Determination of the Absolute Configurations of Microtermolides A and B

Article abstract of DOI:10.1021/acs.jnatprod.5b01143

Absolute configurations of the three consecutive chiral centers in the cyclic depsipeptide microtermolide A have been tentatively assigned as 2?R, 3?R, and 4?R. However, on the basis of a structural comparison with vinylamycin, another depsipeptide with a unique 4-amino-2,4-pentadienolate structure, the chiral centers could also be assigned as 2?R, 3?R, and 4?S. Here, the first total synthesis of microtermolide A is reported and the configurations of the three consecutive chiral centers were confirmed to be 2?R, 3?R, and 4?S. A similar approach was used to determine the analogous centers in microtermolide B as 2?R, 3?R, and 4?S.

Full text of DOI:10.1021/acs.jnatprod.5b01143

Note
pubs.acs.org/jnp
Determination of the Absolute Configurations of Microtermolides
A and B
Zhantao Yang,† Meiyan Ma,† Chun-Hua Yang, Yuan Gao, Quan Zhang, and Yue Chen*
The State Key Laboratory of Medicinal Chemical Biology, College of Pharmacy, and Tianjin Key Laboratory of Molecular
Drug Research, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: Absolute configurations of the three consec-
utive chiral centers in the cyclic depsipeptide microtermolide A
have been tentatively assigned as 2‴R, 3‴R, and 4‴R. How-
ever, on the basis of a structural comparison with vinylamycin,
another depsipeptide with a unique 4-amino-2,4-pentadieno-
late structure, the chiral centers could also be assigned as 2‴R,
3‴R, and 4‴S. Here, the first total synthesis of microtermolide A
is reported and the configurations of the three consecutive
chiral centers were confirmed to be 2‴R, 3‴R, and 4‴S. A similar
approach was used to determine the analogous centers in microtermolide B as 2‴R, 3‴R, and 4‴S.
epsipeptides with unique 4-amino-2,4-pentadienolate
D
structures include rakicidins,1−3 BE-43547A1,4 and vinyl-
amycin,5 and these compounds have exhibited cytotoxic or
antibiotic activities (Figure 1). However, the absolute config-
urations of most of the chiral centers in these depsipeptides
have remained unknown. Recently, total syntheses and degra-
dation of these depsipeptides were used to determine their
absolute configurations.6−11 Microtermolides A and B are
also depsipeptides with 4-amino-2,4-pentadienolate moieties,
isolated from Streptomyces sp.12 When assayed, neither com-
pound exhibited cytotoxicity or antimicrobial activity. In addi-
tion, NMR spectroscopy was used to predict the absolute
configurations of the three consecutive chiral centers in each
compound. As a result, the three chiral centers in both micro-
termolides A and B were tentatively assigned as 2‴R, 3‴R, and
4‴R (compounds 1a and 2a, respectively),12 thereby repre-
senting an anti−anti relationship among the chiral centers.
However, vinylamycin, an antimicrobial natural product that
appears to differ from microtermolide A only by the length of
its side chain, had the configurations of its three consecutive
chiral centers determined as 2‴R, 3‴R, and 4‴S, thereby repre-
senting an anti−syn relative relationship.8 Therefore, it was also
possible that microtermolides A and B could have an anti−syn
relative stereochemistry for their three consecutive chiral centers
(compounds 1b and 2b, respectively). In order to unambiguously
assign the absolute configurations of microtermolides A and B,
compounds 1a, 1b, 2a, and 2b were synthesized.
Figure 1. Various proposed structures for microtermolides A (1a, 1b)
and B (2a, 2b).
5a/5b with Fmoc-D-valine 12 produced compounds 6a and 6b.
Deprotection of the Fmoc group in 6a/6b then allowed the
resulting amines to be coupled with Fmoc-L-alanine 13 to
provide compounds 7a/7b. After deprotection of the allyl
groups was completed, the resulting acids were coupled with the
D-serine derivative 11, followed by cleavage of the allyl ester and
Fmoc protecting groups, and then the resulting intermediates
were allowed to undergo a lactamization step based on
previously optimized conditions6 to produce lactams 9a/9b
Briefly, target compounds 1a and 1b were prepared accord-
ing to a synthetic sequence that was previously reported for
the total synthesis of vinylamycin (Scheme 1).8 To achieve
adducts 4a/4b with the desired configurations, these com-
pounds were synthesized with high diastereoselectivity (dr >
10:1) via the Mukaiyama aldol reaction.8 Hydrolysis of 4a/4b
was followed by treatment with cesium carbonate (Cs2CO3)
and allyl bromide to achieve allyl esters 5a/5b. Esterification of
Received: December 23, 2015
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.5b01143
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Note
Scheme 1. Total Syntheses of Microtermolide A and
Scheme 3. Synthesis of Unsaturated Acid 22
C4‴ -epi-Microtermolide A
Known compound 2213 was prepared using our concise
sequence (Scheme 3). Briefly, compound 20 was treated with
oxalyl chloride, followed by an aqueous ammonia solution,
to provide amide 21 in 33% yield. The methyl ester was then
hydrolyzed to generate the corresponding unsaturated acid 22
in 42% yield.
Esterification of alcohols 16a/16b with Fmoc-D-valine 12
produced compounds 23a/23b (Scheme 4). The Fmoc pro-
Scheme 4. Total Syntheses of Microtermolide B and
C4‴-epi-Microtermolide B
with 29% yield in five steps. Exposure of cyclic compounds 9a
and 9b to hydrofluoric acid (HF) provided free alcohols 10a/
10b. Finally, a mesylation step was followed by deprotection
of 4-methoxybenzyl ether (PMB) with 2,3-dichloro-5,6-
dicyanobenzoquinone (DDQ) and elimination of the meth-
anesulfonyl (Ms) group with 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) to produce compounds 1a and 1b, respectively.
To determine the absolute configuration of microtermolide B,
the total synthesis of microtermolide B commenced with
the known compound 14.8 A Mukaiyama aldol condensation
of 14 and 18a/18b produced 15a/15b with high diastereo-
selectivity (dr > 10:1).8 The benzyl group of 15a/15b was then
removed with a palladium on carbon catalyst and hydrogen
at atmospheric pressure (Pd−C/H2). The resulting alcohol was
rapidly converted to cyclic products 16a/16b with silica/MeOH
(Scheme 2).
tecting group was subsequently removed to generate free amines
24a/24b. After 24a/24b were coupled with Fmoc-Ala-OH
13, followed by removal of the Fmoc group, amines 25a/25b
were achieved. In the final step, the secondary amines of 25a/25b
were coupled with unsaturated acid 22 to provide compounds
2a/2b.
Following the preparation of target molecules 1a/2a and
1
1b/2b, the four compounds were subjected to H and 13C
Scheme 2. Synthesis of Dihydrofuran-2(3H)-ones 16a/16b
NMR analyses. These data were compared with previously
reported data for natural microtermolides A and B.12 The data
for compound 1b and microtermolide A matched perfectly,
while the 13C NMR data for compound 1a and microtermolide A
clearly differed at C-3‴, C-4‴, C-5‴, C-7‴, and C-10‴ (refer
to Supporting Information). Thus, the absolute configurations
of the three consecutive chiral centers in microtermolide A
were unambiguously assigned as 2‴R, 3‴R, and 4‴S, thereby
representing an anti−syn relationship. Similarly, NMR data
for compound 2b and microtermolide B matched. In contrast,
the 13C NMR data for compound 2a and microtermolide B
clearly differed at C-3‴, C-4‴, C-5‴, and C-10‴. Thus, the
B
DOI: 10.1021/acs.jnatprod.5b01143
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Note
1
Table 1. H and 13C NMR Data Comparison (MeOD-d4)
lit. microtermolide A
synthetic microtermolide 1a
synthetic microtermolide 1b
assignment 13C ppm 1H ppm [mult, J (Hz)] 13C ppm Δppm 1H ppm [mult, J (Hz)]
Δ
13C ppm
Δ
1H ppm [mult, J (Hz)]
Δ
C1, CO
C2, CH
C3, CH
C4, CH3
C5, CH3
C1′, CO
C2′, CH
C3′,CH3
C1″, CO
C2″, CH
C3″, CH
C4″, C
170.4
59.2
170.6
59.0
0.2
−0.2
0
170.3
59.2
−0.1
0
4.42 (m)
4.48 (d, 6.6)
2.17−2.09 (m)
0.96 (d, 5.7)
0.99 (d, 5.6)
0.06
0.06
0
4.43 (d, 7.3)
0.01
33.4
2.07 (dq, 13.7, 6.8)
0.96 (d, 6.5)
0.96 (d, 7.0)
33.4
33.4
0
2.07 (dq, 14.1, 7.1)
0.96 (d, 6.6)
0
19.7
19.9
0.2
−0.1
−0.2
0.1
0.1
0
19.9
0.2
0
0
18.5
18.4
0.03
18.5
0.95 (d, 6.6)
−0.01
175.2
52.6
175.0
52.7
174.9
52.8
−0.3
0.2
0.1
0
4.40 (m)
4.41−4.35 (m)
1.45 (d, 7.0)
−0.02
0.01
4.36 (dd, 10, 6.9)
1.45 (d, 6.9)
−0.04
0.01
18.5
1.44 (d, 7.0)
18.6
18.6
169.2
118.2
141.6
138.6
119.3
169.2
118.3
141.7
138.7
119.6
169.2
118.3
141.6
138.6
119.6
6.24 (d, 15.3)
7.11 (d, 14.7)
0.1
0.1
0.1
0.3
6.20 (d 15.1)
7.14 (d 15.1)
−0.04
0.03
0.1
0
6.20 (d, 15.1)
7.13 (d, 15.2)
−0.04
0.02
0
C5″, CH2
5.59 (s)
5.52 (s)
5.60 (s)
5.54 (s)
0.01
0.02
0.3
5.59 (s)
5.54 (s)
0
0
C1‴, CO
C2‴, CH
C3‴, CH
C4‴, CH
C5‴, CH2
174.3
46.9
78.0
34.9
37.3
174.1
46.7
80.1
33.4
35.3
−0.2
−0.2
2.1
174.1
47.0
78.1
34.9
37.3
−0.2
0.1
0.1
0
3.00 (td, 10.3, 4.1)
5.48 (dd, 10.3, 2.1)
1.88 (m)
3.02 (td, 10.3, 4.7)
5.42 (d, 9.9)
0.02
−0.06
0.05
2.97 (td, 10.6, 4.9)
5.49 (d, 10.3)
1.92−1.85 (m)
1.36−1.32(m)
1.20−1.11(m)
1.44−1.36 (m)
0.90 (t, 7.0)
−0.03
0.01
−1.5
−2.0
1.96−1.89 (m)
1.37−1.29 (m)
0
1.34 (m)
−0.01
0
0
1.16 (m)
−0.01
−0.02
0
0
0
0
0
C6‴, CH2
C7‴, CH3
C8‴, CH2
C9‴, CH2
21.2
14.2
33.4
59.9
1.42 (m)
21.3
16.7
33.4
59.9
0.1
2.5
0
1.37−1.29 (m)
0.95, t (5.6)
21.3
14.2
33.4
59.9
0.1
0
0.90 (t, 7.0)
1.80 (m)
0.05
0.01
0.01
0.01
1.86−1.77 (m)
3.70−3.64(m)
3.61−3.54 (m)
1.80−1.77 (m)
0
1.84−1.76 (m)
3.66 (td, 10.2, 4.5)
3.58−3.55 (m)
1.06 (d, 6.7)
3.66 (m)
0
0
3.56 (m)
C10‴, CH3
13.4
1.06 (d, 7.0)
14.3
0.9
13.4
0
dd = doublet of doublets, t = triplet, td = triplet of doublets, m =
multiplet), coupling constant (Hz), and integration. High-resolution
mass spectrometry (HRMS) was performed on a QTOF ESI or
MALDI spectrometer. Commercially available dry solvents were used
for DMF, CH2Cl2, THF, and MeOH. The starting materials 12 and 13
were purchased from Energy Chemical Co.Ltd.
(3R,6S,14R,15R,E)-14-(2-Hydroxyethyl)-3-isopropyl-6-meth-
yl-11-methylene-15-((R)-pentan-2-yl)-1-oxa-4,7,12-triazacyclo-
pentadec-9-ene-2,5,8,13-tetraone (1a). To a solution of 10a
(10 mg, 0.017 mmol) in CH2Cl2 (1 mL) were added triethylamine
(2.6 mg, 0.025 mmol) and MsCl (2.3 mg, 0.02 mmol) at 0 °C. After
being stirred for 0.5 h, the reaction solution was quenched by addition
of water (1 mL). The aqueous phase was extracted with EtOAc.
The combined organic phases were washed with brine, dried over
Na2SO4, filtrated, and concentrated under reduced pressure. The crude
mixture was further purified through column chromatography (ethyl
acetate/CH3OH, 10:1) to afford the compound as a solid.
absolute configurations of the three consecutive chiral centers
in microtermolide B were unambiguously assigned as 2‴R,
3‴R, and 4‴S, which also represents an anti−syn relationship.
Taken together, these results indicate that the structures of
compounds 1b and 2b represent the structures of the natural
microtermolides A and B, respectively, and assignments of the
three chiral centers of each compound have been determined.
Depsipeptides with 4-amino-2,4-pentadienolate structures
exhibited important biological activities1−5,11,12,14,15 and are
suitable for further investigation of structure activity relation-
ships (SAR).16 The determination of the absolute configura-
tion of microtermolides supplied not only two more examples
of an anti−syn relative stereochemistry in depsipeptides with
4-amino-2,4-pentadienolate moieties but also valuable informa-
tion for further SAR study of this type of depsipeptides.
To a solution of the compound above (9 mg, 0.013 mmol) in
CH2Cl2 (1.4 mL) and PBS (0.1 mL, pH 7.0) was added DDQ (4.4 mg,
0.02 mmol) at 20 °C. After 2 h, the mixture was quenched with
saturated NaHCO3 solution and extracted with EtOAc. The organic
phase was washed by brine and then dried over Na2SO4, filtrated, and
concentrated under reduced pressure. The crude product was purified
by column chromatography on silica gel (ethyl acetate/methanol,
1:0 to 10:1) to give the primary alcohol.
EXPERIMENTAL SECTION
■
General Methods and Materials. Thin-layer chromatography
(TLC) was performed on 0.25 mm silica gel plates (60-F254). Proton
and carbon magnetic resonance spectra (1H and 13C NMR) were
recorded on a 400 MHz spectrometer, as specified. Spectra were
acquired in CDCl3 and referenced to the solvent peak at 7.26 ppm
(1H) and 77.16 ppm (13C) for CDCl3. H NMR data are reported
1
as follows: chemical shift, multiplicity (abbreviations: d = doublet,
C
DOI: 10.1021/acs.jnatprod.5b01143
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Note
1
Table 2. H and 13C NMR Data Comparison (MeOD-d4)
lit. microtermolide B
synthetic microtermolide 2a
synthetic microtermolide 2b
assignment 13C ppm
1H ppm [mult, J (Hz)]
13C ppm Δppm
1H ppm [mult, J (Hz)]
Δ
13C ppm
Δ
1H ppm [mult, J (Hz)]
Δ
C1, CO
168.7
134
168.2
133.8
134.1
166.0
174.4
50.3
−0.5
−0.2
−0.2
−0.2
−0.1
−0.2
−0.1
0
168.7
134.0
134.3
166.2
174.5
50.5
0
0
C2, CH
6.90 (d 15.3)
6.94 (d 15.3)
6.90 (d, 15.3)
6.96 (d, 15.3)
0
6.90 (d, 15.3)
6.96 (d, 15.3)
0
C3, CH
134.3
166.2
174.5
50.5
18.2
172.2
59.5
31.0
20.0
18.3
178.6
41.7
77.1
35.8
36.6
0.02
0
0.02
C4, CO
0
C1′, CO
C2′, CH
C3′, CH3
C1″, CO
C2″, CH
C3″, CH
C4″, CH3
C5″, CH3
C1‴,CO
C2‴, CH
C3‴, CH
C4‴, CH
C5‴, CH2
0
4.56 (q, 7.0)
1.39 (d, 7.0)
4.55 (q, 7.0)
1.39 (d, 7.1)
−0.01
0
0
4.56 (q, 7.1)
1.39 (d, 7.0)
0
0
18.1
18.2
0
172.2
59.5
172.3
59.4
0.1
−0.1
0.3
0
4.35 (d, 6.5)
2.21 (m)
0
4.34−4.28 (d, 6.1)
2.24−2.16 (m)
0.99 (d, 6.8)
−0.04
−0.01
0
4.35 (d, 6.1)
2.25−2.17 (m)
0.98 (d, 6.8)
0.95 (d, 6.8)
0
31.1
0.1
31.3
0
0.99 (d, 7.0)
0.95 (d, 7.0)
19.7
−0.3
0
20.0
−0.01
0
18.3
0.96 (d, 6.8)
0.01
18.3
0
178.2
41.5
−0.4
−0.2
1.1
178.6
41.6
0
3.03 (ddd, 10.7, 9.0, 6.7)
5.08 (dd, 7.0, 5.3)
1.94 (m)
3.08 (ddd, 10.3, 9.1, 6.7)
5.01 (dd, 7.3, 5.1)
2.12−2.01 (m)
1.31−1.27 (m)
1.15−1.06 (m)
1.31−1.27 (m)
0.88 (t, 6.7)
0.05
−0.07
0.12
−0.1
0
3.03 (ddd, 10.7, 9.1, 6.9)
5.08 (dd, 6.8, 5.2)
1.98−1.89 (m)
1.33−1.29 (m)
1.17−1.09 (m)
1.37−1.32 (m)
0.87 (t, 7.1)
0
78.2
77.1
0
34.6
−1.2
−1.4
35.8
0
0
1.32 (m)
35.2
36.6
0
0
1.13 (m)
−0.02
0
C6‴, CH2
C7‴, CH3
C8‴, CH2
21.0
14.5
27.0
1.34 (m)
20.5
14.4
26.4
−0.5
−0.1
−0.6
21.0
14.5
27.0
0
0
0
0
0.88 (t, 7.3)
2.33 (m)
0
−0.01
0.02
0
−0.01
0
2.36−2.28 (m)
2.12−2.01 (m)
4.34−4.28 (m)
4.18 (td, 9.2, 7.0)
0.92 (d, 6.9)
2.37−2.29 (m)
2.09−1.99 (m)
4.31 (td, 8.8, 2.7)
4.17 (td, 9.3, 6.8)
0.91 (d, 6.8)
2.04 (m)
0
C9‴, CH2
67.6
14.5
4.31 (td, 8.7, 2.6)
4.18 (td, 9.1, 7.0)
0.92 (d 7.0)
67.6
16.0
0
67.6
14.5
0
0
0
0
−0.01
−0.01
C10‴, CH3
1.5
0
To a solution of the primary alcohol above in CH2Cl2 (0.5 mL) was
added DBU (14 mg, 0.09 mmol) at 25 °C. After being stirred for 2 h
at this temperature, 1% HCl was added to quench the reaction.
The aqueous phase was extracted with EtOAc. The combined organic
phases were washed with brine, dried over Na2SO4, filtrated, and
concentrated under reduced pressure. The crude product was purified
by flash chromatography on silica gel (MeOH/CH2Cl2, 1:50) to
(1R,2R)-2-Methyl-1-((R)-2-oxotetrahydrofuran-3-yl)pentyl
((E)-4-amino-4-oxobut-2-enoyl)-L-alanyl-D-valinate (2a). To a
solution of the amine (20 mg, 0.056 mmol) and the unsaturated
acid (10 mg, 0.087 mmol) in DMF/CH2Cl2 (0.3 mL/1.2 mL) was
added DIPEA (56 mg, 0.435 mmol), followed by HATU (66 mg,
0.174 mmol). After 1 h at room temperature, 1% HCl was added to
quench the reaction, which was washed with saturated NaHCO3 and
brine and evaporated in vacuo. The residue was further purified
through chromatography (CH2Cl2/CH3OH, 5%) to give the title
product 2a (15 mg, 60%) as a solid: [α]2D9 = −10.3 (c 0.6, CHCl3); IR
(KBr, cm−1) νmax 3350, 2959, 2872, 1772, 1726, 1517, 1260, 1100,
1
obtain the title 1a (3 mg, 41% for three steps) as a solid: H NMR
(400 MHz, MeOD) δ 7.14 (d, J = 15.1 Hz, 1H), 6.20 (d, J = 15.1 Hz,
1H), 5.60 (s, 1H), 5.54 (s, 1H), 5.42 (d, J = 9.9 Hz, 1H), 4.58
(brs, 1H), 4.48 (d, J = 6.6 Hz, 1H), 4.41−4.35 (m, 1H), 3.70−3.64
(m, 1H), 3.61−3.54 (m, 1H), 3.02 (td, J = 10.3, 4.7 Hz, 1H), 2.17−2.09
(m, 1H), 1.96−1.89 (m, 1H), 1.86−1.77 (m, 2H), 1.61−1.52 (m, 1H),
1.45 (d, J = 7.0 Hz, 3H), 1.37−1.29 (m, 3H), 1.01 (d, J = 6.7 Hz, 3H),
0.99 (d, J = 5.6 Hz, 3H), 0.96 (d, J = 5.7 Hz, 3H), 0.95 (t, J = 5.6 Hz,
3H); 13C NMR (100 MHz, MeOD) δ 175.0, 174.1, 170.6, 169.2,
141.7, 138.7, 119.6, 118.3, 80.1, 59.9, 59.0, 52.7, 46.7, 35.3, 33.4,
33.4, 33.2, 21.3, 19.9, 18.6, 18.4, 16.7, 14.3; [M + Na]+ calcd for
C23H37N3O6Na+, 474.2580; found, 474.2578.
1
872; H NMR (400 MHz, MeOD) δ 6.96 (d, J = 15.3 Hz, 1H), 6.90
(d, J = 15.3 Hz, 1H), 5.01 (dd, J = 7.3, 5.1 Hz, 1H), 4.55 (q, J =
7.0 Hz, 1H), 4.34−4.28 (m, 2H), 4.18 (td, J = 9.2, 7.0 Hz, 1H), 3.12−
3.05 (m, 1H), 2.36−2.28 (m, 1H), 2.19 (dt, J = 12.8, 6.6 Hz, 1H),
2.12−2.01 (m, 2H), 1.39 (d, J = 7.1 Hz, 3H), 1.31−1.27 (m, J =
3.4 Hz, 2H), 1.25−1.18 (m, 1H), 1.15−1.06 (m, 1H), 0.99 (d, J =
6.8 Hz, 3H), 0.96 (d, J = 6.8 Hz, 3H), 0.92 (d, J = 6.9 Hz, 3H), 0.88
(t, J = 6.7 Hz, 3H); 13C NMR (100 MHz, MeOD) δ 178.2, 174.4,
172.2, 168.6, 166.0, 134.1, 133.8, 78.2, 67.6, 59.5, 50.3, 41.5, 35.2, 34.6,
31.1, 26.4, 20.5, 19.7, 18.3, 18.1, 16.0, 14.4; HRMS−MALDI (m/z)
[M + Na]+ calcd for C22H35N3O7Na+, 476.2373; found, 476.2369.
2b: [α]2D9 = −4.2(c 0.7, CHCl3); IR (KBr, cm−1) νmax 3294, 2963,
2875, 1771, 1623, 1538, 1166, 1028, 844; 1H NMR (400 MHz,
MeOD) δ 6.96 (d, J = 15.3 Hz, 1H), 6.90 (d, J = 15.3 Hz, 1H), 5.08
(dd, J = 6.8, 5.2 Hz, 1H), 4.56 (q, J = 7.1 Hz, 1H), 4.35 (d, J = 6.1 Hz,
1H), 4.31 (td, J = 8.8, 2.7 Hz, 1H), 4.17 (td, J = 9.3, 6.8 Hz, 1H),
3.03 (ddd, J = 10.7, 9.1, 6.9 Hz, 1H), 2.37−2.29 (m, 1H), 2.25−2.17
(m, 1H), 2.09−1.99 (m, 1H), 1.98−1.89 (m, 1H), 1.39 (d, J = 7.1 Hz,
3H), 1.37−1.32 (m, 2H), 1.33−1.29 (m, 1H), 1.17−1.09 (m, 1H),
0.98 (d, J = 6.8 Hz, 3H), 0.95 (d, J = 6.8 Hz, 3H), 0.91 (d, J = 6.8 Hz,
3H), 0.87 (t, J = 7.1 Hz, 3H); 13C NMR (400 MHz, MeOD) δ
178.6, 174.5, 172.3, 168.7, 166.2, 134.3, 134.0, 77.1, 67.6, 59.4, 50.5,
1b: [α]1D9 = +83.6 (c 0.28, MeOH); IR (KBr, cm−1) νmax 3245, 2960,
2873, 1733, 1671, 1520, 1244, 1038, 982; 1H NMR (400 MHz,
MeOD) δ 7.13 (d, J = 15.2 Hz, 1H), 6.20 (d, J = 15.1 Hz, 1H), 5.59
(s, 1H), 5.54 (s, 1H), 5.49 (d, J = 10.3 Hz, 1H), 4.43 (d, J = 7.3 Hz,
1H), 4.36 (q, J = 6.8 Hz, 1H), 3.66 (td, J = 10.2, 4.5 Hz, 1H), 3.57
(dd, J = 16.4, 8.7 Hz, 1H), 2.97 (td, J = 10.6, 4.9 Hz, 1H), 2.07 (dq,
J = 14.1, 7.1 Hz, 1H), 1.92−1.85 (m, 1H), 1.84−1.76 (m, 2H),
1.45 (d, J = 6.9 Hz, 3H), 1.44−1.36 (m, 3H), 1.20−1.11 (m, 1H),
1.06 (d, J = 6.7 Hz, 3H), 0.96 (d, J = 6.6 Hz, 3H), 0.95 (d, J = 6.6 Hz,
3H), 0.90 (t, J = 7.0 Hz, 3H); 13C NMR (100 MHz, MeOD) δ 174.9,
174.1, 170.3, 169.2, 141.6, 138.6, 119.6, 118.3, 78.1, 59.9, 59.2, 52.8,
47.0, 37.3, 34.9, 33.4, 21.3, 19.9, 18.6, 18.5, 14.2, 13.3; [M + Na]+ calcd
for C23H37N3O6Na+, 474.2580; found, 474.2576.
D
DOI: 10.1021/acs.jnatprod.5b01143
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Note
41.6, 36.6, 35.8, 31.3, 27.0, 21.0 20.0, 18.3, 18.2, 14.5, 14.5; HRMS−
MALDI (m/z) [M + Na]+ calcd for C22H35N3O7Na+, 476.2373; found,
476.2369.
(16) Sang, F.; Ding, Y. H.; Wang, J. H.; Sun, B. X.; Sun, J. L.; Geng,
Y.; Zhang, Z.; Ding, K.; Wu, L. L.; Liu, J. W.; Ba, C. G.; Yang, G.;
Zhang, Q.; Li, L. Y.; Chen, Y. J. Med. Chem. 2016, 59, 1184.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnatprod.5b01143.
1H and 13C NMR, HRMS−MALDI, and NMR spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*Tel/Fax: +86 2285358386. E-mail: yuechen@nankai.edu.cn.
Author Contributions
†Z. Yang and M. Ma contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (NSFC) (Nos. 21372129 and 81573282
to Y.C. and Nos. 81370086 and 81573308 to Q.Z.), Program
for New Century Excellent Talents in University to Y.C.,
and Hundred Young Academic Leaders Program of Nankai
University to Y.C.
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DOI: 10.1021/acs.jnatprod.5b01143
J. Nat. Prod. XXXX, XXX, XXX−XXX