coupling constants were determined di-
rectly from 1H and 13C NMR spectra. The chemical shifts are
Scheme 1. Synthesis of seco acid 6.
silver salt,8 we executed the KitaeTrost method.9 Treatment of 6
with ethoxyacetylene (3.0 equiv) in the presence of [RuCl2(p-cym-
ene)]2 (5 mol %) afforded the corresponding ethoxyvinyl ester in-
termediate 7. To a solution of CSA (10 mol %) in 1,2-dichloroethane,
a dilute solution of 7 in 1,2-dichloroethane was slowly added over
7 h at 80 ꢀC, followed by stirring for 24 h. The desired reaction
proceeded to afford dilactone 8 in 59% yield (Scheme 2).
reported in
d
(ppm) values relative to CHCl3 (
d
7.26 ppm for 1H NMR
and
d
77.0 ppm for 13C NMR) and Me4Si (
d
0.00 ppm for 1H NMR).
Mass spectra were obtained on a JEOL JMS-700T (EI, CI, FAB) or
a Bruker solariX (ESI) spectrometer. IR spectra were recorded on
a JASCO FT/IR-4600 spectrometer. Optical rotations were measured
on a PERKIN-ELMER 241 polarimeter with a path length of 1 dm or
on a JASCO P-1030 with a path length of 0.1 dm at ambient tem-
perature; the concentrations are reported in g dLꢁ1. Melting points
were determined on a Yanaco MP-I3 micro melting point apparatus
and the thermometer was used without correction. All air- and
moisture-sensitive reactions were conducted in a flame-dried, ar-
gon-flushed, two-necked flask sealed with a rubber septa, and dry
solvents and reagents were introduced using a syringe. Tetrahy-
drofuran (THF) was freshly distilled under an argon atmosphere
from sodium benzophenone ketyl. Dichloromethane (CH2Cl2) was
freshly distilled from phosphoric pentoxide (P2O5). Flash column
chromatography was conducted on a Kanto Chemical silica gel 60N
(spherical, neutral, 40e50
mm), and pre-coated Merck silica gel
Scheme 2. Construction of the nine-membered dilactone core.
plates (Art5715 Kieselgel 60F254, 0.25 mm) were used for thin-layer
chromatography (TLC). TLC visualization was accompanied by the
use of a UV lamp (254 nm) or a charring solution (ethanolic p-
anisaldehyde, ethanolic phosphomolybdic acid).
To complete the synthesis of 1, the TBS group was removed by
treatment of 8 with an HF$pyridine complex in a mixture of pyri-
dine and THF to afford 9, which was condensed with isobutyric acid
in the presence of EDCI$HCl (3.0 equiv)eDMAP (0.30 equiv) to af-
ford 10 (Scheme 3). The N-Boc moiety was removed with TFA in
dichloromethane to afford 11. Amide formation of 11 with acid 12
was achieved in DMF with EDCl$HCl, HOBt, and NMM in 91% yield.
Hydrogenolysis of 13 with Pd(OH)2 in EtOH afforded splenocin B (1)
in 98% yield. The spectral data of synthetic 1 were identical to those
reported for the natural sample. The optical rotation of synthetic 1
4.2. (2R,3R,4S)-2-Benzyl-4-[(2S,3R)-3-benzyloxy-2-tert-butox-
ycarbonylamino-propionyloxy]-3-(tert-butyl-dimethyl-sila-
nyloxy)-pentanoic acid benzyl ester (5)
To a stirred, cooled (0 ꢀC) solution of freshly prepared 26b
(454 mg, 1.06 mmol) and N-Boc-L-Thr (OBn)
4 (989 mg,
([
a
]
D þ24.8, c 0.11, CHCl3) was in agreement with that of the natural
3.20 mmol) in CH2Cl2 (8 mL), DMAP (39 mg, 0.32 mmol) and EDCI
HCl (613 mg, 3.20 mmol) were added successively. After stirring
overnight at 0 ꢀC to rt, the resulting mixture was diluted with
Et2Oehexane (1:2, 30 mL) and filtered through a short-pass silica
gel column. The filtrate was concentrated and purified by silica gel
column chromatography (EtOAcehexane) to provide 5 (614 mg,
product ([
a
]
D þ21.2, c 0.01, CHCl3).1a
0.975 mmol, 92%) as a clear oil. [
(600 MHz, CDCl3)
a
]
þ6.7 (c 1.3, MeOH); 1H NMR
D
d
0.05 (s, 3H), 0.070 (s, 3H), 0.92 (s, 9H), 1.23 (d,
J¼6.7 Hz, 6H), 1.43 (s, 9H), 2.78e2.84 (m, 2H), 3.09e3.16 (m, 1H),
4.01e4.04 (m, 1H), 4.09e4.14 (m, 1H), 4.31 (dd, J¼9.8, 2.0 Hz, 1H),
4.36 (d, J¼11.7 Hz, 1H), 4.50 (d, J¼11.7 Hz, 1H), 4.89e4.95 (m, 1H),
4.92 (s, 2H), 5.30 (d, J¼9.8 Hz, 1H), 7.03 (d, J¼7.3 Hz, 1H), 7.04 (d,
J¼5.3 Hz, 1H), 7.10 (d, J¼6.7 Hz, 2H), 7.18 (t, J¼6.8 Hz, 2H), 7.18 (t,
J¼7.3 Hz, 1H), 7.18e7.20 (m, 1H), 7.20e7.26 (m, 4H), 7.26e7.29 (m,
1H), 7.33 (t, J¼7.1 Hz, 2H); 13C NMR (150 MHz, CDCl3)
d
ꢁ4.52,
ꢁ4.05, 14.28, 16.27, 18.31, 25.94, 28.27, 35.23, 52.14, 58.35, 66.44,
Scheme 3. Final steps in the synthesis of 1.

9628
K.-ichi Yoshida et al. / Tetrahedron 71 (2015) 9626e9629
70.87, 74.09, 74.89, 75.15, 79.73, 126.31, 127.63, 127.72, 128.07,
128.27, 128.38, 128.41, 128.49, 128.87, 135.33, 137.85, 139.03, 155.79,
170.37, 173.00; IR(KBr) nmax 2931, 1781, 1722, 1497, 1171, 778, 748,
699 cmꢁ1; HRFABMS calcd for C41H58NO8Si [MþH]þ 720.3931:
found 720.3929.
concentrated, and purified by silica gel column chromatography
(EtOAcehexane) to provide 9 (75.7 mg, 0.186 mmol, 93%) as a clear
oil. [a
]D¼þ55 (c 0.29, CHCl3); 1H NMR (400 MHz, CDCl3)
d 1.15 (d,
J¼6.6 Hz, 3H), 1.43 (s, 9H), 1.45 (d, J¼6.5 Hz, 3H), 2.66 (ddd, J¼10.8,
9.6, 3.6 Hz 1H), 2.98 (dd, J¼13.4, 10.8 Hz, 1H), 3.18 (dd, J¼13.4,
3.6 Hz, 1H), 3.69 (t, J¼9.6 Hz, 1H), 4.75e4.87 (m, 2H), 5.17e5.27 (m,
1H), 5.41 (m, 1H), 7.14e7.29 (m, 5H); 13C NMR (150 MHz, CDCl3)
4.3. (2R,3R,4S)-2-Benzyl-3-(tert-butyl-dimethyl-silanyloxy)-4-
hydroxy-pentanoic acid (10R,20S)-2-tert-butox-
d
14.38, 18.38, 28.24, 31.58, 35.06, 53.90, 54.32, 71.06, 75.95, 80.36,
ycarbonylamino-2-carboxy-1-methyl-ethyl ester (6)
128.48, 128.53, 128.84, 138.67, 154.85, 170.65, 172.96; IR(KBr) nmax
3396, 1687, 1544, 1509, 1135, 1044, 698 cmꢁ1; HRFABMS calcd for
To a stirred solution of 5 (328 mg, 0.52 mmol) in EtOH (15 mL),
10% Pd(OH)2 (51 mg) was added. The resulting suspension was
placed under H2 gas (1 atm) and stirred vigorously at rt for several
hours. Then, the mixture was filtered through a pad of Celite. The
filtrate was concentrated to provide crude seco acid 6 (276 mg,
C
21H28NO7 [MꢁH]ꢁ 406.1866: found 406.1861.
4.6. Isobutyric acid (3S, 4R, 7R, 8R, 9S)-[7-benzyl-3-tert-bu-
toxycarbonylamino-4,9-dimethyl-2,6-dioxo-[1,5]dioxonan-8-
yl] ester (10)
0.511 mmol, 98%) as a pale yellow oil. [
NMR (400 MHz, CDCl3)
a
]D¼ꢁ7.8 (c 0.66, MeOH). 1H
d
0.10 (s, 3H), 0.12 (s, 3H), 0.94 (s, 9H), 1.21
To a stirred, cooled (0 ꢀC) solution of 9 (17.8 mg, 44
isobutyric acid (18 L) in CH2Cl2 (0.5 mL) was added DMAP (2 mg,
16 mol) and EDCI HCl (36 mg, 18 mol) successively. After stirring
mmol) and
(d, J¼5.6 Hz, 3H), 1.29 (d, J¼6.3 Hz, 3H), 1.44 (s, 9H), 2.74e2.89 (m,
m
2H), 3.02e3.08 (m, 1H), 4.02 (dd, J¼6.1, 4.4 Hz, 1H), 4.22e4.34 (m,
m
m
2H), 4.99 (br s, 1H), 5.43 (br d, J¼9.3 Hz, 1H), 7.16e7.28 (m, 5H). 13
C
overnight at rt, the resulting mixture was poured into H2O and
extracted with EtOAc (3ꢃ). The combined extracts were washed with
H2O (2ꢃ) and brine, dried over Na2SO4, concentrated, and purified by
silica gel column chromatography (20% EtOAcehexane) to give 10 as
NMR (100 MHz, CDCl3)
d
ꢁ4.51, ꢁ4.14, 15.63, 18.18, 19.55, 25.86,
28.21, 34.17, 51.84, 58.80, 74.13, 75.00, 77.38, 126.45, 128.48, 128.90,
139.03, 156.14, 170.71, 177.17; IR(KBr) nmax 3423, 2932, 1717, 1509,
1164, 778, 700 cmꢁ1; HRFABMS calcd for C27H44NO8Si [MꢁH]ꢁ
538.2837: found 538.2838.
a white solid (18.4 mg, 39
m
mol, 88%). Mp 122 ꢀC; [
1.15 (d, J¼6.6 Hz, 3H), 1.217 (d,
a
]D¼þ57 (c 0.11,
CHCl3); 1H NMR (400 MHz, CDCl3)
d
J¼6.8 Hz, 3H), 1.224 (d, J¼6.8 Hz, 3H), 1.30 (d, J¼6.3 Hz, 3H), 1.43 (s,
9H), 2.59 (sept, J¼6.8 Hz, 1H), 2.66 (dd, J¼3.2,13.4 Hz, 1H), 2.85 (ddd,
J¼11.6, 10.0, 3.2 Hz, 1H), 2.97 (dd, J¼11.6, 13.4 Hz, 1H), 4.83e4.97 (m,
2H), 5.17 (t, J¼10.0 Hz, 1H), 5.16e5.19 (m, 1H), 5.42e5.45 (m, 1H),
4.4. (3S, 4R, 7R, 8R, 9S)[7-Benzyl-8-(tert-butyl-dimethyl-sila-
nyloxy)-4,9-dimethyl-2,6-dioxo-[1,5]dioxonan-3-yl]-carbamic
acid tert-butyl ester (8)
7.07e7.30 (m, 5H); 13C NMR (100 MHz, CDCl3)
d 14.45, 17.77, 18.86,
Under Ar atmosphere, ethoxyacetylene (40 wt. % solution in
28.15, 34.03, 34.45, 51.75, 54.27, 71.28, 74.08, 75.25, 126.59, 128.54,
128.76, 138.15, 154.62, 170.67, 171.99, 175.73; IR(KBr) nmax 3368, 1741,
1699, 1510, 1369, 1149, 743, 704 cmꢁ1; HRFABMS calcd for C25H36NO8
[MþH]þ 478.2441: found 478.2438.
hexanes, 221
[RuCl2(p-cymene)]2 (9.3mg,15
m
L, 0.924 mmol) was slowly added to a solution of
m
mol) inacetone (1.5mL) at0 ꢀC. After
being stirred at 0 ꢀC for 5 min, 6 (166 mg, 0.308 mmol) in acetone
(1.5 mL) was slowly added to the solution at 0 ꢀC. The resulting
mixture was stirred at rt for 1 h and then filtered through a short
neutral SiO2 pad column elucidated with EtOAc. The filtrate was
concentrated to afford the corresponding ethoxyvinyl ester (EVE) 7,
which was used without further purification. A solution of crude EVE
7 in 1,2-dichloroethane (31 mL) was slowly added to a highly diluted
(ꢂ)-10-camphorsulfonic acid (0.05 M in 1,2-dichloroethaneeCH3CN
4.7. Isobutyric acid (3S, 4R, 7R, 8R, 9S)-[3-amino-7-benzyl-4,9-
dimethyl-2,6-dioxo-[1,5]dioxonan-8-yl] ester (11)
To a solution of 10 (9.2 mg, 19 mmol) in CH2Cl2 (1 mL), trifluoro-
acetic acid (0.5 mL, 0.759 mmol) was added dropwise. After stirring
at rt for 2 h, the resulting mixture was concentrated, diluted with
saturated aq NaHCO3, and extracted with EtOAc (2ꢃ). The combined
extracts were washed with brine, dried over Na2SO4, and concen-
trated to produce crude 11 (8.5 mg, 23 mmol, 98%) as a white solid.
This was used for the next reaction without further purification. Mp
1:1, 620
at 80 ꢀC. Stirring was continued at 80 ꢀC for an additional 17 h. The
resulting mixture was cooled to rt before the addition of Et3N (42 L,
30 mol). Concentration and purification by silica gel column chro-
mL, 31 mmol) solution in 1,2-dichloroethane (94 mL) over 7 h
m
m
matography (EtOAcehexane) provided 8 as a pale yellow solid
124 ꢀC; [
a
]D¼þ0.54 (c 0.56, MeOH); 1H NMR (400 MHz, CDCl3)
d 1.20
(87.0 mg, 0.167 mmol, 59%). Mp 96 ꢀC; [
NMR (400 MHz, CDCl3)
a
]D¼þ49 (c 0.13, CHCl3); 1H
(d, J¼6.3 Hz, 3H),1.218 (d, J¼6.8 Hz, 3H), 1.224 (d, J¼6.8 Hz, 3H), 1.29
(d, J¼6.3 Hz, 3H), 2.58 (sept, J¼6.8 Hz, 1H), 2.65 (dd, J¼2.8, 13.6 Hz,
1H), 2.88 (ddd, J¼11.2,10.0, 2.8 Hz,1H), 2.99 (dd, J¼13.6,11.2 Hz,1H),
4.13 (d, J¼8.1 Hz,1H), 4.82 (dq, J¼10.0, 6.3 Hz,1H), 5.13 (t, J¼10.0 Hz,
1H), 5.17e5.20 (m,1H), 7.11e7.30 (m, 5H); 13C NMR (150 MHz, CDCl3)
d
0.16 (s, 3H), 0.21 (s, 3H), 0.96 (s, 9H),1.09 (d,
J¼6.6 Hz, 3H),1.38 (d, J¼6.8 Hz, 3H),1.42 (s, 9H), 2.63 (ddd, J¼10.4, 9.2,
2.8 Hz,1H), 2.79 (dd, J¼12.8,10.4 Hz,1H), 3.08 (dd, J¼12.8, 2.8 Hz,1H),
3.77 (t, J¼9.2 Hz, 1H), 4.70e4.90 (m, 2H), 5.20 (d, J¼7.6 Hz, 1H),
5.32e5.45 (m, 1H), 7.12 (d, J¼7.3 Hz, 2H), 7.11e7.31 (m, 3H); 13C NMR
d
13.00, 18.93, 18.96, 29.69, 34.11, 34.56, 51.64, 53.82, 72.08, 73.86,
(100 MHz, CDCl3)
d
ꢁ3.17, ꢁ3.14, 14.60, 18.68, 18.99, 25.93, 28.15,
75.26, 126.50, 128.49, 128.73, 138.28, 171.51, 174.14, 175.75; IR(KBr)
35.48, 54.67, 55.30, 71.32, 76.94, 77.54,126.42,128.40,128.85,138.84,
154.93,170.68,173.77; IR(KBr) nmax 3367,1739,1709,1521,1454,1361,
1249,1169,1095, 780 cmꢁ1; HRFABMS calcd for C27H44NO7Si [MþH]þ
522.2887: found 522.2880.
nmax 3397, 2983, 1741, 1454, 1177, 702 cmꢁ1; HRFABMS calcd for
C
20H28NO6 [MþH]þ 378.1916: found 378.1921.
4.8. Isobutyric acid (3S, 4R, 7R, 8R, 9S)-7-benzyl-3-(3-
formylamino-2-benzyloxy-benzoylamino)-4,6-dimethyl-2,6-
dioxo-[1,5]dioxonan-8-yl ester (13)
4.5. (3S, 4R, 7R, 8R, 9S)-(7-Benzyl-8-hydroxy-4,9-dimethyl-2,6-
dioxo-[1,5]dioxonan-3-yl)-carbamic acid tert-butyl ester (9)
To a stirred solution of 11 (8.5 mg, 23
mol) in DMF (0.5 mL), HOBt (9.3 mg, 69
(13.2 mg, 69 mol) and NMM (5.5 L, 6.9 mol) were added suc-
cessively. After stirring for 18 h at rt, the mixture was poured into
H2O and extracted with EtOAc (3ꢃ). The combined extracts were
washed with H2O (2ꢃ) and brine, dried over Na2SO4, concentrated,
m
mol) and 12 (18.7 mg,
Dilactone 8 (104 mg, 0.200 mmol) was treated with (HF$pyr-
idine complex)-pyridine-THF (5:6:8, 3.2 mL) at rt until the starting
material disappeared. The mixture was diluted with EtOAc, poured
into stirred saturated aq NaHCO3, and extracted with EtOAc (2ꢃ).
The combined extracts were washed with brine, dried over Na2SO4,
69
m
mmol), EDCI HCl
m
m
m

K.-ichi Yoshida et al. / Tetrahedron 71 (2015) 9626e9629
9629
and purified by silica gel column chromatography (40%
Analytical Division, Osaka City University for NMR
measurements.
EtOAcehexane) to produce 13 (13.2 mg, 21 mmol, 91%) as a pale
yellow oil.10
[
a
]D¼þ24 (c 0.78, MeOH); 1H NMR (600 MHz, CDCl3)
d
1.14 (d, J¼6.7 Hz, 3H), 1.23 (d, J¼7.0 Hz, 3H), 1.24 (d, J¼7.0 Hz, 3H),
Supplementary data
1.32 (d, J¼6.3 Hz, 3H), 2.61 (sept, J¼7.0 Hz, 1H), 2.69 (dd, J¼13.5,
3.0 Hz, 1H), 2.89 (ddd, J¼11.4, 10.3, 3.0 Hz, 1H), 3.00 (dd, J¼13.5,
11.4 Hz, 1H), 4.82 (A of ABq, J¼11.5 Hz, 1H), 4.99e5.09 (m, 1H), 5.15
(B of ABq, J¼11.5 Hz, 1H, Dn¼198.5 Hz) 5.21 (t, J¼10.3 Hz, 1H), 5.33
(dd, J¼7.8, 7.2 Hz, 1H), 5.54e5.61 (m, 1H), 7.13 (d, J¼7.8 Hz, 2H),
7.18e7.39 (m, 5H), 7.71 (d, J¼7.8 Hz, 1H), 8.02 (d, J¼7.8 Hz, 1H), 8.13
Supplementary data (1H and 13C NMR spectra) associated with
this article can be found in the online version, at http://dx.doi.org/
10.1016/j.tet.2015.10.075.
(s, 1H), 8.42 (d, J¼7.8 Hz, 1H); 13C NMR (150 MHz, CDCl3)
d 14.87,
References and notes
17.91, 18.95, 22.63, 34.54, 51.88, 53.91, 71.35, 74.35, 78.76, 121.44,
124.78,125.50,126.42,126.59,128.52,128.72,128.74,129.10,129.40,
131.31, 135.10, 138.03, 145.94, 158.44, 160.93, 164.68, 170.47, 171.93,
175.59; IR(KBr) nmax 3368, 2934, 1736, 1626, 1545, 1059, 739,
696 cmꢁ1; HRFABMS calcd for C35H37N2O9 [MꢁH]ꢁ 629.2499:
found 629.2488.
1. Isolation and structure elucidation of splenocin’s. (a) Fenical, W.; Strangman,
W. K.; Kwon, H. K.; Broide, D.; Jensen, P. R. J. Med. Chem. 2009, 52, 2317e2327
Biosynthesis of splenocin’s; (b) Chang, C.; Huang, R.; Yan, Y.; Ma, H.; Dai, Z.;
Zhang, B.; Deng, Z.; Liu, W.; Qu, X. J. Am. Chem. Soc. 2015, 137, 4183e4190.
2. Isolation and structure elucidation of AA. (a) Leben, C.; Keitt, G. W. Phytopa-
thology 1948, 38, 899e906; (b) Dunshee, B. R.; Leben, C.; Keitt, G. W.; Strong, F.
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4.9. Splenocin B (1)
To a stirred solution of 13 (2.0 mg, 3.2 mmol) in ethanol (0.5 mL),
10% Pd(OH)2 (3.0 mg) was added. The resulting suspension was
placed under H2 gas at 1 atm and stirred vigorouslyat rt for 15 h. Next,
the mixture was filtered through a pad of Celite and concentrated to
produce splenocin B (1.6 mg, 3.0 mmol, 98%) as white amorphous
powder.10 Mp 137 ꢀC;11
[a
]D¼þ63 (c 0.10, MeOH), natural:1a
:
[a
]D¼þ68 (c 0.1, MeOH); 1H NMR (600 MHz, CDCl3)
1.16 (d, J¼7.0 Hz,
d
3H), 1.215 (d, J¼7.1 Hz, 3H), 1.221 (d, J¼6.9 Hz, 3H), 1.33 (d, J¼6.2 Hz,
3H), 2.61 (sept, J¼7.0 Hz, 1H), 2.70 (dd, J¼2.0, 12.4 Hz, 1H), 2.91 (ddd,
J¼2.5, 9.2, 11.3 Hz, 1H), 3.00 (dd, J¼11.5, 12.5 Hz, 1H), 5.02e5.05 (m,
1H), 5.22 (t, J¼9.5 Hz, 1H), 5.27 (dd, J¼7.6, 7.6 Hz, 1H), 5.62 (quint,
J¼6.9 Hz, 1H), 6.91 (t, J¼8.1 Hz, 1H), 6.99 (d, J¼7.7 Hz, 1H), 7.13 (d,
J¼7.2 Hz, 2H), 7.19 (d, J¼7.4 Hz, 1H), 7.20 (d, J¼8.4 Hz, 1H), 7.24 (t,
J¼7.5 Hz, 2H), 7.89 (s, 1H), 8.50 (s, 1H), 8.54 (d, J¼7.2 Hz, 1H), 12.60 (s,
6. (a) Shimano, M.; Shibata, T.; Kamei, N. Tetrahedron Lett. 1998, 39, 4363e4366;
(b) Shimano, M.; Kamei, N.; Shibata, T.; Inoguchi, K.; Ito, N.; Ikari, T.; Senda, H.
Tetrahedron 1998, 54, 12745e12774.
7. For a recent review on MNBA, see Shiina, I. Bull. Chem. Soc. Jpn. 2014, 87,
196e233.
1H);13C NMR (150MHz, CDCl3)
d 14.71,17.81,18.95, 34.11, 34.53, 51.91,
53.46, 70.92, 74.78, 75.06, 112.54, 118.94, 120.08, 124.80, 126.65,
127.44, 128.55, 128.72, 137.90, 150.62, 159.06, 169.33, 170.07, 171.94,
175.65; IR(KBr) nmax 3352, 2932,1737,1524,1376,1151, 749, 703 cmꢁ1
HRESIMS calcd for C28H31N2O9 [MꢁH]ꢁ 539.2035: found 539.2035.
;
8. For a recent review on macrolactonization, see Parenty, A.; Moreau, X.; Niel, G.;
Campagne, J.-M. Chem. Rev. 2013, 113, PR1ePR40.
9. (a) Kita, Y.; Maeda, H.; Omori, K.; Okuno, T.; Tamura, Y. Synlett 1993, 273e274;
(b) Trost, B. M.; Chisholm, J. D. Org. Lett. 2002, 4, 3743e3745; (c) Trost, B. M.;
Harrington, P. E.; Chisholm, J. D.; Wrobleski, S. T. J. Am. Chem. Soc. 2005, 127,
13598e13610; (d) Ohba, Y.; Takatsuji, M.; Nakahara, K.; Fujioka, H.; Kita, Y.
Chem.dEur. J. 2009, 3526e3537.
Acknowledgements
10. 1H and 13C NMR spectra indicated that it existed as a mixture of two rotamers.
11. Mp was not reported in ref 1a.
This paper is dedicated to the memory of late Professor Kozo
Shibata, Osaka City University. We thank Dr. Matsumi Doe,