Cyclopent-4-ene-1,3-diones: A new class of herbicides acting as potent photosynthesis inhibitors

Article abstract of DOI:10.1021/jf5014605

In a recent paper, we reported the synthesis and photosynthesis-inhibitory activity of a series of analogues of rubrolides. From quantitative structure-activity relationship (QSAR) studies, we found that the most efficient compounds are those having higher ability to accept electrons. On the basis of those findings, we directed our effort to synthesize new analogues bearing a strong electron-withdrawing group (nitro) in the benzylidene ring and evaluate their effects on photosynthesis. However, the employed synthetic approach led to novel cyclopent-4-ene-1,3-diones as major products. Here, we report the synthesis and mechanism of action of such cyclopent-4-ene-1,3-diones as a new class of photosynthesis inhibitors. These compounds block the electron transport at the Q_B level by interacting at the D1 protein at the reducing side of Photosystem II and act as Hill reaction inhibitors, with higher activity than the corresponding rubrolides. To the best of our knowledge, this is the first report on the photosynthesis inhibitory activity of cyclopentenediones.

Full text of DOI:10.1021/jf5014605

Article
pubs.acs.org/JAFC
Cyclopent-4-ene-1,3-diones: A New Class of Herbicides Acting as
Potent Photosynthesis Inhibitors
Jodieh O. S. Varejao,† Luiz C. A. Barbosa,*,†,‡ Eduardo V. V. Varejao,† Celia R. A. Maltha,†
́
̃
̃
Beatriz King-Díaz,§ and Blas Lotina-Hennsen§
†Department of Chemistry, Federal University of Vico
̧
sa, Avenida Peter Henry Rolfs, s/n, 36570-000 Vico
̧ sa, Minas Gerais, Brazil
‡Department of Chemistry, Universidade Federal de Minas Gerais, Avenida Presidente Anto
̂
nio Carlos, 6627, Campus Pampulha,
31270-901 Belo Horizonte, Minas Gerais, Brazil
§Department of Biochemistry, School of Chemistry, National Autonomous University of Mexico, Mexico City 04510, Mexico
S
* Supporting Information
ABSTRACT: In a recent paper, we reported the synthesis and photosynthesis-inhibitory activity of a series of analogues of
rubrolides. From quantitative structure−activity relationship (QSAR) studies, we found that the most efficient compounds are
those having higher ability to accept electrons. On the basis of those findings, we directed our effort to synthesize new analogues
bearing a strong electron-withdrawing group (nitro) in the benzylidene ring and evaluate their effects on photosynthesis.
However, the employed synthetic approach led to novel cyclopent-4-ene-1,3-diones as major products. Here, we report the
synthesis and mechanism of action of such cyclopent-4-ene-1,3-diones as a new class of photosynthesis inhibitors. These
compounds block the electron transport at the QB level by interacting at the D1 protein at the reducing side of Photosystem II
and act as Hill reaction inhibitors, with higher activity than the corresponding rubrolides. To the best of our knowledge, this is
the first report on the photosynthesis inhibitory activity of cyclopentenediones.
KEYWORDS: analogues of rubrolides, cyclopentenediones, chlorophyll a fluorescence, polarography, PSII inhibitors,
Hill reaction inhibitors
nitro electron-withdrawing group at the para position in the
benzylidene ring (Figures 1, compounds 2a and 2b) showed the
higher activities, with IC50 1 order of magnitude higher than
that of commercial herbicides Diuron. In the same study,
quantitative structure−activity relationship (QSAR) analyses
indicated that the most efficient compounds are those having
higher ability to accept electrons.
On the basis of these findings, we envisaged that rubrolide
analogues bearing electron-withdrawing groups, such as nitro,
for example, on the meta position of the benzylidene ring
(Figure 1, compounds 3a and 3b), would be active as
photosynthesis inhibitors. Because a nitro group acts as an
electron-withdrawing group, its presence reduces the redox
potential of the compound, making the molecule more able to
accept an electron and interfere with the photosynthetic
electron transport.23 We then tried to prepare compounds 3a
and 3b; however, this attempt led to unexpected and novel
compounds, identified as 4-bromo-5-(2-methoxyphenyl)-2-(3-
nitrophenyl)cyclopent-4-ene-1,3-dione (4a) and 4-bromo-5-(4-
fluoro-2-methoxyphenyl)-2-(3-nitrophenyl) cyclopent-4-ene-
1,3-dione (4b) (Figure 1), as major products. These
compounds were assayed for their inhibitory activity on the
photosynthetic electron transport chain and showed high
INTRODUCTION
■
Weed infestations constitute a major cause of crop losses
worldwide, making agricultural yields highly dependent upon
the management of such invasive plants.1,2 Since the discovery
of the herbicidal property of 2,4-dichlorophenoxyacetic acid
(2,4-D) in the mid-1940s,3 chemical control has been the most
effective method for weed management. However, the
continuous and repetitive use of herbicides has exerted selective
pressure, leading to the replacement of sensitive by herbicide-
resistant biotypes.4−7 This problem has been worsened by the
wide adoption of herbicide-resistant crops, which has
contributed to an indiscriminate use of broad activity spectrum
herbicides.8−10 As a whole, this scenario has made the
development of new herbicides a great concern and an
important area of activity in industry and academy.
The use of phytotoxic natural products as models for the
development of novel active molecules have been regarded as a
promising strategy in the search for new herbicides.11−14
Examples of the potential of such an approach are the
commercial herbicides cinmethylin and mesotrione, derivatives
from phytotoxic plant metabolites, and bialafos, synthesized
from a phytotoxin isolated from species of the genera
Streptomyces.13,15 In line with such tendency, our research
group have been using several natural compounds as models for
the development of new bioactive molecules.16−22
In a recent paper, we reported the synthesis and photosyn-
thesis-inhibitory activity of a series of analogues of rubrolides
(Figure 1), a family of natural butenolides isolated from marine
ascidians.23 In that work, we found that derivatives bearing a
Received: March 31, 2014
Revised: June 3, 2014
Accepted: June 9, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Figure 1. Rubrolide N, rubrolide analogues, and derived ciclopent-4-ene-1,3-diones.
residue. FTIR (KBr) νmax, cm−1: 3068, 2923, 2847, 1771, 1606, 1531,
1491, 1281, 988, 750. 1H NMR, δH (300 MHz, CDCl3): 3.86 (s, 3H),
5.98 (s, 1H), 7.08−7.16 (m, 2H), 7.27 (dd, 1H, J = 8.0 and 1.5 Hz),
7.52−7.60 (m, 2H), 8.14−8.17 (m, 1H), 8.22 (brd, 1H, J = 8.1 Hz),
8.39−8.40 (m, 1H). 13C NMR δC (75 MHz, CDCl3): 55.6, 110.3,
111.7, 113.7, 117.1, 120.8, 123.4, 125.0, 129.9, 130.2, 132.3, 134.3,
135.7, 148.4, 149.4, 152.2, 156.5, 164.5. EM, m/z (%): 403 (98) [M +
2]+ •, 401 (100) (C18H12BrNO5) [M+ •], 322 (46), 294 (50), 281
(20), 279 (21), 277 (20), 219 (25), 205 (20), 189 (21), 176 (26), 163
(58), 131 (65), 119 (36), 115 (36), 103 (48), 102 (26), 95 (36), 91
(21), 89 (64), 88 (64), 77 (70), 63 (36), 51 (25), 39 (48).
4-Bromo-5-(2-methoxyphenyl)-2-(3-nitrophenyl)cyclopent-
4-ene-1,3-dione (4a). The compound was a yellow amorphous
residue. FTIR (NaCl, film) νmax, cm−1: 3080, 2955, 2923, 2852, 1750,
1709, 1604, 1529, 1486, 1463, 1347, 1265, 1144, 1021, 735, 732, 691.
1H NMR, δH (300 MHz, CDCl3): 3.89 (s, 3H), 4.38 (s, 1H), 7.05−
7.14 (m, 2H), 7.37 (dd, 1H, J = 7.8 and 1.8 Hz), 7.50−7.58 (m, 1H),
7.60 (brd, 1H, J = 7.8 Hz), 7.67 (brd, 1H, J = 7.8 Hz), 8.16−8.17 (m,
1H), 8.22 (dt, 1H, J = 8.1 and 1.5 Hz). 13C NMR, δC (75 MHz,
CDCl3): 55.8, 56.1, 111.6, 117.1, 120.7, 123.3, 123.4, 130.0, 132.8,
133.7, 135.3, 145.1, 148.5, 156.8, 156.9, 191.8, 194.1. EM, m/z (%):
403 (38) [M + 2]+ •, 401 (38) (C18H12BrNO5) [M+ •], 322 (28), 294
(28), 219 (22), 150 (41), 149 (20), 135 (21), 131 (84), 124 (31), 119
(36), 115 (34), 109 (23), 103 (39), 102 (19), 97 (23), 95 (27), 94
(24), 91 (21), 89 (61), 88 (79), 86 (58), 84 (87), 77 (100), 76 (26),
75 (31), 71 (20), 63 (49), 61 (28), 57 (34), 51 (81), 50 (24). High-
resolution mass spectrometry (HRMS) (ESI) calculated for
[C18H12BrNNaO5]+, 423.9791; found, 423.9787.
activities. Their synthesis and photosynthesis-inhibitory activ-
ities are presented herein.
MATERIALS AND METHODS
■
General Experimental Procedures. All reagents and solvents
were prepared following the procedure already reported in the
literature24 or were purchased from commercially available suppliers
and used without any further purification. Compounds 5a and 5b were
prepared employing a synthetic procedure previously reported.23
Melting points are uncorrected and were obtained from a MQAPF-
301 melting point apparatus (Microquimica, Brazil). Analytical thin-
layer chromatography analysis was conducted on aluminum-packed
pre-coated silica gel plates. Column chromatography was performed
over silica gel 230−400 mesh. All compounds were fully characterized
by infrared (IR), Electron ionization−mass spectrometry (EI−MS),
1H and 13C nuclear magnetic resonance (NMR), correlation
spectroscopy (COSY), heteronuclear correlation (HETCOR), and
nuclear Overhauser effect difference (NOEDIFF) NMR spectroscopy.
IR spectra were recorded on a PerkinElmer Paragon 1000 fourier
Transform infrared (FTIR) spectrophotometer using potassium
bromide (1%, w/w) disks or as a thin liquid film on NaCl plates.
Mass spectra were recorded on a Shimadzu GCMS-QP5050A
instrument by direct insertion, using EI mode (70 eV). High-
resolution mass spectra were recorded on a Bruker MicroToF
[resolution = 10 000 full width at half maximum (fwhm)] under
electrospray ionization (ESI) and are given to four decimal places. The
1H and 13C NMR spectra were recorded on a Varian Mercury 300
spectrometer at 300 and 75 MHz, respectively, using CDCl3 as the
solvent and tetramethylsilane (TMS) as the internal reference, unless
otherwise stated. The energy gap between highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
of the compounds was calculated with MOPAC25 software using the
AM1 method.26,27
Synthesis of Compound 6. Compound 5b (100 mg, 0.37 mmol),
3-nitrobenzaldehyde (67 mg, 0.44 mmol), dichloromethane (3 mL),
TBDMSOTf (101 μL, 0.44 mmol), and DIPEA (130 μL, 0.74 mmol)
were added to a two-necked round-bottom flask (25 mL), under a
nitrogen atmosphere. The resulting mixture was stirred at room
temperature for 1 h. The resulting organic phase was washed with 3
mol L−1 HCl (2 × 25 mL) and brine (2 × 25 mL). The combined
organic layers were dried over magnesium sulfate, filtered, and
concentrated under reduced pressure. The resulting residue was
purified by column chromatography on silica gel eluted with hexane/
dichlorometane (1:1, v/v). Compound 6 was obtained with 80% yield.
3-Bromo-5-((tert-butyldimethylsilyloxy)(3-nitrophenyl)-
methyl)-4-(4-fluoro-2-methoxyphenyl)furan-2(5H)-one (6). The
compound was a white solid. Melting point (mp): 161.2−162.8 °C.
FTIR (KBr) νmax, cm−1: 2957, 2929, 2856, 2855, 1763, 1609, 1528,
Synthesis of Compounds 3a and 4a. In a two-necked round
bottomed flask (25 mL) under a nitrogen atmosphere were added 3-
bromo-4-(2-methoxyphenyl)furan-2(5H)-one (5a) (100 mg, 0.37
mmol), 3-nitrobenzaldehyde (67 mg, 0.44 mmol), dichloromethane
(3 mL), tert-butyldimethylsilyl trifluoromethanesulfonate
(TBDMSOTf) (101 μL, 0.44 mmol), and N,N-diisopropylethylamine
(DIPEA) (130 μL, 0.74 mmol). The resulting mixture was stirred at
room temperature for 1 h. After this period, 1,8-diazabicyclo[5.4.0]-
undec-7-ene (DBU) (110 μL, 0.74 mmol) was added, and the
resultant mixture was refluxed for 1 h before the addition of
dichloromethane (70 mL). The organic phase was washed with 3 mol
1
1500, 1469, 1353, 1286, 1127, 1020, 837, 781. H NMR, δH (300
MHz, CDCl3): −0.38 (s, 3H), −0.28 (s, 3H), 0.78 (s, 9H), 3.92 (s,
3H), 4.84 (d, 1H, J = 2.4 Hz), 5.83 (d, 1H, J = 2.4 Hz), 6.72 (dd, 1H, J
= 2.4 and 10.5 Hz), 6.80−6.86 (m, 1H), 7.50−7.54 (m, 1H), 7.66−
7.74 (m, 2H), 8.03−8.05 (m, 1H), 8.13−8.16 (m, 1H). 13C NMR, δC
(75 MHz, CDCl3): −5.7, −4.6, 17.8, 25.4, 55.9, 73.0, 86.1, 100.0 (d, J
= 26.0 Hz), 108.0 (d, J = 21.7 Hz), 110.3, 114.7 (d, J = 3.3 Hz), 121.5,
123.3, 129.3, 132.3 (d, J = 10.4 Hz), 133.4, 141.9, 147.8, 156.1, 158.3
(d, J = 10.3 Hz), 165.2 (d, J = 251.9 Hz), 168.2. Electron microscopy
(EM), m/z (%) (molecular ion does not appear): 496 (22), 494 (21),
415 (15), 345 (13), 343 (13), 342 (13), 267 (10), 266 (48), 264 (15),
150 (12), 115 (20), 75 (20), 73 (100), 59 (10), 45 (11).
L
−1 HCl (2 × 25 mL) and brine (2 × 25 mL). The combined organic
layers were dried over magnesium sulfate, filtered, and concentrated
under reduced pressure in a rotary evaporator. The residue obtained
was purified by column chromatography on silica gel eluted with
hexane/ethyl acetate (3:1, v/v), affording compound 4a with a 42%
yield. Compound 3a was isolated in a 30% yield, but although
observed as a unique peak in gas chromatography−mass spectrometry
(GC−MS) analysis, it was unstable in CDCl3.
(Z)-3-Bromo-4-(2-methoxyphenyl)-5-(3-nitrobenzylidene)-
furan-2(5H)-one (3a). The compound was a yellow amorphous
B
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Scheme 1. Synthesis of Compounds 3a and 4a
Synthesis of Compounds 3b and 4b. Compound 6 (100 mg,
0.18 mmol), dichloromethane (3 mL), and DBU (49 μL, 0.36 mmol)
were added to a two-necked round-bottom flask (25 mL), under a
nitrogen atmosphere. The resultant mixture was refluxed for 1 h.
Dichloromethane (70 mL) was added, and the organic phase was
washed with 3 mol L−1 HCl (2 × 25 mL) and brine (2 × 25 mL). The
organic layer was dried over magnesium sulfate, filtered, and
concentrated under reduced pressure. The residue obtained was
purified by column chromatography on silica gel eluted with hexane/
ethyl acetate (3:1, v/v) to afford compounds 3b and 4b in 35 and 49%
yields, respectively.
(Z)-3-Bromo-4-(4-fluoro-2-methoxyphenyl)-5-(3-
nitrobenzylidene)furan-2(5H)-one (3b). The compound was a
yellow solid. mp: 156.1−158.6 °C. FTIR (KBr) νmax, cm−1: 3033,
2997, 2967, 2935, 1769, 1617, 1597, 1527, 1499, 1347, 1281, 987, 839.
1H NMR, δH (300 MHz, CDCl3): 3.86 (s, 3H), 5.96 (s, 1H), 6.80−
Chl and Fluorescence Determinations. Chl a fluorescence
transients of Photosystem II (PSII) were measured in thylakoids with a
Handy plant efficient analyzer (PEA, Hansatech, King’s Lynn, Norfolk,
U.K.) as previously described.31,32 The maximum fluorescence yield
from the sample was generated by illumination for 2 s with continuous
light (650 nm peak wavelength, intensity equivalent of 2830 μmol of
photons m−2 s−1, and gain of 0.7) provided by an array of three light-
emitting diodes. The reaction medium set was that employed in basal
non-cyclic electron transport measurements without methyl viologen
(MV). To monitor Chl and fluorescence transients, aliquots of dark-
adapted thylakoids for 5 min containing 60 μg of Chl were transferred
to a filter paper by gravity with a dot-blot apparatus (Bio-Rad,
Hercules, CA) to ensure a homogeneous and reproducible distribution
of thylakoids. The filter paper was dipped immediately in 3 mL of the
medium with different concentrations of the tested compounds, and
the control contained medium plus the amount of dimethyl sulfoxide
(DMSO) employed for dissolving each compound at the different
used concentrations. Chloroplasts infiltrated with 50 μM 3-(3,4-
dichlorophenyl)-1,1-dimethylurea (DCMU) were used as a positive
control.
Measurement of Adenosine Triphosphate (ATP) Synthesis.
To measure ATP synthesis, intact chloroplasts (20 μg/mL of Chl)
were broken by osmotic rupture in non-buffered solution with 100
mM sorbitol, 10 mM KCl, 5 mM MgCl2, 0.5 mM KCN, 50 μM MV
(MV was used as an electron acceptor), 1 mM Na+−tricine (pH 8.0),
and 1 mM ADP (pH 6.7). The pH was adjusted to 8.0 with 50 mM
KOH, and the mixture was illuminated for 1 min. The synthesized
ATP was calculated as micromoles of ATP per milligram of Chl per
hour.29,33
Measurement of the Rate of Non-cyclic Electron Transport.
Electron flow (basal, phosphorylating, and uncoupled) from water to
MV was monitored with a Yellow Springs Instrument (YSI) oxygraph
using a Clark-type electrode.29,34 Light-induced non-cyclic electron
transport determinations were performed in the presence of 50 μM
MV. Chloroplasts were efficiently lysed to yield free thylakoids prior to
each experiment by incubating them in the following basal electron
transport medium: 100 mM sorbitol, 10 mM KCl, 5 mM MgCl2, 0.5
mM KCN, and 30 mM N-tris[hydroxymethyl]methylglicine (tricine)
buffer (pH 8.0 with the addition of KOH). A total of 20 μg of Chl/mL
of medium was illuminated for 1 min. Phosphorylating non-cyclic
electron transport was measured as the basal non-cyclic electron
transport medium in the addition of 1 mM ADP and 3 mM KH2PO4.
Uncoupled electron transport was tested in the basal transport
medium by adding 6 mM NH4Cl as the uncoupler agent.
Uncoupled PSII and Photosystem I (PSI) Electron Flow
Measurements. These experiments were performed in an uncoupled
non-cyclic electron transport assay. The activities were monitored with
a YSI oxygen monitor, model 5300A, using a Clark-type electrode.
Uncoupled PSII from water to 2,5-dichloro-1,4-benzoquinone
(DCBQ)35 was monitored using the basal electron transport medium,
and 1 μM 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone
(DBMIB), 100 μM DCBQ, and 6 mM NH4Cl were added. MV was
omitted. DCBQ accepts electrons at the D1 protein. Partial reaction of
PSII electron transport from water to sodium silicomolybdate (SiMo)
was determined as in PSII with the same basal electron transport
medium without MV, and 50 μM SiMo and 10 μM DCMU were
added.35 These electron flow activities were monitored with a YSI
oxygen monitor, model 5300, using a Clark-type electrode.
6.89 (m, 2H), 7.23−7.28 (m, 1H), 7.58 (t, 1H, J = 8.1 Hz), 8.15−8.19
(m, 1H), 8.22 (d, 1H, J = 8.1 Hz), 8.39−8.40 (m, 1H). 13C NMR, δC
(75 MHz, CDCl3): 56.0, 100.3 (d, J = 25.9 Hz), 107.8 (d, J = 22.0
Hz), 110.3, 112.6, 113.1 (d, J = 3.3 Hz), 123.6, 125.1, 129.9, 131.4 (d, J
= 10.6 Hz), 134.2, 135.8, 148.4, 149.3, 151.2, 158.1 (d, J = 10.4 Hz),
164.3, 165.2 (d, J = 249.9 Hz). EM, m/z (%): 421 (99) [M + 2]+ •
,
419 (99) (C18H11BrFNO5) [M+ •], 340 (30), 312 (30), 238 (25), 237
(36), 230 (23), 228 (23), 223 (23), 181 (65), 177 (27), 153 (27), 149
(100), 137 (28), 133 (37), 118 (20), 106 (43), 101 (93), 89 (56), 39
(38). HRMS (ESI) calculated for [C18H11BrFNNaO5]+, 441.9697;
found, 441.9700.
4-Bromo-5-(4-fluoro-2-methoxyphenyl)-2-(3-nitrophenyl)-
cyclopent-4-ene-1,3-dione (4b). The compound was a yellow
solid. mp: 152.1−153.4 °C. FTIR (NaCl, film) νmax, cm−1: 3095, 2977,
2945, 2895, 1752, 1708, 1608, 1526, 1492, 1348, 1279, 1193, 1104,
1028, 952, 842, 808, 725, 670, 595, 488, 447. 1H NMR, δH (300 MHz,
CDCl3): 3.88 (s, 3H), 4.37 (s, 1H), 6.77−6.86 (m, 2H), 7.36 (dd, 1H,
J = 8.4 and 6.3 Hz), 7.57−7.62 (m, 1H), 7.64−7.68 (m, 1H), 8.14−
8.15 (m, 1H), 8.22 (ddd, 1H, J = 8.1, 2.4, and 1.5 Hz). 13C NMR, δC
(75 MHz, CDCl3): 56.0, 56.1, 100.2 (d, J = 26.2 Hz), 107.9 (d, J =
22.5 Hz), 113.1 (d, J = 3.1 Hz), 123.3, 130.0, 131.5 (d, J = 10.5 Hz),
133.6, 135.1, 145.0, 148.5, 158.0, 158.6 (d, J = 11.3 Hz), 165.7 (d, J =
251.0 Hz), 191.6, 194.0. EM, m/z (%): 421 (40) [M+ • + 2], 419 (40)
(C18H11BrFNO5) [M+ •], 181 (29), 153 (20), 149 (78), 133 (42), 118
(22), 101 (76), 89 (100), 75 (23), 74 (12), 63 (90), 62 (31), 51 (28),
50 (24), 39 (64), 30 (69). HRMS (ESI) calculated for
[C18H11BrFNNaO5]+, 441.9697; found, 441.9692.
Study of the Conversion of Rubrolide Analogues to
Cyclopentenediones. To verify the hypothesis that cyclopent-4-
ene-1,3-dione was formed through rearrangement of the correspond-
ing rubrolide analogues, we have tried to obtain compound 4b by
treating compound 3b with DBU (2.0 equiv) in dichloromethane
under reflux for 1 h. The resulting mixture was worked-up as described
for the synthesis from compound 6. Compound 4b was obtained with
50% yield, and 50% of compound 3b was recovered.
Chloroplast Isolation and Total Chlorophyll (Chl) Determi-
nation. Intact chloroplasts were obtained from market spinach leaves
(Spinacea oleraceae L.) as published28,29 by homogenization and
differential centrifugation, and they were resuspended in a small
volume of the following medium: 400 mM sucrose, 5 mM MgCl2, 10
mM KCl, and 30 mM tricine−KOH (pH 8.0). The Chl concentration
was determined as reported.29,30
C
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Scheme 2. Synthesis of Compounds 3b and 4b
applied for the preparation of other rubrolide analogues, and no
rearranged products were reported.23,36,37
RESULTS AND DISCUSSION
■
Synthesis. In attempting to synthesize rubrolide analogues
bearing a nitro group at the meta position (3a and 3b), we
Base-catalyzed rearrangement of alkylidenebutenolides to the
isomeric cyclopentenediones has already been documented in
the literature,38−40 and this approach has been used for the
synthesis of a range of natural cyclopent-4-ene-1,3-diones and
derivatives.40,41 Commonly, this procedure is carried out by
treating butenolide with sodium methoxy in methanol. The
suggested mechanism involves the electrophilic attack to
carbon of the carbonyl group of benzylidene, which results in
the opening of the ring, followed by a new cyclization to the
cyclopentenedione ring.40
To verify if cyclopentenedione 4b was in fact formed by
means of a rearrangement of rubrolide 3b, instead of being
produced directly from the sililated intermediate, the former
was reacted with DBU, in the same reaction conditions,
affording compound 4b with 50% yield (Scheme 2). Despite
the strong Lewis basic property of DBU, it is not a good
nucleophile and unlikely to be responsible for the opening of
the butenolide ring.42 A good example is the synthetic
procedure reported by Tan and co-workers41 for the synthesis
of the natural cyclopentenedione known as methyl linderone.
In this approach, the reaction of a precursor butenolide with
sodium methoxy led to the desired cyclopent-4-ene-1,3-dione.
However, treatment of the same precursor with DBU resulted
in a basic catalyzed elimination reaction but not to the
conversion of butenolide to cyclopentenedione. When the
product of this elimination was reacted with NaOMe,
cyclopentenedione was obtained with 71% yield.41
Figure 2. Fluorescence rise kinetics of freshly lysed broken
□
○
chloroplasts infiltrated with ( ) compound 4a and ( ) compound
■
4b and compared to the ( ) control chloroplasts. Chl a fluorescence
induction curves were measured at room temperature. Details are in
the Materials and Methods. Data are averages of three replicates.
employed the same approach used for the preparation of
rubrolide analogues 2a and 2b.23 Briefly, lactone 5a (Scheme 1)
was treated with tert-butyldimethylsilyl trifluoromethanesulfo-
nate and N,N-diisopropylethylamine to yield the corresponding
tert-butyldimethylsilyl intermediate, which was reacted in situ
with 3-nitrobenzaldehyde. The silylated adducts were treated
with DBU to afford the desired rubrolide analogue 3a (30%
yield). However, cyclopent-4-ene-1,3-dione 4a was isolated as a
major product (42% yield). This result was surprising and
unexpected because this methodology has been previously
Thus, in view of the poor nucleophilic property of DBU, we
supposed that the rearrangement of the rubrolide analogues to
cyclopent-4-ene-1,3-diones could be caused by the presence of
traces of water in DBU. In fact, Cota et al.42 demonstrated that
the DBU−water system acts as a basic catalyst for aldol
D
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Table 1. Experimental Average Values Plus Standard Deviation of Parameters Obtained from Measurements of the Chl a
Fluorescence Induction Curve in Control and Treated Chloroplasts with Different Concentrations of Compounds 4a and 4b
a
compound
control
F0
F1
F2
F3
F4
F5
FM
area
149 4.0
159 2.5
167 5.7
208 6.1
368 14.5
385 14.1
741 15.3
29234 503
[4a] (μM)
25
153 7.6
132 7.8
158 7.8
142 8.9
168 8.1
154 4.8
224 13.2
209 5.5
423 21.0
403 17.3
525 18.2
481 16.4
694 34.2
584 11.3
59,683 12,230
16,000 529
50
[4b] (μM)
12.5
149 4.7
161 4.7
182 1.4
199 4.2
170 4.6
226 6.6
260 1.4
277 12.0
418 8.4
478 2.8
505 24.0
518 12.2
580 4.2
592 24.0
694 8.5
709 7.1
661 15.6
32,000 16,116
33100 24,749
7600 848
25.0
167
0
193
0
50.0
185 4 0.2
213 11.3
50 μM
DCMU
control
[4a] (μM)
205
5
224 3.01
295 5.7
247 6.1
317 5.5
346 11.5
422 5.9
616 18.0
665 7.5
716 22.5
912 27.0
723 22
1333 115
269 4.8
301 9.9
312 13.8
1273 23
140,960 19,963
100
[4b] (μM)
100
335 13.4
364 16.3
368 19.4
500 21.2
485 33.8
851 14.1
801 55.7
1003 0.7
964 32.5
1120 44
947 71.1
14,600 656
2067 808
341 16.1
a
DCMU was used as a positive control.
□
■
○
△
Figure 3. (A) OJIP curves normalized from F1 to FM at ( ) 25 μM, ( ) 50 μM, and ( ) 100 μM compound 4a in comparison to the ( ) control
●
and ( ) 50 μM DCMU. (B) Calculated J band near 2 ms, as a result of the difference between the normalized treated and controls of the Chl a
transients.
□
■
○
△
Figure 4. (A) OJIP curves normalized from F1 to FM at ( ) 12.5 μM, ( ) 25 μM, and ( ) 50 μM of compound 4b in comparison to the ( )
●
control and ( ) 50 μM DCMU. (B) J band near 2 ms, as a result of the difference between the normalized treated and controls of Chl a transients.
condensation reactions. By means of NMR studies, the authors
showed that the reaction with water changes the DBU structure
to a second resonance structure, which contains hydroxide ions
responsible for the nucleophilic attack in the aldol condensation
(see Scheme S1 of the Supporting Information).42
The presence of water in DBU used was confirmed by IR
spectroscopy. Thus, we suppose that the water−DBU system is
responsible for the rearrangement of rubrolide to cyclo-
pentenedione. Also, this rearrangement can be facilitated by
the presence of the electron-withdrawing NO2 group in the
benzylidene ring, which makes the lactone carbonyl more
E
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Table 2. Derived Parameters, Their Description, and Their Formula Using Data Extracted from the Chl a Fluorescence (OJIP)
Transient Presented in This Work
fluorescence parameters derived from the extracted data
M0 = 4(F300 μs − F0)/(FM − F0)
Sm = (area)/(FM − F0)
approximated initial slope (in ms−1) of the fluorescence transient V = f(t)
normalized total complementary area above the OJIP transient (reflecting multiple turnover QA reduction
events)
yields or flux ratios
φPo = TR0/ABS = [1 − F0/FM]
φEo = ET0/ABS = [1 − FJ/FM)]
ψEo = ET0/TR0 = (1 − VJ)
φDo = 1 − φPo = (F0/FM)
maximum quantum yield of primary photochemistry at t = 0
quantum yield for electron transport at t = 0
−
probability (at t = 0) that a trapped exciton moves an electron into the electron transport chain beyond QA
quantum yield (at t = 0) of energy dissipation
δR0 = RE0/ET0 = (1 − VI)/(1 − VJ)
efficiency with which an electron can move from the reduced intersystem electron acceptors to the PSI end
of electron acceptors of PSI
φR0 = RE0/ABS = φPo*ψEo*δR0 = 1 − (FI/FM) quantum yield for the reduction of end of acceptors of PSI per photon absorbed
RE0/TR0 = ψEoδR0
efficiency with which a trapped exciton moves an electron into the electron transport chain from QA− to the
PSI end of electron acceptors
−
specific energy fluxes or activities [per reaction QA reducing the PSII reaction center (RC)]
ABS/RC = M0(1/VJ)(1/φPo)
TR0/RC = M0/VJ
absorption per RC
trapped energy flux per RC (at t = 0)
electron transport flux per RC (at t = 0)
ET0/RC = M0(1/VJ)(ψEo)
phenomenological energy fluxes [per excited cross-section (CS)]
ABS/CS0
absorption flux per CS
TR0/CS0
trapped energy flux per CS (at t = 0)
electron transport flux per CS (at t = 0)
ET0/CS0
de-excitation rate constants
Kp
photochemical de-excitation rate constant
Kn
non-photochemical de-excitation rate constant
sum of photochemical and non-photochemical rate constants
sum K
performance index at t = 0
performance index on absorption basis
φPo
ψEo
RC
PIABS
=
ABS 1 − φPo 1 − ψEo
monitored by thin-layer chromatography (TLC), we observed a
degradation of the starting material and no signs of the
corresponding cyclopent-4-ene-1,3-dione. We suppose then
that the reason for this degradation could be attributed to the
higher electron-withdrawing effect of the nitro group in the
para position, which, by means of the resonance effect, makes
the lactone carbonyl even more susceptible to nucleophilic
attack, resulting in different products. The formation of acyclic
products from some other butenolides from the treatment with
a strong base has already been reported.40
Chl and Fluorescence Transient Measurements in the
Presence of Compounds 3b, 4a, and 4b. Chl a
fluorescence measurements on photosynthetic organisms
under different conditions, including stress, can be analyzed
to provide detailed information about the structure, con-
formation, and function of the photosynthetic apparatus,
especially of PSII and electron transport activity.32 On this
premise, different concentrations of compounds 3b, 4a, and 4b
were assayed on Chl a fluorescence of freshly lysed spinach
chloroplasts. Compound 3a was not tested because of
Figure 5. Radar graph showing the effects of compound 4a (dotted
line) and compound 4b (black line) at 50 μM in comparison to 50 μM
DCMU (gray line) on the derived parameters calculated from the
OJIP transients of Chl a presented by dark-adapted freshly lysed
chloroplasts. The control is represented by a dashed black line at the
center of the radar and is equal to the unity.
1
degradation, as assessed by H NMR experiments. Figure 2
shows the effects of compounds 4a and 4b at 100 μM on the
induction curve of fluorescence of Chl a. Compound 3b did not
show any modification in the transients (data not shown).
OJIP transients were analyzed according to the JIP test, and
the measured parameters are presented in Table 1: fluorescence
intensity level (F0) when plastoquinone electron acceptor pool,
QA, is fully oxidized and fluorescence of the Chl a level when
QA is transiently fully reduced (FM). F1 is the fluorescence Chl a
electrophilic and susceptible to a nucleophilic attack. This
supposition led us to speculate why cyclopentenedione was not
isolated in our previous work23 as a co-product of the reaction
that afforded compounds 2a and 2b. Then, we decided to treat
compound 2b with the same DBU used in the present work,
under the same reaction conditions. As the reaction was
F
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
■
●
▲
Figure 6. Effect of compound 4a on electron transport flow: ( ) basal, ( ) phosphorylating, and ( ) uncoupled conditions. Control values in =
100% = 450, 880, and 1200 μequiv of e− mg−1 of Chl h−1, respectively. Each curve is the average of three replicates. Other conditions are indicated in
the Materials and Methods.
parameters are shown as a radar plot in Figure 5 (the effects at
other concentrations are not shown for graph clarity). The
normalized area (Sm), the electron transport per reaction
center and per cross-section, and the performance index on an
absorption basis were decreased by the compounds, while M0
and φDo were increased by both compounds 4a and 4b.
However, ABS/CS0 and TR0/CS0 were increased only by
compound 4b and DCMU. These results suggest that
compounds 4a and 4b inhibit the photosynthetic electron
transport in the same site as herbicide DCMU, with compound
4b being more potent than compound 4a.
Effect of Cyclopent-4-ene-1,3-diones 4a and 4b on
the Electron Transport Rate from Water to MV. To
corroborate the site and mechanisms of action of cyclo-
■
●
pentenediones on electron transport flow, different concen-
trations of compounds 4a and 4b were assayed by polarography
on the electron transport under basal, phosphorylating, and
uncoupled conditions. Because compound 3b did not modify
the Chl a fluorescence measurements, it was not subjected to
this test. Panels A and B of Figure 6 show that all three
conditions of the electron transport flow were inhibited by both
compounds. The IC50 values (i.e., the concentration required to
inhibit 50% of the activity) for the uncoupled electron flow
were 9.5 and 8.4 μM for compounds 4a and 4b, respectively.
These results indicate that both compounds act as Hill reaction
inhibitors. DCMU inhibits both oxygen evolution and ATP
synthesis at 5 μM;44 therefore, compounds 4a and 4b have
similar potency as an inhibitor to the PSII reducing side, just
like DCMU.
To localize the target of compounds 4a and 4b on the
thylakoid electron transport chain, effects on partial reactions of
PSII and PSI electron transport were determined using
appropriate artificial electron donors, electron acceptors, and
electron transport inhibitors.31,45 Figure 7 shows that
compounds 4a and 4b inhibited PSII measured from water to
DCBQ, with I50 values of 50 and 44 μM, respectively.
Contrarily, these compounds showed no effects on PSI (data
not shown). Furthermore, the activity of H2O to SiMo was not
affected by these compounds, indicating that the interaction
and inhibition site of both compounds 4a and 4b are located in
the PSII electron transport chain. These data corroborate the
results obtained from measurements of the Chl a fluorescence
transient.
Figure 7. Effects of ( ) compound 4a and ( ) compound 4b on
uncoupled electron transport flow of PSII measured from water to
DCBQ. Control values = 100% = 300 μequiv of e− mg−1 of Chl h−1,
respectively. Each curve is the average of three replicates.
intensity level at 0.05 ms; F2 is the fluorescence Chl a intensity
level at 0.1 ms; F3 is the fluorescence Chl a intensity level at 0.3
ms; F4 or FJ is the fluorescence Chl a intensity level at 2 ms;
and F5 is the fluorescence Chl a intensity level at 30 ms. The
area is the area over the curve between F0 and FM and relates
the pool size of PSII electron transport acceptors.43,45
The OJIP data were normalized using the equation Vt = (Ft
− F0)/(FM − F0) and are presented as the kinetics of relative
variable fluorescence between F0 and FM (Figures 3A and 4A).
The difference between the normalized treated transients minus
the control transients showed an increase in the 2−4 ms range,
a J band (Figures 3B and 4B). The fluorescence rise to the J
band provides information about single-turnover events of the
primary reactions of the photochemistry, mainly QA reduction.
−
This reflects light-driven accumulation of QA with QB,
indicating that the electron transport beyond QA is blocked at
the QB level, just like the commercial herbicide DCMU. These
finds suggest that both compounds (4a and 4b) interact at the
D1 protein site and the unused energy is dissipated as heat.32
Using the program Biolyzer HP3 (from a laboratory at the
University of Geneva; available at http://www.unige.ch/
sciences/biologie/bioen), data from Table 1 were used to
calculate other parameters related to the electron transport
activity, such as the yield ratios φPo, ψEo, φR0, and RE0/TR0.32
These derived parameters are depicted in Table 2, and the
effects of compounds 4a and 4b and DCMU at 50 μM on these
The higher activity of compound 4b when compared to
compound 4a may be related to the presence of the fluorine
G
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
Funding
substituent at the benzene ring of the first. Because of its high
electronegativity, fluorine acts as an electron-withdrawing group
and contributes to reduce the redox potential of compound 4b
relative to compound 4a, thus enhancing the ability of
compound 4b to act as electron acceptors and inhibit the
photosynthetic electron transport.
The authors are grateful to the following Brazilian agencies:
Conselho Nacional de Desenvolvimento Cientifico e Tecnolo-
́
́
gico (CNPq) and Coordenaca
̧
o de Aperfeico
̧
amento de Pessoal
de Nivel Superior (CAPES) for research fellowships (to Celia
R. A. Maltha, Eduardo V. V. Varejao, Jodieh O. S. Varejao, and
̃
́
́
̃
In a previous work, a QSAR study showed that the
photosynthetic inhibition ability of rubrolide analogues was
mainly correlated to the HOMO and LUMO energies.23
However, using those data in attempting to explain the higher
activity of cyclopentenediones is hindered by the difference in
the structural skeleton between these two classes of
compounds. On the other hand, a series of studies has
concluded that biological activities (including herbicide proper-
ties) of various classes of organic compounds are related to the
high-energy gap between HOMO and LUMO. For many of
these studies, the higher the HOMO−LUMO energy gap of the
compounds, the higher their biological activity.46−50 Thus, we
decided to calculate the HOMO−LUMO energy gap for
compounds 3a, 3b, 4a, and 4b. Results obtained showed that
cyclopentenediones 4a and 4b, which have higher biological
activities, also have higher energy gaps (7.940 and 7.950 eV,
respectively) than the corresponding rubrolides 3a and 3b
(7.577 and 7.580 eV, respectively). This finding may suggest
that the ability to accept electrons and interfere in the
photosynthetic electron transport of these compounds may be
related to higher energy gaps between HOMO and LUMO.
Also, the energy gap between HOMO and LUMO for
compound 4b was found to be higher than that for compound
4a, thus helping to explain the higher activity of the first in
comparison to the former.
In this work, we have shown that rubrolide analogues bearing
a nitro group at the benzylidene ring (meta position) can be
converted into cyclopent-4-ene-1,3-diones using excess DBU
with traces of water in the reaction medium. When assayed on
Chl a fluorescence of freshly lysed spinach chloroplasts, these
cyclopent-4-ene-1,3-diones were more active than the precursor
rubrolides, displaying the same degree of potency as the
commercial herbicide DCMU. Also, the presence of a fluorine
atom at one of the benzene rings enhances the interference of
cyclopent-4-ene-1,3-diones in the Chl a transients. By means of
the polarography assay, we have further demonstrated that
cyclopent-4-ene-1,3-diones act as Hill reaction inhibitors by
binding the D1 protein at the reducing side of the PSII. These
findings clearly show that cyclopent-4-ene-1,3-diones bearing
electron-withdrawing groups at the aromatic rings can be
regarded as promising candidates for the development of novel
photosynthesis-inhibiting herbicides.
Luiz C. A. Barbosa) and Fundaca̧ o de Amparo a Pesquisa de
Minas Gerais (FAPEMIG) for supporting our research in this
area. The authors also gratefully acknowledge the financial
̀
̃
support from Direccion
Academ
́
General de Asuntos del Personal
́
ico (DGAPA)−National Autonomous University of
Mexico (UNAM) Grants PAPIT IT102012-3 and PAIP 4290-
03 from Faculty of Chemistry, UNAM.
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Oerke, E. C. Crop losses to pests. J. Agric. Sci. 2006, 144, 31−43.
(2) Neve, P.; Vila-Aiub, M.; Roux, F. Evolutionary-thinking in
agricultural weed management. New Phytol. 2009, 184, 783−793.
(3) Wood, H. E.; Olson, P. J. A New Weed Killer, 2,4-D; Manitoba
Department of Agriculture and Immigration: Winnipeg, Manitoba,
Canada, 1946; 201.
(4) Trezzi, M. M.; Felippi, C. L.; Mattei, D.; Silva, H. L.; Nunes, A.
L.; Sebastiani, C.; Vidal, R. A.; Marques, A. Multiple resistance of
acetolactate synthase and protoporphyrinogen oxidase inhibitors in
Euphorbia heterophylla biotypes. J. Environ. Sci. Health, Part B 2005, 40,
101−109.
(5) Vila-Aiub, M. M.; Vidal, R. A.; Balbi, M. C.; Gundel, P. E.;
Trucco, F.; Ghersa, C. M. Glyphosate-resistant weeds of South
American cropping systems: An overview. Pest Manage. Sci. 2008, 64,
366−371.
(6) Powles, S. B. Evolved glyphosate-resistant weeds around the
world: Lessons to be learnt. Pest Manage. Sci. 2008, 64, 360−365.
(7) Beckie, H. J. Herbicide-resistant weed management: Focus on
glyphosate. Pest Manage. Sci. 2011, 67, 1037−1048.
(8) Duke, S. O.; Powles, S. B. Glyphosate: A once-in-a-century
herbicide. Pest Manage. Sci. 2008, 64, 319−325.
(9) Duke, S. O.; Powles, S. B. Glyphosate-resistant crops and weeds:
Now and in the future. AgBioForum 2009, 12, 346−357.
(10) Duke, S. O. Why have no new herbicide modes of action
appeared in recent years? Pest Manage. Sci. 2012, 68, 502−512.
(11) Dayan, F. E.; Cantrell, C. L.; Duke, S. O. Natural products in
crop protection. Bioorg. Med. Chem. 2009, 17, 4022−4034.
(12) Cantrell, C. L.; Dayan, F. E.; Duke, S. O. Natural products as
sources for new pesticides. J. Nat. Prod. 2012, 75, 1231−1242.
(13) Huter, O. F. Use of natural products in the crop protection
̈
industry. Phytochem. Rev. 2011, 10, 189−194.
(14) Barbosa, L. C. A.; Teixeira, R. R.; Montanari, R. M. In Current
Trends in Phytochemistry; Epifano, F., Ed.; Research Singpost: Kerala,
India, 2009; pp 21−59.
ASSOCIATED CONTENT
* Supporting Information
■
(15) Mitchell, G.; Bartlett, D. W.; Fraser, T. E. M.; Hawkes, T. R.;
Holt, D. C.; Townson, J.; Wichert, R. A. Mesotrione: A new selective
herbicide for use in maize. Pest Manage. Sci. 2001, 57, 120−128.
(16) Chaves, F. C.; Barbosa, L. C. A.; Demuner, A. J.; Silva, A. A.
New helminthosporal analogues with plant-growth regulatory property
synthesized via oxyallyl cation. Z. Naturforsch. 2006, 61, 1287−1294.
(17) Barbosa, L. C. A.; Ferreira, M. L.; Demuner, A. J.; Silva, A. A.;
Pereira, R. C. Preparation and phytotoxicity of sorgoleone analogues.
Quim. Nova 2001, 24, 751−755.
(18) Barbosa, L. C. A.; Costa, A. V.; Veloso, D. O.; Lopes, J. L. C.;
Hernandez-Terrones, M. G.; King−Diaz, B.; Lotina-Hennsen, B.
Phytogrowth-inhibitory lactones derivatives of glaucolide B. Z.
Naturforsch. 2004, 59c, 803−810.
(19) Barbosa, L. C. A.; Demuner, A. J.; Alvarenga, E. S.; Oliveira, A.;
King-Diaz, B.; Lotina-Hennsen, B. Phytogrowth- and photosynthesis-
S
1H and 13C NMR spectra of compounds 3b, 4a, and 4b
(Figures S1−S4, S7, and S8), COSY and HETCOR spectra of
the compounds 4a and 4b (Figures S5, S6, S9, and S10),
COLOQ spectrum of compound 4b (Figure S11), and
mechanistic proposal for the synthesis of compounds 4a and
4b (Scheme S1). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +55-31-34093396. E-mail: lcab@ufmg.br and/or
lcab@outlook.com.
■
H
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
inhibiting properties of nostoclide analogues. Pest Manage. Sci. 2006,
62, 214−222.
(20) Barbosa, L. C. A.; Teixeira, R. R.; Forlani, G.; Veloso, D. P.;
Carneiro, J. W. M. Synthesis of photosynthesis-inhibiting nostoclide
analogues. J. Agric. Food Chem. 2008, 56, 2321−2329.
(40) Gedge, D. R.; Pattenden, G. Rearrangement of 4-ylidenebute-
nolides to cyclopentene-1,3-diones: Synthesis of calythrone and
related compounds. J. Chem. Soc., Chem. Commun. 1978, 20, 880−882.
(41) Tan, H.; Zheng, C.; Liu, Z.; Wang, D. Z. Biomimetic total
syntheses of linderaspirone A and Bi-linderone and revisions of their
biosynthetic pathways. Org. Lett. 2011, 13, 2192−2195.
(42) Cota, J.; Chimentao, R.; Sueiras, J.; Medina, F. The DBU−H2O
complex as a new catalyst for aldol condensation reactions. Catal.
Commun. 2008, 9, 2090−2094.
(43) Jiang, H.-X.; Chen, L.-S.; Tang, N.; Smith, B. R. Aluminum-
induced effects on photosystem II photochemistry in citrus leaves
assessed by the chlorophyll a fluorescence transient. Tree Physiol.
2008, 28, 1863−1871.
(44) Giaquinta, R. T.; Dilley, R. A.; Crane, F. L.; Barr, R.
Photophosphorylation not coupled to DCMU-insensitive photosystem
II oxygen evolution. Biochem. Biophys. Res. Commun. 1974, 59, 985−
991.
(45) Allen, J. F.; Holmes, N. G. In Photosynthesis, Energy Transduction.
A Practical Approach; Hipkinns, M. F., Baker, N. R., Eds.; IRL Press:
Oxford, U.K., 1986; pp 103−141.
(46) Refat, M. S.; El-Sayed, M. Y.; Adam, A. M. A.; Saad, H. A.;
Eldaroti, H. H. Charge transfer complexes as a semiconductor models:
Outline of spectroscopic studies on electron donor−acceptor
complexes of hexane-1,6-diol with different π-acceptors. Int. J.
Electrochem. Sci. 2013, 8, 4234−4259.
(47) Mahalakshmi, G.; Balachandran, V. FT-IR, FT-Raman and DFT
study of 3,3′-bis(trifluoromethyl)benzophenone and its biological
activity with other halogen (Cl, Br) atoms. Spectrochim. Acta, Part A
2014, 124, 328−340.
(48) Lamchouri, F.; Toufik, H.; Bouzzine, S. M.; Hamidi, M.;
Bouachrine, M. Experimental and computational study of biological
activities of alkaloids isolated from Peganum harmala seeds. J. Mater.
Environ. Sci. 2010, S1, 343−352.
(49) Wang, Z.; Song, J.; Han, Z.; Jiang, Z.; Zheng, W.; Chen, J.; Song,
Z.; Shang, S. Quantitative structure−activity relationship of terpenoid
aphid antifeedants. J. Agric. Food Chem. 2008, 56, 11361−11366.
(50) Balasubramanian, K. Molecular orbital basis for yellow curry
spice curcumin’s prevention of Alzheimer’s disease. J. Agric. Food
Chem. 2006, 54, 3512−3520.
(21) Barbosa, L. C. A.; Nogueira, L. B.; Maltha, C. R. A.; Teixeira, R.
R.; Silva, A. A. Synthesis and phytogrowth properties of oxabicyclic
analogues related to helminthosporin. Molecules 2009, 14, 160−173.
(22) Barbosa, L. C. A.; Varejao, J. O. S.; Petrollino, D.; Pinheiro, P.
̃
F.; Demuner, A. J.; Maltha, C. R.; Forlani, G. Tailoring nostoclide
structure to target thechloroplastic electron transport chain. ARKIVOC
2012, iv, 15−32.
(23) Barbosa, L. C. A.; Maltha, C. R.; Lage, M. R.; Barcelos, R. C.;
Dona, A.; Carneiro, J. W. M.; Forlani, G. Synthesis of rubrolide
analogues as new inhibitors of the photosynthetic electron transport
chain. J. Agric. Food Chem. 2012, 60, 10555−10563.
(24) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 5th ed.; Butterworth−Heinemann: Oxford, UK, 2003; p
609.
(25) Stewart, J. J. P. MOPAC 2007, Version 7; Stewart Computational
Chemistry: Colorado Springs, CO, 2007.
(26) Dewar, M. J. S.; Thiel, W. Ground states of molecules. 38. The
MNDO method. Approximations and parameters. J. Am. Chem. Soc.
1977, 99, 4899−4907.
(27) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F. AM1: A new
quantum chemical molecular model. J. Am. Chem. Soc. 1985, 107,
3902−3909.
(28) Mills, J. D.; Mitchell, P.; Schurmann, P. Modulation of coupling
ATPase activity in intact chloroplasts. FEBS Lett. 1980, 191, 144−148.
(29) Morales-Flores, F.; Aguilar, M. I.; King-Díaz, B.; Lotina-
Hennsen, B. Derivatives of diterpen labdane-8α,15-diol as photo-
synthetic inhibitorsin spinach chloroplasts and growth plant inhibitors.
J. Photochem. Photobiol., B 2013, 125, 42−50.
(30) Strain, H. H.; Cope, T.; Svec, M. A. Analytical procedures for
the isolation, identification, estimation and investigation of thechlor-
ophylls. Methods Enzymol. 1971, 23, 452−466.
(31) Veiga, T. A. M.; King-Díaz, B.; Marques, A. S. F.; Sampaio, O.
M.; Vieira, P. C.; da Silva, M. F. G. F.; Lotina-Hennsen, B.
Furoquinoline alkaloids isolated from Balfourodendron riedelianum as
photosynthetic inhibitors in spinach chloroplasts. J. Photochem.
Photobiol., B 2013, 120, 36−43.
(32) Strasser, R. J.; Tsimilli-Michael, M.; Srivastava, A. In Chlorophyll
Fluorescence: A Signature of Photosynthesis, Papageorgion, G. C.,
Govindjee, Eds.; Kluwer Academic Publishers: Dordrecht, Nether-
lands, 2004; pp 321−362.
(33) Dilley, R. A. Ion transport (H+, K+, Mg2+ exchange phenomena).
Methods Enzymol. 1972, 24, 68−74.
(34) Izawa, S.; Hind, G. The kinetics of the pH rise in illuminated
chloroplasts suspensions. Biochim. Biophys. Acta 1967, 143, 377−390.
(35) Yruela, I.; Montoya, G.; Alonso, P. J.; Picorel, R. Identification
of the pheophytin-QA-Fe domain of the reducing side of the
photosystem II as the Cu(II)-inhibitory binding site. J. Biol. Chem.
1991, 266, 22847−22850.
(36) Boukouvalas, J.; Lachance, N.; Oullet, M.; Trudeau, M. Facile
access to 4-aryl-2(5H)-furanones by Suzuki cross coupling: Efficient
synthesis of rubrolides C and E. Tetrahedron Lett. 1998, 39, 7665−
7668.
(37) Boukouvalas, J.; McCann, L. C. Synthesis of the human aldose
reductase inhibitor rubrolide L. Tetrahedron Lett. 2010, 51, 4636−
4639.
́ ̌ ́ ́ ̌
(38) Tichotova, L.; Matousova, E.; Spulak, M.; Kunes, J.; Buchta, V.;
Pour, I. M. Synthesis and biological activity of desmethoxy analogues
of coruscanone A. Bioorg. Med. Chem. Lett. 2011, 21, 6062−6066.
(39) Krayushkin, M. M.; Pashchenko, D. V.; Lichitsky, B. V.; Irie, M.;
Vorontsova, L. G.; Starikova, Z. A. Synthesis of photochromic
dihetarylethenes with a cyclopentenedione bridge fragment. Mendeleev
Commun. 2007, 17, 301−302.
I
dx.doi.org/10.1021/jf5014605 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX