Isolation of 2-Alkyl-4-quinolones with Unusual Side Chains from a Chinese Pseudomonas aeruginosa Isolate

Article abstract of DOI:10.1021/acs.jnatprod.0c00026

Chemical investigation of a Pseudomonas aeruginosa strain isolated from Hebei, China, led to the isolation of a suite of quinolones, quinolone-N-oxides, and phenazines, the structures of which were elucidated by detailed spectroscopic analysis. Most notable among the secondary metabolites isolated was an unprecedented 4-quinolone containing an S-methyl group in the side chain and a new derivative including a phenyl ring in the side chain, which expand significantly the variety of structural motifs found in the quinolones and raise interesting questions about their biosynthesis.

Full text of DOI:10.1021/acs.jnatprod.0c00026

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
pubs.acs.org/jnp
Note
Isolation of 2‑Alkyl-4-quinolones with Unusual Side Chains from a
Chinese Pseudomonas aeruginosa Isolate
Jianye Li, Weiwei Sun, Muhammad Saalim, Guixiang Wei, Diana A. Zaleta-Pinet, and Benjamin R. Clark*
Cite This: https://dx.doi.org/10.1021/acs.jnatprod.0c00026
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Chemical investigation of a Pseudomonas aeruginosa
strain isolated from Hebei, China, led to the isolation of a suite of
quinolones, quinolone-N-oxides, and phenazines, the structures of
which were elucidated by detailed spectroscopic analysis. Most
notable among the secondary metabolites isolated was an
unprecedented 4-quinolone containing an S-methyl group in the
side chain and a new derivative including a phenyl ring in the side
chain, which expand significantly the variety of structural motifs
found in the quinolones and raise interesting questions about their
biosynthesis.
unprecedented S-methylated example, from a Chinese P.
he Pseudomonas genus comprises Gram-negative, rod-
Tshaped bacteria that are capable of colonizing a wide aeruginosa isolate.
range of environments, including the human body.1 Species of
Pseudomonas are well known for the capability to produce
diverse secondary metabolites, which have been studied at
both a chemical and genetic level.2,3 These secondary
metabolites may play a vital role for microbes in their nutrient
acquisition, virulence, and defense against other competitors in
the microbial community.2 In Pseudomonas aeruginosa, an
opportunistic pathogen in humans, one of the most important
of these compound classes is the alkyl-quinolones.4 The best
known of these metabolites are 3,4-dihydroxyheptylquinone,
also known as the pseudomonas quinolone signal (PQS),5 and
its biosynthetic precursor, 4-hydroxy-2-heptylquinoline
(HHQ), which both play a role in quorum sensing behavior
of P. aeruginosa.6 The third major class of quinolone-type
molecules from Pseudomonas species is the quinolone-N-
oxides, which have been largely reported to have antimicrobial
properties.7 Between these three major structural classes, there
are estimated to be more than 50 quinolone derivatives
produced by P. aeruginosa strains.8
While the alkylquinolones have attracted particular attention
due to their quorum sensing and antimicrobial activities, these
compounds possess a wide range of other pharmacologically
relevant properties. In 2012, it was reported that two
quinolones from a marine Alteromonas strain possessed
significant activity against a range of harmful bloom-forming
algae,9 while in a 2016 report, a suite of quinolones with
antimalarial and cytotoxic activity were isolated from a P.
aeruginosa strain.10 However, despite this considerable
diversity of activity, the structural motifs found in this
compound class are relatively limited. In the current study,
we report the isolation of six new quinolones, including an
The microbial strain, designated BD06-03, was isolated from
a soil sample collected from a park in Baoding City (Hebei
Province, China) and identified as Pseudomonas aeruginosa by
16S rDNA sequence analysis. Large-scale culture of P.
aeruginosa BD06-03 was carried out in ISP-4 medium. After
repeated solvent extraction and fractionation by a combination
of HP-20 resin column chromatography, open-column silica
chromatography, and semipreparative reversed-phase HPLC,
29 compounds were obtained, the structures of which were
elucidated by spectroscopic analysis. The isolated compounds
consisted of six new quinolone natural products (1−6) in
addition to a range of known quinolones, quinolone-N-oxides,
a small suite of phenazines, and N-(2-hydroxyphenyl)-
acetamide. Structures of the known compounds were
confirmed by comparison of NMR data with literature values
(Supporting Information); however, elucidation of the new
compounds 1−6 required a more detailed spectroscopic
analysis.
1
The H NMR data of compound 1 (Table 1) showed one
exchangeable proton, seven deshielded protons (δH 8.02, 7.69,
7.61, 7.55, 7.26, 6.17, 6.16), and one methyl singlet (δH 2.44).
The 13C NMR, together with the HMBC spectrum, displayed
12 carbons including one carbonyl, three additional non-
protonated carbons, seven methine carbons, and one methyl
Received: January 9, 2020
© XXXX American Chemical Society and
American Society of Pharmacognosy
https://dx.doi.org/10.1021/acs.jnatprod.0c00026
J. Nat. Prod. XXXX, XXX, XXX−XXX
A

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of Natural Products
pubs.acs.org/jnp
Note
Figure 1. Key COSY and HMBC correlations in the structure
elucidation of new compounds 1−6.
chemical shifts of δH 2.44 and δC 13.7 for the methyl group
suggested that it may be bonded to a sulfur atom.11 The
molecular formula C12H11NOS, obtained from HRESIMS,
supported this assignment, and the structure was assigned as
shown. The presence of this unusual substituent in a quinolone
is totally unprecedented and raises questions about the
molecule’s biosynthesis.
carbon. COSY correlations between δH 8.20 (H-5), 7.26 (H-
6), 7.61 (H-7), and 7.55 (H-8), together with HMBC
correlations from H-5 to C-4/C-8a, H-8 to C-4a, and H-3 to
C-2/C-4a, revealed the presence of a 2-substituted-4-
quinolone moiety (Figure 1). The other two protons at δH
6.17 and 7.69 were coupled to each other with a distinctive
coupling constant (J = 15 Hz) showing the existence of a trans
double bond in the side chain, and were assigned as H-1′ and
H-2′, respectively. HMBC showed correlations from δH 6.17
(H-1′) to C-3 and C-2′, from δH 7.69 (H-2′) to C-2 and C-4′,
and from a methyl singlet at δH 2.44 to C-2′. All of these
HMBC correlations suggested that a methyl group was linked
to the double bond through a heteroatom. The distinctive
The molecular formula of compound 2 was deduced to be
1
C16H21NO from HRESIMS data. The H NMR spectrum
showed five aromatic protons, which suggested the presence of
the same 4-quinolone nucleus observed for 1; however,
significant differences were observed in the signals associated
with the side chain, which included several signals consistent
with the presence of an alkyl chain. The signal at δH 0.87
showed a 6H doublet, suggesting the presence of two
equivalent methyl groups at the terminus of the alkyl chain.
1
Table 1. H and 13C NMR Data of Quinolones 1−6
a
a
b
a
a
a
1
(DMSO-d6)
2
(CDCl3)
3
(methanol-d4)
4
(methanol-d4)
δH, (J in
5
(methanol-d4)
6 (CDCl3)
δH, (J in
δH, (J in
δH, (J in
no.
δC, type
NH
147.3, C
Hz)
11.34, br
δC, type
NH
Hz)
δC, type
Hz)
δC, type
NH
154.0, C
Hz)
δC, type
NH
154.5, C
δH, (J in Hz)
n/o
δC, type
NH
152.5, C
δH, (J in Hz)
8.74, br
c
1
2
3
4
8.08 (br) NH
155.6, C
n/o
n/o
152.8, C
105.2, CH 6.16, s
177.0, CO
109.5, CH 6.18, s
178.9, CO
107.2, CH 6.26, s
n/o
108.5, CH 6.21, s
n/o
109.8, CH 6.29, s
180.0, CO
109.4, CH 6.18, s
178.7, CO
4a 124.9, C
125.2, C
123.9, C
124.6, C
125.5, C
125.8, C
5
125.2, CH 8.20,dd
(8.1, 1.3)
123.1, CH 7.26,ddd
126.4, CH 8.34, d
(8.1)
124.6, CH 8.25, d
(9.5)
124.7, CH 8.25, d
(7.7)
125.7, CH 8.22, dd (8.2, 0.9) 126.2, CH 8.35, d (8.1)
6
123.5, CH 7.32, t
(7.5)
123.7, CH 7.44, t
(8.1)
123.8, CH 7.44, t
(7.6)
124.8, CH 7.40, ddd (8.5, 7.2, 123.7, CH 7.32, m
1.0)
(9.5, 7.0,
0.8)
7
131.8, CH 7.61,ddd
131.8, CH 7.57,t
(8.4)
132.0, CH 7.73, t
(8.4)
132.2 CH 7.74, m
133.1, CH 7.70, d (8.3)
131.9, CH 7.57, t (7.3)
(8.2, 7.0,
1.6)
8
118.2, CH 7.55,d
(8.2)
117.0, CH 7.28, s
117.6, CH 7.64, d
(8.4)
117.9, CH 7.64, d
(8.4)
119.1, CH 7.60, ddd (8.7, 7.0, 116.9, CH 7.32, m
0.7)
8a 140.6, C
139.9, C
141.7, C
136.6, C
141.7, C
139.6, C
1′
116.9, CH 6.17,d
(15.9)
34.6, CH2 2.62,
31.8, CH2 3.52, d
(7.3)
39.2, CH2 4.12, s
42.7, CH2 2.89,dd (13.9,
4.3); 2.74,dd
34.1, CH2 2.62, s
(13.9, 8.5)
2′
135.8, CH 7.69, d
(15.5)
28.5, CH2 1.72, m
116.4, CH 5.43, m
142.7, C
129.2, C
71.4, CH
3.96, m
28.2, CH2 1.81, m
3′
4′
S
29.3, CH2 1.39, m
27.1, CH2 1.23, m
129.2, CH 7.37, m
128.6, CH 7.39, m
38.2, CH2 1.55, m
31.7, CH2 2.08, m
128.6, CH 5.37, m
13.7, CH3 2.44, s
31.9, CH2 2.17, q
(7.5)
32.7, CH2 1.25−1.40, m
5′
6′
7′
27.8, CH
1.51, m
11.6, CH3 1.11, t
(7.5)
126.9, CH 7.32, m
128.6, CH 7.39, m
129.2, CH 7.37, m
30.4, CH2 1.25−1.40, m
30.1, CH2 1.25−1.40, m
26.5, CH2 1.25−1.40, m
132.4, CH 5.44, m
32.2, CH2 1.98, m
31.8, CH2 1.18-1.41, m
22.5, CH3 0.87, d
(6.6)
14.9, CH3 1.81, s
22.5, CH3 0.87, d
(6.6)
8′
9′
23.5, CH2 1.25−1.40, m
14.1, CH3 0.89, t (6.8)
22.4, CH2 1.18−1.41, m
14.1, CH3 0.88, t (7.0)
a
b
c
1
1
600 MHz for H NMR and 125 MHz for 13C NMR. 400 MHz for H NMR and 100 MHz for 13C NMR. n/o not observed.
B
https://dx.doi.org/10.1021/acs.jnatprod.0c00026
J. Nat. Prod. XXXX, XXX, XXX−XXX

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of Natural Products
pubs.acs.org/jnp
Note
Correlations obtained from the COSY, HSQC, and HMBC
spectra (Figure 1) defined compound 2 as 2-(5-methyl)hexyl-
4-quinolone.
however only weak activity against Staphylococcus aureus and
Staphylococcus epidermidis was observed.
All quinolone side chains reported to date possess a limited
set of structural motifs, all of which consist of saturated or
unsaturated alkyl chains, with the most interesting example
being an unusual cyclopropyl ring-containing example reported
from a plant-derived Pseudomonas strain.10 Thus, the S-methyl
derivative 1 is unprecedented among the quinolones,
representing the first example of a sulfur-containing side
chain at the 2-position. Similarly, the branched chain
quinolones 2, 3, and 2-(6-methylheptyl)-4-quinoline are also
unprecedented as microbial metabolites, although the latter
compound has been reported previously as a plant
metabolite.12 Quinolones with branched-chain alkyl groups at
the 2-position are very uncommon from microbes, with only
two prior reports: a 1998 report describing several
alkylquinones with isoprenoid-derived alkyl side chains13 and
a 2016 report describing the isolation of a pair of branched
chain quinolones from a marine Pseudoalteromonas strain.14
The discovery of these highly unusual quinolones raises
questions about the biosynthesis of quinolones in P. aeruginosa
BD06-03. Recent studies have suggested that the quinolone
HHQ is biosynthesized by the condensation of anthranilic
acid, malonate, and octanoic acid, through a process catalyzed
by the enzymes PqsA−D.15 Compounds 1 and 4 could be
formed from 3-(methylthio)acrylic acid and phenylacetic acid,
respectively, by an analogous process. Both 3-(methylthio)-
acrylic acid16,17 and phenylacetic acid18 have been reported as
natural products previously in a number of microbial strains,
though the former has never been reported from a
Pseudomonas isolate. In order to test this hypothesis, 3-
(methylthio)acrylic acid and phenylacetic acid were synthe-
sized and small-scale precursor incubation studies were carried
out; however, no significant changes in metabolite profile were
observed. This might indicate that a more highly activated
substrate is necessary for incorporation or that an alternative
biosynthetic pathway is used in the synthesis of these
compounds. Future studies will focus on unravelling these
questions.
The 1H NMR spectrum of compound 3 also revealed signals
consistent with a quinolone nucleus, in addition to a single
deshielded multiplet (δH 5.43) suggesting a trisubstituted
double bond. A deshielded methyl singlet peak (δH 1.81) was
also observed, while a quartet and triplet (δH q 2.16, t 1.11)
also indicated the presence of an ethyl group. COSY
correlations between δH 5.43 (H-2′) and 3.51 (H-1′), together
with HMBC correlations from H-1′ to C-2/C-3/C-2/3′, H-4′
to C-2, H-5′ to C-3′, and H-6′ to C-2′/C-3′, linked these
groups to the double bond as shown (Figure 1) and revealed
compound 3 as 2-(E-3-methylpent-2-enyl)-4-quinolone. The
molecular formula C15H17NO, obtained from the HRESIMS
data, supported this assignment.
Compound 4 was also a 4-quinolone-type compound, but
displayed five additional proton resonances in the aromatic
region (δH 7.32−7.38, 5H), consistent with the presence of a
monosubstituted benzene ring. A distinctive 2H singlet at δH
4.12 showed clear correlations to C-2/C-3 in the quinolone
nucleus and to two additional aromatic carbons (δC 128.6,
136.4) (Figure 1), confirming the side chain as a benzyl group.
The molecular formula C15H17NO, obtained from HRESIMS
at m/z 236.0516 [M + H]+, supported this assignment.
Compound 5 also shared a 4-quinolone nucleus. In the
HSQC spectrum, a proton was observed at δH 3.96, correlating
with a deshielded methine carbon (δC 71.4); in combination,
these data suggested the presence of a hydroxylated methine
group. Two diastereotopic protons at δH 2.89 and 2.76 were
attributed to methylene carbon C-1′. 1H−1H COSY data
displayed correlations between the diastereotopic methylene
δH 2.89/2.74 (H-1a′/H-1b′) and the hydroxylated methine,
which also showed COSY correlations to another methylene
group (δH 1.55, H-3′) (Figure 1). HMBC data showed
correlations from H-3 to C-1′ and H-1′ to C-2/C-3/C-3′. The
remaining methylene protons were contained within a
methylene envelope at 1.25−1.40 ppm, suggesting they were
part of a linear alkyl chain. Thus, in combination these data
suggested that a hydroxy group was present at C-2′ of a nine-
carbon linear alkyl chain. The molecular formula C18H25NO2
was obtained from HRESIMS and further supported the
assignment. Therefore, compound 5 was identified as 2-(2′-
hydroxyl)nonyl-4-quinolone. Given the low amount of material
isolated, the absolute configuration of the C-3′stereogenic
center was not determined.
EXPERIMENTAL SECTION
■
General Experimental Procedures. Optical rotations were
measured on a Rudolph Research Analytical Autopol IV polarimeter
(λ 589) using a 700 μL cell with a path length of 1 dm. UV−vis
spectra were obtained on an Agilent 8453 UV−vis spectropho-
tometer. NMR spectra were recorded on a Bruker Avance III 400
spectrometer and a Bruker Avance III 600 spectrometer with
tetramethylsilane as an internal reference. HRESIMS spectra were
obtained from an Agilent 1290 UPLC system with a Bruker
microTOF-Q II mass spectrometer or an Agilent 6230 HPLC system
with a Bruker TOF mass spectrometer. Methanol-d4, chloroform-d,
and dimethyl sulfoxide-d6 (Cambridge Isotope Lab.) were used in the
NMR experiments. HPLC analysis was performed on a Agilent 1260
series HPLC system equipped with a G1311B quaternary pump, a
G1315D photodiode array detector, a G1316C thermostated column
compartment, and a G1329B autosampler and using a reversed-phase
ZORBAX SB-C18 column (3.5 μm, 4.6 × 150 mm; Agilent
Technologies). Semipreparative HPLC separation was carried out
on a Shimadzu LC-20AR series HPLC instrument equipped with a
UV detector and a reversed-phase C18 column (ZORBAX SB-C18, 5
μm, 9.4 × 250 mm). Column chromatography was performed using
45−75 μm silica gel (Qingdaodingkang) and Diaion HP20 macro-
porous resin (Mitsubishi). Thin-layer chromatography (TLC) was
performed on precoated sheets of silica gel 60 F254 (EMD Millipore
1
The H NMR spectrum of compound 6 revealed signals
associated with a 4-quinolone. Two deshielded multiplets at
approximately δH 5.5 showed the existence of a single double
bond, which was not directly connected with the quinolone
nucleus. Sequential COSY correlations (Figure 1) revealed the
double bond was located at the C-4′ position and together
with HSQC correlations showed a total of nine carbons in the
side chain. The two protons at δH 5.37 and 5.44 coupled to
each other (J = 14.4 Hz), showing the existence of a trans
double bond. Thus, compound 6 was identified as 2-((E)-non-
4′-enyl)-4-quinolone. The molecular formula, C18H23NO,
obtained from HRESIMS, supported this assignment. Selected
compounds were tested for cytotoxic activity against the HeLa
cervical cancer cell line; however, none of these displayed
significant activity (Table S1). Preliminary antimicrobial
testing was conducted against 16 microbial test strains;
C
https://dx.doi.org/10.1021/acs.jnatprod.0c00026
J. Nat. Prod. XXXX, XXX, XXX−XXX

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of Natural Products
pubs.acs.org/jnp
Note
Corp.). Reagents and solvents used in this study were commercially
available (Concord Tianjin).
spectra, see Table 1; HRESI(+)MS m/z 509.3123 [2M + Na]+ (calcd
for C32H42N2O2Na, 509.3138).
2-(E-3-Methylpent-2-enyl)-4-quinolone (3): colorless solid; UV−
vis (CH3OH) λmax (log ε) 235 (2.11), 315 (1.59), and 328 (1.54)
nm; 1H NMR (methanol-d4, 600 MHz) and 13C NMR (methanol-d4,
150 MHz) spectra, see Table 1; HRESI(+)MS m/z 228.1380 [M +
H]+ (calcd for C15H18NO, 228.1383).
2-Benzyl-4-quinolone (4): colorless solid; UV−vis (CH3OH) λmax
(log ε) 235 (2.13), 315 (1.42), and 328 (1.37) nm; 1H NMR
(methanol-d4, 600 MHz) and 13C NMR (methanol-d4, 150 MHz)
spectra, see Table 1; HRESI(+)MS m/z 236.1066 [M + H]+ (calcd
for C16H14NO, 236.1070).
Microbial Strains. The microbial strain BD06-03 was isolated
from a soil sample collected from a park in Baoding City (Hebei
Province, China) at GPS coordinates 38.8747 N; 115.5349 E. The
species of the microbial strain BD06-03 were identified by 16S rDNA
sequence analysis as Pseudomonas aeruginosa (Figure S1), and the
sequence was submitted to GenBank under accession number
MT109313. DNA isolation, amplification, and sequence analysis
was carried out using an adaption of existing protocols (Supporting
Information).19 All microbial strains for antimicrobial testing were
obtained from ATCC (Table S2 of the Supporting Information).
Medium Selection and Fermentation. For this microbe,
optimization of culture media was carried out to improve the
production and increase the diversity of secondary metabolites. P.
aeruginosa BD06-03 was initially grown on TSA media and inoculated
into five 250 mL conical flasks containing 50 mL of different liquid
medium including TSB, PDB, NB, ISP4, and CZB separately (Figure
S2). Then the microbe was grown in a shaking incubator, shaking at
200 rpm at 30 °C for 4 days. After 4 days, CH2Cl2 was used to extract
the fermentation broth three times, and then then the extract was
evaporated and concentrated by a rotary evaporator. Using 4 mL of
MeOH, the residue was redissolved and 0.5 mL removed for HPLC
analysis. HPLC analysis was performed with a standard analytical
method using a reversed-phase C18 column (ZORBAX SB-C18, 4.6 ×
150 mm, 3.5 μm; Agilent Technologies, USA).
2-(2-Hydroxynonyl)-4-quinolone (5): colorless solid; [α]D +5.2 (c
0.05, MeOH); UV−vis (CH3OH) λmax (log ε) 234 (2.35), 314
1
(2.03), and 328 (1.91) nm; H NMR (methanol-d4, 600 MHz) and
13C NMR (methanol-d4, 150 MHz) spectra, see Table 1; HRESI-
(+)MS m/z 288.1963 [M + H]+ (calcd for C18H26NO2, 288.1964).
2-(E-Non-4-enyl)-4-quinolone (6): colorless solid; UV−vis
1
(CH3OH) λmax (log ε) 231 (2.03) and 314 (1.05) nm; H NMR
(CDCl3, 600 MHz) and 13C NMR (CDCl3, 150 MHz) spectra, see
Table 1; HRESI(+)MS m/z 561.3457 [2M + Na]+ (calcd for
C36H46N2O2Na, 561.3457).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jnatprod.0c00026.
■
sı
Based on the optimization results for the culture media, ISP4
medium (composed of casein 10 g, KH2PO4 1.0 g, MgSO4 0.49 g,
NaCl 1.0 g, (NH4)2SO4 2.0 g, trace element solution 1.0 mL) was
selected as the most suitable culture medium for maximum
production of secondary metabolites in a large-scale culture. P.
aeruginosa BD06-03 was inoculated into 1 L of ISP4 medium (× 10
flasks) and incubated in a rotary incubator shaker, shaking at 200 rpm
at 30 °C for 7 days.
HPLC data for BD06-03 grown in various media, a
phylogenetic tree showing closely related microbes,
experimental data for known compounds, synthetic
procedures, precursor incubation studies, and NMR
and MS data for all isolated compounds (PDF)
Extraction and Separation. After 7 days of cultivation, all
culture flasks were combined and extracted with equal volumes of
CH2Cl2, EtOAc, and n-butanol (BuOH) three times each (10 L total
each). The solvent was removed under vacuum (Savant SC 210A,
Ameritech) to yield dry solvent extracts: CH2Cl2 extract (1.1 g),
EtOAc extract (0.5 g), and BuOH extract (0.8 g).
The CH2Cl2 extract (1.1 g) was fractionated through HP-20 resin
in an open column, eluting with H2O−MeOH by a stepwise elution
gradient (100:0, 80:20, 60:40, 40:60, 20:80, 0:100). Fractions of 60%
MeOH (Fr.1), 80% MeOH (Fr. 2), and 100% MeOH (Fr.3) were
selected as the primary fractions for the continued separation.
Fr.2 (185 mg) was fractionated again through HP-20 resin open
column chromatography, eluting with H2O−MeOH by a four-step
elution gradient (60:40, 40:60, 20:80, 0:100), to give four subfractions
(Fr.2-1, Fr.2-2, Fr 2-3, and Fr.2-4). Then subfractions Fr.2-2 and Fr.2-
3 were separated by semipreparative HPLC (MeCN−H2O, 15:85,
isocratic elution) to yield compounds 1 (1.0 mg), compound 3 (1.1
mg), and compound 4 (0.8 mg).
Fr.3 (1.4 g) was subjected to silica gel column chromatography,
eluting with CH2Cl2−MeOH (100:0 → 0:100), to afford seven
subfractions (Fr.3-1 to Fr.3-5). Subfractions Fr.3-2, Fr.3-3, and Fr.3-4
were purified by semipreparative HPLC (ZORBAX SB-C18), eluting
with 25% MeCN for 20 min and 38% MeCN for 25 min, then 44%
MeCN for 30 min and then 55% MeCN for 15 min, then 20 min to
100% MeCN for 30 min to yield compound 2 (1.0 mg), compound 5
(0.7 mg), compound 6 (1.8 mg), and a range of known compounds
(Supporting Information)
AUTHOR INFORMATION
Corresponding Author
■
Benjamin R. Clark − School of Pharmaceutical Science and
Technology, Tianjin University, Tianjin 300092, People’s
Republic of China; orcid.org/0000-0001-8987-7092;
Phone: +86-22-87401830; Email: bclark@tju.edu.cn
Authors
Jianye Li − School of Pharmaceutical Science and Technology,
Tianjin University, Tianjin 300092, People’s Republic of China
Weiwei Sun − School of Pharmaceutical Science and Technology,
Tianjin University, Tianjin 300092, People’s Republic of China
Muhammad Saalim − School of Pharmaceutical Science and
Technology, Tianjin University, Tianjin 300092, People’s
Republic of China
Guixiang Wei − School of Pharmaceutical Science and
Technology, Tianjin University, Tianjin 300092, People’s
Republic of China
Diana A. Zaleta-Pinet − School of Pharmaceutical Science and
Technology, Tianjin University, Tianjin 300092, People’s
Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jnatprod.0c00026
2-(E-2-Methylthiovinyl)-4-quinolone (1): colorless solid; UV−vis
(CH3OH) λmax (log ε) 228 (2.38), 288 (2.71), and 328 (1.58) nm;
1H NMR (d6-DMSO, 600 MHz) and 13C NMR (d6-DMSO, 150
MHz) spectra, see Table 1; HRESI(+)MS m/z 240.0457 [M + Na]+
(calcd for C12H11NNaOS, 240.0459).
2-(5-Methylhexyl)-4-quinolone (2): colorless solid; UV−vis
(CH3OH) λmax (log ε) 234 (2.14), 314 (1.32), and 326 (0.89) nm;
1H NMR (CDCl3, 600 MHz) and 13C NMR (CDCl3, 150 MHz)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded in part by the award from the Research
Fund for International Young Scientists (81850410550) from
the National Science Foundation of China.
D
https://dx.doi.org/10.1021/acs.jnatprod.0c00026
J. Nat. Prod. XXXX, XXX, XXX−XXX