the asparaginyl-tRNA synthetase (AsnRS) in B. malayi (i)
is highly expressed in all stages of the parasite life cycle, (ii)
is biochemically and structurally well characterized, and
(iii) shows significant structural differences in comparison
to human and other eukaryotic AARS.5
affording pure 1 (13.1 mg), 2 (5.2 mg), 3 (3.9 mg), and 4
(3.2 mg) as white powders, respectively [see Supporting
Information (SI)]. Analysis of high resolution ESI-MS
(HRESIMS) data and 1H and 13C NMR spectra of 1 and
its triacetyl derivative 1a suggested 1 to be WS9326A, a
depsipeptide that has been previously isolated from Strepto-
myces violaceoniger no. 9326.7 The identity of 1 was unam-
bigously confirmed by extensive 1D and 2D NMR analysis
of 1 and 1a (Tables S1 and S2), as well as comparison to the
spectroscopic data reported previously.7
In an effort to discover new antifilarial drug leads, we
recently completed a high throughout screening campaign,
targeting the BmAsnRS. Of the ∼73000 extracts from a
collection of 36720 microbial strains screened, we identi-
fied 177 active strains. Previously, we reported the discov-
ery of the tirandamycins (TAMs) from Streptomyces
sp. 17944 and showed that TAM B was a potent and
specific BmAsnRS inhibitor that efficiently killed the adult
B. malayi parasite.6 We now report bioassay-guided frac-
tionation of another active strain, Streptomyces sp. 9078,
leading to the discovery of three new congeners of
the known depsipeptide WS9326A (1),7 WS9326C (2),
WS9326D (3), and WS9326E (4) (Figure 1). Importantly,
3, a moderate BmAsnRS inhibitor, efficiently kills the
adult B. malayi parasite, representing another lead scaffold
for antifilarial drug discovery.
The molecular formula of 2 was determined to be
C53H66N8O13 by HRESIMS, affording an [M þ H]þ ion at
m/z 1023.4842 (calculated [M þ H]þ ion at m/z 1023.4827)
and indicating that 2 differs from 1 (C54H68N8O13) by the
absence of a CH2 unit. The 1H and 13C NMR spectra of 2
indicated that 2 exists as a mixture of two conformers in
solution. A similar conformational mixture was known for
1; however its triacetyl derivative (1a) afforded a single
conformer.7 Thus, 2 was simiarly converted into its triace-
1
tyl derivative (2a), with H and 13C NMR spectra con-
firming it, in CDCl3, as a single conformer. The structure
of 2 was then established by careful comparison of the 1H
and 13C NMR data between 2a and 1a (Tables 1, S1, and S2).
The absence of a doublet methyl signal [δC 17.1 and δH 1.40
(3H, d, 6.30 Hz)] and a methine signal [δC 71.0 and δH 5.47
(1H, m)] with the concomitant presence of one new methy-
lene signal [δC 63.8 and δH 4.43 (1H, m), 4.68 (1H, m)] led to
theconclusionthat the1Thr residue in 1a was substitutedby
a 1Ser residue in 2a (Figure 1). This conclusion was further
supported by key correlations observed in gHMBC,
COSY, and NOESY experiments of 2a (Figure 2A). Since
the absolute stereochemistry of 1 was known,7 2, named
as WS9326C, was assigned the same stereochemistry as
that of 1 on the basis of its biosynthetic origin (Figure 1).
The molecular formula of 3 was determined to be
C47H59N5O10 by HRESIMS, affording an [M þ H]þ ion at
m/z 854.4357 (calculated [M þ H]þ ion at m/z 854.4340).
Analysis of the 1H and 13C NMR spectra indicated that3 is
a linear lipopeptide consisted of five amino acids. The struc-
tures of the five amino acids were determined by 1D and 2D
NMR, the sequence of which assigned in the order -NH-
1Thr-2ΔMeTyr-3Leu-4(D)Phe-5alloThr-CO2H with the as-
sistance of gHMBC and NOESY (Table 1 and Figures 1
and 2B). The identity of the 3-[2-(1(Z)-pentenyl)phenyl]-
2(E)-propenoyl moiety was readily evident upon analysis of
Figure 1. Structures of depsipeptide WS9326A (1)7 and the three
new congeners, WS9326C (2), WS9326D (3), and WS9326E (4),
from Streptomyces sp. 9078.
1
its H and 13C NMR spectra (Table 1) and in comparison
The crude extract from a 9.6-L fermentation culture
of Streptomyces sp. 9078 was subjected to sequential
chromatography over SiO2 and Sephadex LH-20 columns,
followed by semipreparative HPLC over a C-18 column.
Natural product isolation was guided by bioassay for in-
hibitory activity against the recombinant BmAsnRS,
with the same moiety in 1 and 2 (Tables S1 and S2). The
regiochemistry of the acyl moiety was assigned on the basis of
key gHMBC correlations between HR-1Thr [δH 5.01 (1H, m)]
and C-1 [δC 165.3 (s)] of the acyl unit of 3, establishing
that the acyl moiety is attached to R-C of 1Thr via an amide
linkage (Figure 2B). Thus 3, named as WS9326D, could be
envisaged as a biosynthetic intermediate of 1, thereby
sharing the same absolute stereochemistry (Figure 1).
The molecular formula of 4 was determined to be
C46H57N5O10 by HRESIMS, yielding an [M þ H]þ ion
at m/z 840.4194 (calculated [M þ H]þ ion at m/z 840.4183)
and differing from 3 by the absence of a CH2 unit. The
structure of 4 was established by direct comparison of the
(6) Yu, Z.; Vodanovic-Jankovic, S.; Ledeboer, N.; Huang, S.; Rajski,
S. R.; Kron, M.; Shen, B. Org. Lett. 2011, 13, 2034–2037.
(7) (a) Hayashi, K.; Hashimoto, M.; Shigematsu, N.; Nishicawa, M.;
Ezaki, M.; Yamashita, M.; Kiyoto, S.; Okuhara, M.; Kohsaks, M.;
Imanaka, H. J. Antibiot. 1992, 45, 1055–1063. (b) Hashimoto, M.;
Hayashi, K.; Murai, M.; Fujii, T.; Nishikawa, M.; Kiyoto, S.; Okuhara,
M.; Kohsaka, M.; Imanaka, H. J. Antibiot. 1992, 45, 1064–1070. (c)
Shigematsu, N.; Hayashi, K.; Kayakiri, N.; Takase, S.; Hashimoto, M.;
Tanaka, H. J. Org. Chem. 1993, 58, 170–175.
Org. Lett., Vol. 14, No. 18, 2012
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Yu, Zhiguo
