Synthesis of rubrolide analogues as new inhibitors of the photosynthetic electron transport chain

Article abstract of DOI:10.1021/jf302921n

Many natural products have been used as a model for the development of new drugs and agrochemicals. Following this strategy 11 rubrolide analogues, bearing electron-withdrawing and -donating groups at both benzene rings, were prepared starting from commercially available mucobromic acid. The ability of all compounds to inhibit the photosynthetic electron transport chain in the chloroplast was investigated. The rubrolide analogues were effective in interfering with the light-driven ferricyanide reduction by isolated chloroplasts. The IC₅₀ values of the most active derivatives are in fact only 1 order of magnitude higher than those of commercial herbicides sharing the same mode of action, such as Diuron (0.27 μM). QSAR studies indicate that the most efficient compounds are those having higher ability to accept electrons, either by a reduction process or by an electrophilic reaction mechanism. The results obtained suggest that the rubrolide analogues represent promising candidates for the development of new active principles targeting photosynthesis to be used as herbicides.

Full text of DOI:10.1021/jf302921n

Article
pubs.acs.org/JAFC
Synthesis of Rubrolide Analogues as New Inhibitors of the
Photosynthetic Electron Transport Chain
Luiz C. A. Barbosa,*,† Celia R. A. Maltha,† Mateus R. Lage,† Rosimeire C. Barcelos,† Alice Dona,#
́
̀
Jose W. M. Carneiro,§ and Giuseppe Forlani*,#
́
†Department of Chemistry, Federal University of Vico
§Department of Inorganic Chemistry, Federal University Fluminense, Outeiro de Sao Joao Batista s/n, Centro, CEP 24020-141
̧
sa, Avenida P. H. Rolfs, CEP 36570-000 Vico
̧ sa, MG, Brazil
̃
̃
́
Niteroi, RJ, Brazil
#Department of Life Science and Biotechnology, University of Ferrara, via L. Borsari 46, I-44121 Ferrara, Italy
S
* Supporting Information
ABSTRACT: Many natural products have been used as a model for the development of new drugs and agrochemicals.
Following this strategy 11 rubrolide analogues, bearing electron-withdrawing and -donating groups at both benzene rings, were
prepared starting from commercially available mucobromic acid. The ability of all compounds to inhibit the photosynthetic
electron transport chain in the chloroplast was investigated. The rubrolide analogues were effective in interfering with the light-
driven ferricyanide reduction by isolated chloroplasts. The IC50 values of the most active derivatives are in fact only 1 order of
magnitude higher than those of commercial herbicides sharing the same mode of action, such as Diuron (0.27 μM). QSAR
studies indicate that the most efficient compounds are those having higher ability to accept electrons, either by a reduction
process or by an electrophilic reaction mechanism. The results obtained suggest that the rubrolide analogues represent promising
candidates for the development of new active principles targeting photosynthesis to be used as herbicides.
KEYWORDS: γ-alkylidenebutenolides, rubrolides, QSAR, Hill reaction inhibitors
using the naturally occurring nostoclides I and II (1, 2) as
models (Figure 1).
INTRODUCTION
■
During the past decades synthetic herbicides have been playing
a very important role in controlling weeds in agriculture,
contributing to the increase in crop productivity. Due to the
development of herbicide resistance in weeds, the discovery of
new active molecules continues to be an active area of research.
In this endeavor old herbicides such as 2,4-dichlorophenoxy-
acetic acid have recently been modified to produce some new
very active phosphorus derivatives that inhibit the pyruvate
dehydrogenase complex.1,2 The old class of diphenyl ether
herbicides has also been the subject of recent investigation, and
new compounds bearing an unsaturated carboxylate group have
been discovered. This group of compounds constitutes the first
commercial class of herbicides that inhibit the protoporphyr-
ogen IX oxidase (PPO) enzyme.3 In the search for new “green
herbicides” the acetohydroxyacid synthase enzyme (AHAS) has
been targeted, and novel disulfide-containing [1,2,4]triazole
groups and isatin derivatives were produced. Such compounds
constitute new candidates for the discovery of new herbicides
for controlling weeds resistant to current commercial AHAS
inhibitors.4,5
Several of them were found to inhibit the light-driven
reduction of ferricyanide by isolated spinach chloroplasts, the
so-called Hill reaction.11 Further investigation led to the
preparation of analogues of 3 bearing a chlorine at the 3-
position of the benzyl ring. Most of these compounds were
effective inhibitors of the growth of Sorghum bicolor and
showed moderate activity against Cucumis sativus.12 Introduc-
tion of a bromine on the benzyl ring at the para position
resulted in a decrease of activity.13 A QSAR analysis
demonstrated that their effectiveness as Hill reaction inhibitors
is in part associated with the presence of electron-withdrawing
groups at the benzylidene unit.14 The introduction of an
isopropyl unit at position 4 of the lactone ring did not improve
the inhibitory potential.15 Although the synthetic analogues of
nostoclides interfere with the photosynthetic apparatus at
micromolar concentrations, their activity was limited by poor
water solubility.
In view of the above promising results, we envisaged that
rubrolides (Figure 1), due to their structural resemblance with
nostoclides, could also represent a model for the development
of new inhibitors targeting the photosynthetic electron
transport chain. The rubrolides represent a large family of
butenolides first isolated from the colonial tunicate Ritterella
In line with the international community and within the
framework of long-established research on substances with
potential use as agrochemicals, especially for weed control, we
have been using natural products isolated from microorgan-
isms6,7 and plants8,9 as a model for the development of new
biologically active compounds. This is a widely used strategy for
the development of new agrochemicals.10 In a previous study
we prepared a series of compounds with general structure 3
Received: July 6, 2012
Revised: October 1, 2012
Accepted: October 2, 2012
Published: October 2, 2012
© 2012 American Chemical Society
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Article
Figure 1. Naturally occurring lactones known as nostoclides I and II (1, 2) and 3-benzyl-5-(arylmethylene)furan-2(5H)-ones designed on the basis
of the nostoclides (3) and rubrolide A (4).5
240 (M•+, 9), 215 (7), 213 (15), 211 (8), 163 (98), 161 (100), 133
(23), 131 (20), 119 (27), 117 (28), 107 (16), 105 (23), 81 (15), 79
(13), 53 (33).
rubra.16 Further work resulted in the isolation of six other
rubrolides from the ascidia Synoicum blochmanni.17 Due to their
properties, including anti-inflammatory activity,18 cytotoxicity,
and ability to inhibit human aldose reductase, these compounds
caught the attention of many synthetic chemists, leading to
several total syntheses.19−22 Despite all of these efforts, to the
best of our knowledge no one has to date investigated the
ability of such compounds to inhibit photosynthesis in isolated
chloroplasts. In this work we describe the preparation of several
rubrolide analogues and their ability to act as inhibitors of the
Hill reaction.23 A QSAR study to correlate the inhibitory
potential with theoretical molecular descriptors is also reported.
3-Bromo-4-(2-methoxyphenyl)furan-2(5H)-one (7a). To a two-
necked round-bottom flask (150 mL) under nitrogen atmosphere and
connected to a condenser was added a solution of 3,4-dibromofuran-
2(5H)-one (6) (1.21 g, 5.0 mmol) in anhydrous THF (40 mL). To
this solution were added 4-fluoro-2-methoxyphenylboronic acid (0.94
g, 5.50 mmol), Ag2O (3.48 g, 14.98 mmol), AsPh3 (0.31 g, 1.00
mmol), and PdCl2 (CH3CN)2 (0.07 g, 0.25 mmol). The reaction
mixture was stirred at 65 °C for 24 h, when TLC analysis revealed that
all boronic acid had been consumed. The mixture was cooled to room
temperature, diluted with ethyl acetate (250 mL), and filtered through
a Celite pad. The filtrate was concentrated under reduced pressure to
yield a dark residue. The purification of this residue by silica gel
column chromatography, eluting with a mixture of DCM/hexane of
increasing polarity, produced the required compound 7a with 76%
yield (1.02 g, 3.8 mmol) and a small amount of 2,2′-dimethox-
ybiphenyl 8a (1.07 mmol), resulting from the homocoupling of
MATERIALS AND METHODS
■
General Experimental Procedures. All reactions were carried
out under a protective atmosphere of dry nitrogen or utilizing a
calcium chloride tube. Solvents were purified and dried as described by
Perrin and Armarego.24 All commercially available reagents were
purchased from Aldrich (Milwaukee, WI, USA) and utilized without
further purification. The 1H and 13C NMR spectra were recorded on a
Varian Mercury 300 instrument (300 and 75 MHz, respectively), using
CDCl3 as a solvent and tetramethylsilane (TMS) as internal standard
(δ = 0). IR spectra were obtained using a Perkin-Elmer Paragon 1000
FTIR spectrophotometer, using potassium bromide (1% v/v) disks,
scanning from 600 to 4000 cm−1. Low-resolution mass spectra were
recorded on a Shimadzu GCMS-QP5050A instrument by direct
insertion, EI mode (70 eV). Chemical ionization accurate mass spectra
were recorded on an Apex III FT-ICR-MS spectrometer with high
resolution (resolution = 10000 fwhm). Values quoted are reported as a
ratio of mass to charge in daltons. Melting points are uncorrected and
were obtained from an MQAPF-301 melting point apparatus
(Microquimica, Brazil). Analytical thin layer chromatography analysis
was conducted on aluminum-backed precoated silica gel plates.
Column chromatography separations were performed over silica gel
(60−230 mesh).
boronic acid: white solid; mp 106.9−107.9 °C; IR (KBr) νmax (cm−1)
̅
3070, 2986, 2958, 2930, 2849, 1775, 1617, 1601, 1457, 1439, 1341,
1312, 1266, 1250, 1122, 1067, 1043, 983, 935, 881, 749; δH (CDCl3)
3.87 (s, 3H, 2′-OCH3), 5.26 (s, 2H, H-5), 6.99 (dd, 1H, J3′,4′ = 8.4 Hz,
J
3′,5′ = 1.2 Hz, H-3′), 7.08 (dt, 1H, J5′,4′ = J5′,6′ = 7.5 Hz, J5′,3′ = 1.2 Hz,
H-5′), 7.47 (ddd, 1H, J4′,3′ = 8.4 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz, H-
4′), 7.80 (dd, 1H, J6′,5′ = 7.5 Hz, J6′,4′ = 1.8 Hz, H-6′); δC (CDCl3) 55.8
(2′-OCH3), 73.8 (C-5), 108.1 (C-3), 111.6 (C-3′), 118.8 (C-1′), 121.1
(C-5′), 130.3 (C-6′), 132.9 (C-4′), 157.4/157.8 (C-4/C-2′, 170.1 (C-
2); EI-MS, m/z (%) 270 ([M + 2]•+, C11H9BrO3, 34), 268 ([M]•+,
35), 241 (30), 239 (32), 226 (11), 224 (12), 189 (100), 161 (17), 145
(23), 133 (36), 131 (51), 115 (22), 105 (75), 102 (33), 89 (38), 77
(46).
3-Bromo-4-(2-methoxy-4-fluorophenyl)furan-2(5H)-one (7b).
Compound 7b was prepared using the same procedure described for
7a. It was obtained as a white solid in 76% yield, along with a small
amount of 8b (1.36 mmol), resulting from the homocoupling of the
starting boronic acid. Data for 7b: mp 150.0−151.0 °C; IR (KBr) νmax
Synthetic Procedures. 3,4-Dibromofuran-2(5H)-one (6). To a
250 mL round-bottom flask was added mucobromic acid (5) (7.17 g,
27.8 mmol) in anhydrous methanol (40 mL). After the system had
cooled to 0 °C in an ice bath, NaBH4 (1.58 g, 41.70 mmol) was added
in small portions. The resulting mixture was stirred for 15 min at 0 °C
before a solution of concentrated sulfuric acid (1.5 mL) in methanol
(14 mL) previously cooled to 0 °C was added. After a further 15 min
of stirring, diethyl ether (375 mL) was added. The organic phase was
washed with brine (3 × 30 mL) and dried over magnesium sulfate.
After evaporation of the solvent under reduced pressure, the crude
product was obtained as a solid. This solid was recrystallized in a
mixture of hexane/diethyl ether 1:1 (v/v), resulting in the required
lactone 2 as white crystals (5.37 g, 22.4 mmol, 80% of yield): mp
̅
(cm−1) 3089, 3015, 2995, 2985, 2941, 1757, 1607, 1586, 1495, 1467,
1458, 1349, 1307, 1281, 1158, 1114, 1059, 1031, 987, 955, 835, 752;
δH (CDCl3) 3.87 (s, 3H, 2′-OCH3), 5.24 (s, 2H, H-5), 6.71 (dd, 1H,
J3′,F = 10.5 Hz, J3′,5′ = 2.4 Hz, H-3′), 6.80 (ddd, 1H, J5′,6′ = 8.7 Hz, J5′,F
= 7.8 Hz, J5′,3′ = 2.4 Hz, H-5′), 7.85 (dd, 1H, J6′,5′ = 8.7 Hz, J6′,F = 6.3
Hz, H-6′); δC (CDCl3) 56.2 (2′-OCH3), 73.6 (C-5), 100.1 (d, JC,F
=
25.7 Hz, C-3′), 108.1 (C-3), 108.2 (d, JC,F = 21.8 Hz, C-5′), 115.0 (d,
JC,F = 3.4 Hz, C-1′), 131.8 (d, JC,F = 10.9 Hz, C-6′), 156.6 (C-4), 159.2
(d, JC,F = 9.8 Hz, C-2′), 165.5 (d, JC,F = 251.3 Hz, C-4′), 169.9 (C-2);
EI-MS, m/z (%) 288 ([M + 2]•+, C11H8BrFO3, 40), 286 ([M]•+, 41),
259 (37), 257 (37), 244 (15), 242 (16), 207 (100), 179 (29), 163
(14), 149 (64), 123 (38), 107 (35), 101 (41), 84 (32), 75 (25).
(Z)-5-(Benzo[d][1,3]dioxol-5-ylmethylene)-3-bromo-4-(2-
methoxyphenyl)furan-2(5H)-one (10d). To a two-necked round-
bottom flask (25 mL), under nitrogen atmosphere, were added 3-
bromo-4-(2-methoxyphenyl)furan-2(5H)-one (3) (100 mg, 0.37
89.9−90.5 °C (mp 90−91 °C);25 IR (KBr) νmax (cm−1) 1758, 1606,
̅
1429, 1339, 1216, 1096, 1023, 992, 981; δH (CDCl3) 4.87 (2H, s, H-
5); δC (CDCl3) 74.4 (C-5), 114.9 (C-3), 143.8 (C-4), 166.9 (C-2); EI-
MS, m/z (%) 244 ([M + 4]•+, C4H2Br2O2, 12), 242 ([M + 2]•+, 24),
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Journal of Agricultural and Food Chemistry
Article
mmol), dichloromethane (3 mL), TBDMSOTf (101 μL, 0.44 mmol),
DIPEA (130 μL, 0.74 mmol), and benzo[d][1,3]dioxole-5-carbalde-
hyde (9d) (70 mg, 0.44 mmol). The resulting mixture was stirred at
room temperature for 1 h. After this period, 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 resulting
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 resulting
residue was purified by column chromatography on silica gel eluted
with dichloromethane/hexane (3:2 v/v) to afford the required product
10d (81 mg, 0.2 mmol) as a pale yellow solid in 55% yield. This solid
was recrystallized from a mixture of dichloromethane/hexane (∼1:1 v/
H-3′), 6.85 (dt, 1H, J5′,6′ = J5′,F = 8.1 Hz, J5′,3′ = 2.4 Hz, H-5′), 7.25
(dd, 1H, J6′,5′ = 8.4 Hz, J6′,F = 6.3 Hz, H-6′), 7.88 (d, 2H, J = 8.7 Hz,
H-2″/H-6″), 8.21 (d, 2H, J = 8.7 Hz, H-3″/H-5″); δC (CDCl3) 56.2
(2′-OCH3), 100.6 (d, JC,F = 25.8 Hz, C-3′), 108.1 (d, JC,F = 21.8 Hz, C-
5′), 110.5 (C-6), 113.3 (d, JC,F = 3.2 Hz, C-1′), 113.4 (C-3), 124.2 (C-
3″/C-5″), 131.3 (C-2″/C-6″), 131.6 (d, JC,F = 10.4 Hz, C-6′), 139.1
(C-1″), 147.6 (C-5), 150.3 (C-4″), 151.5 (C-4), 158.5 (d, JC,F = 10.8
Hz, C-2′), 164.5 (C-2), 165.5 (d, JC,F = 250.1 Hz, C-4′); HRMS m/z
+
(M + Na+) calcd for C18H11BrFNNaO5 441.9697, found 441.9700;
EI-MS, m/z (%) 421 ([M + 2]•+, C18H11BrFNO5, 54), 419 ([M]•+,
55), 340 (18), 312 (13), 297 (11), 284 (11), 266 (14), 249 (11), 237
(28), 181 (51), 149 (100), 137 (31), 133 (88), 106 (47), 101 (96), 89
(72), 86 (46), 84 (69), 77 (22), 75 (23), 63 (66), 51 (60), 50 (17).
(Z )-3-Bromo - 4-(4- fl uor o-2-methox yphenyl )-5-(4 -
(trifluoromethyl)benzylidene)furan-2(5H)-one (10c). Product 10c
(91 mg, 0.21 mmol) was obtained as a white solid in 30% yield
after purification by column chromatography on silica gel eluted with
dichloromethane/hexane (2:1 v/v). This solid was recrystallized from
a mixture of dichloromethane/hexane (∼1:1 v/v): mp 131.8−132.8
v). Data for 10d: mp 175.6−176.6 °C; IR (KBr) νmax (cm−1) 3054,
̅
3006, 2962, 2938, 2902, 2836, 2782, 1767, 1644, 1603, 1503, 1489,
1447, 1258, 1109, 1039, 802, 754; δH (CDCl3) 3.85 (s, 3H, 2′-OCH3),
5.86 (s, 1H, H-6), 6.00 (s, 2H, −O−CH2−O−), 6.78 (d, 1H, J5″,6″
8.1 Hz, H-5″), 7.05−7.11 (m, 2H, H-3′, H-6″), 7.10 (dt, 1H, J5′,4′
=
=
=
=
J
5′,6′ = 7.5 Hz, J5′,3′ = 0.9 Hz, H-5′), 7.25 (dd, 1H, J6′,5′ = 7.5 Hz, J6′,4′
°C; IR (KBr) νmax (cm−1) 3089, 3071, 3043, 3017, 2968, 2941, 2879,
1.8 Hz, H-6′), 7.47 (d, 1H, J2″,6″ = 1.8 Hz, H-2″), 7.52 (ddd, 1H, J4′,3′
̅
2841, 1781, 1648, 1613, 1598, 1500, 1465, 1455, 1415, 1325, 1285,
1227, 1195, 1184, 1165, 1115, 1105, 1067, 1031, 1014, 985, 961, 873,
836, 752; δH(CDCl3) 3.85 (s, 3H, 2′-OCH3), 5.92 (s, 1H, H-6), 6.81
(dd, 1H, J3′,F = 10.8 Hz, J3′,5′ = 2.4 Hz, H-3′), 6.85 (dt, 1H, J5′,6′ = J5′,F
= 8.4 Hz, J5′,3′ = 2.4 Hz, H-5′), 7.25 (dd, 1H, J6′,5′ = 8.4 Hz, J6′,F = 6.6
Hz, H-6′), 7.61 (d, 2H, J3″,5″ = 8.4 Hz, H-3″/H-5″), 7.84 (d, 2H, J2″,6″
8.4 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz, H-4′); δC(CDCl3) 55.9 (2′-
OCH3), 101.8 (−O−CH2−O−), 108.8 (C-5″), 109.8 (C-3), 110.3
(C-2″), 111.9 (C-3′), 114.0 (C-6), 118.1 (C-1′), 120.9 (C-5′), 126.8
(C-6″), 127.3 (C-1″), 130.6 (C-6′), 132.1 (C-4′), 146.6 (C-4″), 148.6
(C-3″), 149.1 (C-5), 152.6 (C-4), 156.9 (C-2′), 165.6 (C-2); HRMS
m/z (M + Na+) calcd for C19H13BrNaO5+ 422.9839, found 422.9842;
EI-MS, m/z (%) 402 ([M + 2]•+, C19H13BrO5, 80), 400 ([M]•+, 79),
321(10), 293 (18), 275 (8), 265 (20), 235 (13), 207 (18), 175 (13),
162 (86), 149 (10), 134 (91), 104 (37), 88 (42), 86 (37), 84 (59), 77
(55), 76 (100), 51 (53), 50 (41).
= 8.4 Hz, H-2″/H-6″); δC (CDCl3) 56.2 (2′-OCH3), 100.5 (d, JC,F
=
25.8, C-3′), 108.0 (d, JC,F = 21.8 Hz, C-5′), 111.6 (C-6), 112.5 (C-3),
113.6 (d, JC,F = 2.9 Hz, C-1′), 124.1 (q, JC,F = 270.7 Hz, CF3), 125.9 (q,
JC,F = 4.1 Hz, C-3″/C-5″), 130.9 (q, JC,F = 32.1 Hz, C-4″), 130.9 (C-
2″/C-6″), 131.7 (d, JC,F = 10.3 Hz, C-6′), 136.2 (C-1″), 149.3 (C-5),
151.6 (C-4), 158.5 (d, JC,F = 10.4 Hz, C-2′), 164.9 (C-2), 165.4 (d, JC,F
Compounds 10a−c and 10e−k were prepared employing a
procedure similar to that described for compound 10d, and yields
are presented in results. All compounds were fully characterized by IR,
NMR (1H, 13C), and MS spectrometry. HMBC and HMQC
bidimensional experiments helped in 13C assignments. Structures of
all compounds are supported by the following spectroscopic data.
(Z)-5-(Benzo[d][1,3]dioxol-5-ylmethylene)-3-bromo-4-(4-fluoro-
2-methoxy-phenyl)furan-2(5H)-one (10a). Product 10a (115 mg,
0.28 mmol) was obtained as a white solid in 79% yield after
purification by column chromatography on silica gel eluted with
dichloromethane/hexane (1:1 v/v). This solid was recrystallized from
a mixture of dichloromethane/hexane (∼1:1 v/v): mp 144.5−145.5
=
249.5 Hz, C-4′); HRMS m/z (M +
Na+) calcd for
+
C19H11BrF4NaO3 464.9720, found 464.9707; EI-MS, m/z (%) 444
([M + 2]•+, C19H11BrF4O3, 87), 442 ([M]•+, 100), 425 (10), 423 (9),
363 (28), 335 (31), 318 (17), 307 (21), 230 (23), 228 (24), 186 (24),
181 (58), 177 (25), 158 (61), 153 (22), 149 (97), 137 (32), 133 (26),
109 (21), 106 (28), 101 (68), 86 (39), 84 (62), 75 (15), 63 (17), 57
(31), 55 (25), 51 (77).
(Z)-3-Bromo-4-(2-methoxyphenyl)-5-(4-nitrobenzylidene)furan-
2(5H)-one (10e). Product 10e (123 mg, 0.31 mmol) was obtained as a
yellow solid in 41% yield after purification by column chromatography
on silica gel eluted with dichloromethane/hexane (3:2 v/v). This solid
was recrystallized from a mixture of dichloromethane/hexane (∼1:1 v/
°C; IR (KBr) νmax (cm−1) 3083, 3052, 3010, 2958, 2918, 2848, 2782,
̅
1767, 1613, 1601, 1501, 1489, 1448, 1411, 1285, 1261, 1194, 1155,
1106, 1037, 990, 966, 954, 927, 873, 838, 811, 754; δH (CDCl3) 3.84
(s, 3H, 2′-OCH3), 5.83 (s, 1H, H-6), 6.00 (s, 2H, −O−CH2−O−),
6.77−6.85 (m, 2H, H-3′, H-5′), 6.79 (d, 1H, J5″,6″ = 8.4 Hz, H-5″),
v): mp 181.1−182.0 °C; IR (KBr) νmax (cm−1) 3100, 3070, 3046,
̅
3015, 2961, 2944, 2839, 1781, 1643, 1611, 1592, 1510, 1486, 1462,
1432, 1342, 1301, 1275, 1249, 1181, 1162, 1109, 1044, 1019, 975, 957,
875, 827, 787, 751; δH (CDCl3) 3.86 (s, 3H, 2′-OCH3), 5.98 (s, 1H,
H-6), 7.09 (dl, 1H, J3′,4′ = 8.4 Hz, H-3′), 7.13 (dt, 1H, J5′,4′ = J5′,6′ = 7.5
Hz, J5′,3′ = 0.9 Hz, H-5′), 7.28 (dd, 1H, J6′,5′ = 7.5 Hz, J6′,4′ = 1.8 Hz, H-
6′), 7.56 (ddd, 1H, J4′,3′ = 8.4 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz, H-4′),
7.89 (d, 2H, J2″,6″ = 9.0 Hz, H-2″/H-6″), 8.21 (d, 2H, J3″,5″ = 9.0 Hz,
H-3″/H-5″); δC (CDCl3) 55.9 (2′-OCH3), 110.5 (C-6), 112.0 (C-3′),
113.1 (C-3), 117.4 (C-1′), 121.0 (C-5′), 124.2 (C-3″/C-5″), 130.4
(C-6′), 131.3 (C-2″/C-6″), 132.5 (C-4′), 139.2 (C-1″), 147.5 (C-5),
150.3 (C-4″), 152.5 (C-4), 156.8 (C-2′), 164.7 (C-2); HRMS m/z (M
7.10 (dd, 1H, J6″,5″ = 8.4 Hz, J6″,2″ = 1.8 Hz, H-6″), 7.22 (dd, 1H, J6′,5′
=
8.1 Hz, J6′,F = 6.3 Hz, H-6′), 7.46 (d, 1H, J2″,6″ = 1.8 Hz, H-2″); δC
(CDCl3) 56.2 (2′-OCH3), 100.4 (d, JC,F = 25.7 Hz, C-3′), 101.8
(−O−CH2−O−), 107.8 (d, JC,F = 21.8 Hz, C-5′), 108.9 (C-5″), 110.1
(C-3), 110.3 (C-2″), 113.9 (C-6), 114.1 (d, JC,F = 3.2 Hz, C-1′), 126.8
(C-6″), 127.2 (C-1″), 131.7 (d, JC,F = 10.4 Hz, C-6′), 146.6 (C-5),
148.6 (C-4″), 149.2 (C-3″), 151.6 (C-4), 158.4 (d, JC,F = 9.8 Hz, C-
2′), 165.3 (d, JC,F = 249.0 Hz, C-4′), 165.4 (C-2); HRMS m/z (M +
+
Na+) calcd for C19H12BrFNaO5 440.9744, found 440.9800; EI-MS,
+
+ Na+) calcd for C18H12BrNaO5 423.9791, found 423.9785; EI-MS,
m/z (%) 420 ([M + 2]•+, C19H12BrFO5, 59), 418 ([M]•+, 57), 311
(15), 283 (24), 253 (11), 225 (15), 181 (11), 162 (74), 149 (22), 134
(87), 104 (32), 101 (34), 86 (31), 84 (50), 77 (15), 76 (100), 51 (48),
50 (46).
m/z (%) 403 ([M + 2]•+, C18H12BrNO5, 60), 401 ([M]•+, 59), 322
(51), 294 (26), 281 (13), 266 (10), 248 (16), 231 (17), 219 (32), 205
(23), 189 (22), 176 (25), 163 (39), 131 (100), 119 (50), 115 (49),
103 (41), 102 (44), 89 (99), 88 (85), 77 (98), 63 (84).
( Z) -3 -B ro m o- 4- (4 -fl uo ro-2-metho xyp heny l)-5-(4-
nitrobenzylidene)furan-2(5H)-one (10b). Product 10b (24 mg, 0.57
mmol) was obtained as a yellow solid in 16% yield after purification by
column chromatography on silica gel eluted with dichloromethane/
hexane (3:2 v/v). This solid was recrystallized from a mixture of
dichloromethane/hexane (∼1:1 v/v): mp 222.5−223.6 °C; IR (KBr)
(Z)-3-Bromo-4-(2-methoxyphenyl)-5-(4-(trifluoromethyl)-
benzylidene)furan-2(5H)-one (10f). The product 10f (131 mg, 0.31
mmol) was obtained as a white solid in 41% yield after purification by
column chromatography on silica gel eluted with dichloromethane/
hexane (2:1 v/v). This solid was recrystallized from a mixture of
dichloromethane/hexane (∼1:1 v/v): mp 152.0−152.5 °C; IR (KBr)
νmax (cm−1) 3087, 3009, 2962, 2934, 2902, 2840, 2781, 1766, 1613,
̅
1600, 1501, 1489, 1447, 1411, 1285, 1260, 1194, 1155, 1105, 1036,
989, 966, 953, 927, 873, 838, 809, 754; δH (CDCl3) 3.86 (s, 3H, 2′-
OCH3), 5.95 (s, 1H, H-6), 6.82 (dd, 1H, J3′,F = 10.5 Hz, J3′,5′ = 2.4 Hz,
νmax (cm−1) 3065, 3005, 2985, 2953, 2843, 1785, 1647, 1609, 1578,
̅
1493, 1467, 1453, 1437, 1415, 1325, 1284, 1250, 1226, 1197, 1185,
1166, 1129, 1115, 1068, 1022, 979, 954, 940, 873, 841, 827, 786, 772,
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761, 753; δH (CDCl3) 3.86 (s, 3H, 2′-OCH3), 5.95 (s, 1H, H-6), 7.09
5.85 (s, 1H, H-6), 6.72−6.82 (m, 3H, H3′/H5′/H5″), 6.98 (dl, 1H,
(dl, 1H, J3′,4′ = 8.1 Hz, H-3′), 7.13 (dt, 1H, J5′,4′ = J5′,6′ = 7.5 Hz, J5′,3′
=
J3″, 4″= 8.1 Hz, H3″), 7.10−7.21 (m, 2H, H6′/H4″), 7.34−7.36 (m, 1H,
0.9 Hz, H-5′), 7.28 (ddd, 1H, J6′,5′ = 7.5 Hz, J6′,4′ = 1.8 Hz, J6′,3′ = 0.6
Hz, H-6′), 7.55 (ddd, 1H, J4′,3′ = 8.1 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz,
H-4′), 7.61 (d, 2H, J = 8.4 Hz, H-3″/H-5″), 7.84 (d, 2H, J = 8.4 Hz,
H-2″/H-6″); δC (CDCl3) 55.9 (2′-OCH3), 111.7 (C-6), 111.9 (C-3′),
112.2 (C-3), 117.7 (C-1′), 121.0 (C-5′), 124.2 (q, JC,F = 282.6 Hz,
CF3), 125.9 (q, JC,F = 4.1 Hz, C-3″/C-5″), 130.5 (C-6′), 130.9 (q, JC,F
= 31.7 Hz, C-4″), 130.9 (C-2″/C-6″), 132.4 (C-4′), 136.3 (C-1″),
149.3 (C-5), 152.5 (C-4), 156.9 (C-2′), 164.0 (C-2); HRMS m/z (M
+ Na+) calcd for C19H12BrF3NaO3+ 446.9814, found 446.9800; EI-MS,
m/z (%) 424 ([M + 2]•+, C19H12BrF3O3, 98), 442 ([M]•+, 100), 407
(10), 405 (9), 345 (57), 317 (41), 300 (20), 289 (19), 167 (24), 163
(45), 159 (37), 158 (45), 135 (17), 131 (71), 119 (42), 115 (34), 103
(29), 102 (17), 91 (22), 89 (23), 88 (45), 86 (54), 84 (81), 77 (81),
63 (22), 51 (97).
H6″), 9.85 (s, 1H, OH); δC (CDCl3) 56.1 (2′-OCH3), 100.4 (d, JC,F
=
25.5 Hz, C-3′), 107.8 (d, JC,F = 21.75 Hz, C-5′), 110.5 (C-3), 114.7
(C-6), 115.8 (C1′), 116.7 (C-6″), 117.6 (C-5″), 123.3 (C-3″), 130.0
(C-4″), 131.6 (d, JC,F = 10.4 Hz, C-6′), 133.7 (C-1″), 147.5 (C-5),
152.2 (C-4), 157.5 (C-2′), 158.4 (C2″), 165.1 (C-2), 165.2 (d, JC,F
=
+
249 Hz, C-4′); HRMS m/z (M + Na+) calcd forC18H12BrFNaO4
412.9795, found 412.9789; EI-MS, m/z (%) 392 ([M + 2]•+,
C18H12BrFO4, 76), 390 ([M]•+, 76), 311 (21), 283 (26), 268 (10),
255 (19), 209 (10), 181 (21), 177 (22), 149 (29), 134 (100), 131
(10), 106 (33), 105 (43), 101 (33), 78 (26), 77 (31), 51 (21).
(Z)-3-Bromo-5-(3-hydroxybenzylidene)-4-(2-methoxyphenyl)-
furan-2(5H)-one (10j). Product 10j (81 mg, 0.22 mmol) was obtained
as a white solid in 58% yield after purification by column
chromatography on silica gel eluted with dichloromethane/hexane
(2:1 v/v). This solid was recrystallized from a mixture of dichloro-
methane/hexane (∼1:1 v/v): mp 209.5−213.7 °C (decomp); IR
(Z)-5-Benzylidene-3-bromo-4-(2-methoxyphenyl)furan-2(5H)-
one (10g). Product 10g (42 mg, 0.12 mmol) was obtained as a white
solid in 32% yield after purification by column chromatography on
silica gel eluted with dichloromethane/hexane (1:1 v/v). This solid
was recrystallized from a mixture of dichloromethane/hexane (∼1:1 v/
(KBr) νmax (cm−1) 3394, 3100, 3043, 3014, 2942, 1741, 1633, 1609,
̅
1579, 1491, 1463, 1453, 1368, 1285, 1253, 1220, 1181, 1163, 1047,
1009, 954, 945, 884, 844, 789, 765; δH (CDCl3) 3.79 (s, 3H, 2′-
OCH3), 6.51 (s, 1H, H-6), 6.72 (dd, 1H, J4″, 5″ = 8.4 Hz, J4″,6″ = 1.5 Hz,
H-4″), 6.81−6.89 (m, 1H, H5′), 6.97−7.07 (m, 2H, H3′/H2″), 7.07−
7.15 (m, 1H, H5″), 7.24 (dd, 1H, J6′,5′ = 7.8 Hz, J6′,4′ = 1.5 Hz, H6′),
7.45 (ddd, 1H, J4′,3′ = 8.2 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.5 Hz, H4′), 8.11
(dd, 1H, J6″,5″ = 7.8 Hz, J6″,4″ = 1.5 Hz, H6″); δC (CDCl3) 55.7 (2′-
OCH3), 109.1 (C-6), 109.5 (C-3), 111.8 (C-3′), 115.5 (C-4″), 118.3
(C-1″), 120.4 (C-5′), 120.8 (C-2″), 130.7 (C6′), 131.1 (C-5″), 131.7
(C-6″), 131.9 (C-4′/C1″), 147.1 (C-5), 156.2 (C-4), 156.7 (C-2′),
160.0 (C-3″), 166.0 (C-2); HRMS m/z (M + Na+) calcd
v): mp 147.2−148.2 °C; IR (KBr) νmax (cm−1) 3092, 3067, 3047,
̅
3026, 3008, 2955, 2930, 2836, 1777, 1637, 1609, 1489, 1458, 1434,
1345, 1281, 1253, 1228, 1181, 1111, 1046, 1027, 973, 957, 927, 864,
834, 765, 754; δH (CDCl3) 3.86 (s, 3H, 2′-OCH3), 5.95 (s, 1H, H-6),
7.08 (dl, 1H, J3′,4′ = 8.4 Hz, H-3′), 7.12 (dt, 1H, J5′,4′ = J5′,6′ = 7.5 Hz,
J
5′,3′ = 0.9 Hz, H-5′), 7.28 (dd, 1H, J6′,5′ = 7.5 Hz, J6′,4′ = 1.8 Hz, H-6′),
7.33−7.40 (m, 3H, H-3″, H-4″, H-5″), 7.53 (ddd, 1H, J4′,3′ = 8.4 Hz,
J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz, H-4′), 7.75 (dd, 2H, J2″/6,3″/5″ = 8.4 Hz,
J
2″/6″,4″= 1.8 Hz, H-2″/H-6″); δC (CDCl3) 55.9 (2′-OCH3), 110.9 (C-
3), 111.9 (C-3′), 113.9 (C-6), 118.1 (C-1′), 120.9 (C-5′), 129.1 (C-
3″/C-5″), 129.6 (C-4″), 130.6 (C-6′), 131.0 (C-2″/C-6″), 132.2 (C-
4′), 133.0 (C-1″), 147.9 (C-5), 152.7 (C-4), 156.9 (C-2′), 165.5 (C-
2); HRMS m/z (M + Na+) calcd for C18H13BrNaO3+ 378.9940, found
378.9970; EI-MS, m/z (%) 358 ([M + 2]•+, C18H13BrO3, 65), 356
([M]•+, 62), 277 (30), 249 (23), 231 (15), 221 (18), 202 (13), 178
(20), 163 (27), 131 (56), 119 (32), 118 (80), 115 (52), 105 (17), 103
(23), 194 (19), 91 (46), 90 (100), 89 (74), 88 (54), 86 (42), 84 (63),
77 (77), 63 (27).
+
forC18H12BrFNaO4 412.9795, found 412.9790; EI-MS, m/z (%)
374 ([M + 2]•+, C18H13BrO4, 88), 372 ([M]•+, 86), 293 (100), 278
(26), 247 (15), 234 (43), 221 (33), 161 (15), 159 (40), 133 (21), 131
(74), 115 (29), 106 (26), 105 (24), 78 (55), 77 (86), 51 (40).
(Z )-3-Bromo - 4-(4- fl uor o-2-methox yphenyl )-5-(3 -
hydroxybenzylidene)furan-2(5H)-one (10k). Product 10k (75 mg,
0.19 mmol) was obtained as a white solid in 55% yield after
purification by column chromatography on silica gel eluted with
dichloromethane/hexane (2:1 v/v). This solid was recrystallized from
a mixture of dichloromethane/hexane (∼1:1 v/v): mp 151.1−153.2
(Z)-3-Bromo-5-(2-hydroxybenzylidene)-4-(2-methoxyphenyl)-
furan-2(5H)-one (10h). Product 10h (127 mg, 0.34 mmol) was
obtained as a white solid in 46% yield after purification by column
chromatography on silica gel eluted with dichloromethane/hexane
(2:1 v/v). This solid was recrystallized from a mixture of dichloro-
°C; IR (KBr) νmax (cm−1) 3415, 3070, 2924, 2852, 1739, 1637, 1615,
̅
1582, 1499, 1456, 1409, 1367, 1287, 1255, 1195, 1157, 1103, 1027,
1010, 961, 890, 843, 763; δH (CDCl3) 3.78 (s, 3H, 2′-OCH3), 6.50 (s,
1H, H-6), 6.70−6.78 (m, 3H, H-3′/H5′/H2″), 6.81−6.87 (m, 1H, H-
4″), 7.08−7.14 (m, 1H, H5″), 7.18−7.24 (m, 1H, H-6′), 8.08 (dd, 1H,
J6″,5″ = 8.0 Hz, J6″,4″ = 1.5 Hz, H6″); δC (CDCl3) 56.0 (2′-OCH3),
100.3 (d, JC,F = 25.8 Hz, C-3′), 107.7 (d, JC,F = 21.8 Hz, C-5′), 109.1
(C-6), 109.7 (C-3), 114.2 (C-1′), 115.5 (C-2″), 120.4 (C-4″), 130.9
(C1″), 131.3 (C-5″), 131.6 (C-6″), 131.8 (d, JC,F = 10.3 Hz, C-6′),
methane/hexane (∼1:1 v/v): mp 178.9−180.0 °C; IR (KBr) νmax
̅
(cm−1) 3404, 3072, 3000, 2948, 2836, 1753, 1648, 1593, 1489, 1465,
1452, 1433, 1359, 1345, 1285, 1255, 1168, 1018, 986, 941, 897, 885,
792, 755; δH (CDCl3) 3.77 (s, 3H, 2′-OCH3), 5.86 (s, 1H, H-6), 6.76
(ddd, 1H, J3″,4″ = 8.1 Hz, J3″,5″ = 2.4 Hz, J3″,6″ = 0.9 Hz, H-3″), 6.95−
7.05 (m, 3H, H-3′/H5′/H5″), 7.00−7.15 (m, 1H, H-4″), 7.18 (dd,
1H, J6′,5′ = 7.5 Hz, J6′,4′ = 7.5 Hz, J6′,4′ = 1.8 Hz, H-6′), 7.29−7.34 (m,
1H, H6″), 7.45 (ddd, 1H, J4′,3′ = 8.6 Hz, J4′,5′ = 7.5 Hz, J4′,6′ = 1.8 Hz,
H4′); δC (CDCl3) 55.7 (2′-OCH3), 110.2 (C-3), 111.8 (C-3′), 114.8
(C-6), 116.7 (C-6″), 117.5 (C-3″), 117.8 (C1′), 120.8 (C-5′), 123.2
(C-5″), 130.0 (C-4″), 130.4 (C-6′), 132.2 (C-4′), 133.8 (C-1″), 147.6
(C-5), 153.2 (C-4), 156.8 (C-2′), 157.4 (C2″), 166.3 (C-2); HRMS
147.1 (C-5), 152.2 (C-4), 156.3 (C-3″), 158.4 (C-2′), 165.1 (d, JC,F
=
247.9 Hz, C-4′), 166.2 (C-2); HRMS m/z (M + Na+) calcd for
+
C18H12BrFNaO4 412.9795, found 412.9791; EI-MS, m/z (%) 392
([M + 2]•+, C18H12BrFO4, 88), 390 ([M]•+, 83), 311 (100), 296 (17),
268 (15), 252 (45), 239 (33), 196 (12), 177 (38), 161 (19), 149 (38),
134 (29), 133 (48), 131 (40), 106 (57), 105 (26), 101 (38), 78 (62),
77 (36), 61 (14), 51 (34).
+
m/z (M + Na+) calcd forC18H13BrNaO4 394.9889, found 394.9883;
EI-MS, m/z (%) 374 ([M + 2]•+, C18H13BrO4, 98), 372 ([M]•+, 97),
293 (27), 265 (32), 247 (20), 237 (24), 209 (17), 163 (24), 147 (27),
134 (78), 131 (49), 105 (49), 88 (32), 78 (39), 77 (100), 75 (15), 51
(37).
Biological Assays. The ability to interfere with the photosynthetic
electron transport chain was evaluated on photosynthetically active
thylakoid membranes isolated from market spinach (Spinacia oleracea
L.) leaves. The IC50 for each test compound was determined according
to the procedure previously described by Barbosa et al.13
Computations. The conformational analysis of the 13 rubrolide
analogues was done using the conformer distribution subroutine of the
SPARTAN'06 software.26 The geometry with the lowest energy was
selected for further calculations with the MOPAC code,27 again using
the AM128,29 semiempirical method. After geometry reoptimization
with MOPAC, all structures were confirmed as a local minimum by
calculation of the Hessian matrix force constant (no negative
eigenvector).
( Z) -3 -B ro m o- 4- (4 -fl uo ro-2-metho xyp heny l)-5-(2-
hydroxybenzylidene)furan-2(5H)-one (10i). Product 10i (68 mg, 0.17
mmol) was obtained as a white solid in 50% yield after purification by
column chromatography on silica gel eluted with dichloromethane/
hexane (2:1 v/v). This solid was recrystallized from a mixture of
dichloromethane/hexane (∼1:1 v/v): mp 195−198.3 °C (decomp);
IR (KBr) νmax (cm−1) 3389, 3068, 2979, 2964, 2948, 2886, 1752, 1649,
̅
1313, 1495, 1453, 1409, 1362, 1319, 1278, 1224, 1177, 1153, 1107,
1023, 987, 957, 883, 831, 754; δH (CDCl3) 3.78 (s, 3H, 2′-OCH3),
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Figure 2. Preparation of 3-bromo-4-arylfuran-2(5H)-ones 7a and 7b.
Figure 3. Preparation of (Z)-5-arylidene-3-bromo-4-arylfuran-2(5H)-ones 10a−k.
For the QSAR analysis the outputs from the MOPAC calculations
were used to generate a set of molecular descriptors, using the
CODESSA software.30 Two sets of outputs were obtained with the
MOPAC code. In one of them the following keywords were used:
AM1, PRECISE, XYZ, EF, ENPART, VECTORS, BONDS, PI, and
POLAR. In the second one, which was used to calculate the
thermodynamic properties, the following keywords were used: AM1,
PRECISE, THERMO, ROT=1, and XYZ. Both outputs from MOPAC
were used as input to CODESSA. In the statistical analysis using
CODESSA the following parameters were employed: maximum
number of descriptors per model, 1; criterion of significance of
parameter, 0.01; level of high correlation, 0.99; level of intercorrelation
significance, 0.8.
early stage of our investigation we did not embark on a
production of a large number of compounds with function-
alities at this phenyl ring. Although the choice of the
substituents was arbitrary, we prepared a compound with an
electron-donating group (OMe) at position 2 (7b) and for
comparison another compound containing an extra electron-
withdrawing F atom at position 4 (7a).
These reactions occurred with high degree of regioselectivity,
as described for similar cases,32 and the structures of both
products were confirmed by spectroscopic analysis.
The conversion of compounds 7a and 7b into the
benzylidenes10a−k was accomplished using the same proce-
dure employed for the synthesis of several nostoclide analogues
reported previously.10,11,35,36 Briefly, the two-step transforma-
tion consisted initially of an aldol condensation between the
tert-butyldimethylsilyl derivative of 7a/7b with the aldehydes
9a−f, followed by elimination of TBDMSOH (Figure 3). For
this, lactones 7a/7b were treated with tert-butyldimethylsilyltri-
fluoromethanesulfonate and diisopropylethylamine to yield the
corresponding silyl ethers that were not isolated (Figure 4), but
immediately reacted with the aldehydes to produce the
intermediates of type 11a−k. The formation of such
intermediates has been previously proved,37 but in the present
work they were not isolated but submitted to a reaction with
DBU, to afford the required compounds 10a−k.
RESULTS AND DISCUSSION
■
Synthesis of Analogues. Our approach to the preparation
of rubrolide analogues started with commercially available
mucobromic acid (5), which is very cheap (US $273.00/100 g).
This compound was converted into the more expensive 3,4-
dibromofuran-2(5H)-one (6) (US $169.00/g) by means of a
reduction with sodium boron hydride followed by dehydration
with sulfuric acid (Figure 2). Compound 6 was obtained in
good yield (80%) and on a multigram scale.31 This compound
is a useful template for the preparation of sulfur- and nitrogen-
containing heterocycles including agrochemical precursors.32,33
The introduction of the aryl group at position 4 in lactone 6
was achieved by a selective Suzuki reaction employing Ag2O,
AsPh3, and PdCl2(MeCN)2 conditions as developed by Rossi et
al.34 The reaction was accomplished with 4-fluoro-2-methox-
ybenzaldehyde and 2-methoxybenzaldehyde and produced the
required compounds 7a and 7b. In both cases small and
variable amounts of the homocoupling products 8a (1.07
mmol) and 8b (1.36 mmol) were formed and isolated. At this
The yields for this reaction varied from 16 to 79%, with the
aldehydes containing electron-withdrawing groups such as NO2
being less efficient.
The confirmation of the structures of 10a−k was based on
their spectroscopic data, including nuclear Overhauser effect
difference spectroscopy (NOEdiff) that revealed a correlation
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no correlation between polarity and the ability of rubrolide
analogues to inhibit the photosynthetic electron transport chain
in isolated chloroplasts.
Although the number of compounds tested is limited, the
scaffold of rubrolide seems therefore a promising lead structure
toward the development of new active compounds targeting the
chloroplast electron transport chain. The IC50 values of the
most active derivatives are in fact only 1 order of magnitude
higher than those of commercial herbicides sharing the same
mode of action, such as Diuron (0.27 μM) and Hexazinone
(0.11 μM).39
Figure 4. Structure of the silyl ethers not isolated.
To better understand the relationship between the molecular
structure and the biological activity, QSAR studies were carried
out. To determine the correlation between molecular
descriptors and the efficiency of the set of rubrolides, we
tested a set of models as discussed below.
At first, a structural analysis of the studied compounds was
carried out. The geometries of the 13 rubrolide analogues were
obtained from AM1 semiempirical calculations, after a
conformational analysis using the conformer distribution
subroutine of the SPARTAN'06 software. The geometry with
the lowest energy for each derivative was then reoptimized with
the MOPAC software, again using the AM1 method. The
outputs from the MOPAC calculation were used to generate a
set of molecular descriptors (see the Supporting Information),
using the CODESSA software.
CODESSA may generate about 400 molecular descriptors
associated with molecular constitution, geometry, and topology
and with electrostatic, thermodynamic, and quantum chemical
parameters. The molecular descriptors generated by CODESSA
were correlated to the biological efficiency using the heuristic
method.40,41 After elimination of descriptors with low variance
or high intercorrelation, the best correlations were found with
the molecular descriptors HOMO, LUMO, LUMO+1, and
FNSA3.42,43 HOMO, LUMO, and LUMO+1 are the energies
of the well-known frontier orbitals. FNSA3 is a molecular
descriptor that reports the fractional negatively charged partial
surface area and is associated with the polarity of the molecule.
The values of this descriptor are obtained as a quotient of two
other descriptors, PNSA3 (charge weighted partial negative
surface area) at the numerator and TMSA (total molecular
surface area) at the denominator, as described below:44
between H-6 and H-6′ in the aromatic ring, in agreement with
the Z stereochemistry for the exo double bond.
Biological Assays and QSAR Studies. To evaluate
whether the intermediates 7a and 7b, lacking the arylidene
unit, and the final products 10a−k are able to interfere with the
photosynthetic electron transport chain, the rate of ferricyanide
reduction by isolated spinach chloroplasts was evaluated in the
presence of a given compound at levels ranging from 1 to 100
μM. Results were summarized as the concentrations able to
inhibit by 50% the rate measured in untreated controls (Table
1).
Table 1. Concentrations of Rubrolides Able To Inhibit by
50% (IC50) the Photosynthetic Electron Transport Chain in
Isolated Spinach Chloroplasts
a
b
compd
IC50 (μM)
log(1/IC50) (M)
logP
c
7a
ne
1.92
1.77
3.39
2.97
3.19
3.13
3.23
4.11
4.27
2.80
2.80
2.96
2.96
7b
ne
10b
10d
10g
10a
10e
10f
10c
10j
10h
10k
10i
1.1 0.2
39.5 8.6
20.5 5.2
12.6 0.9
1.3 0.3
8.1 2.1
3.5 0.4
>100
5.96
4.40
4.69
4.90
5.89
5.09
5.46
12.6 1.2
9.2 4.7
5.9 0.9
4.90
5.04
5.54
a
b
For Diuron IC50 = 0.27 μM. The values were obtained with Spartan
PNSA3
FNSA3 =
software.38 ne, not effective at the highest dose tested. Each treatment
was carried out in triplicate, and the values are the mean SD.
c
TMSA
where
Compounds 7a and 7b were ineffective, clearly demonstrat-
ing that the benzylidene moiety is essential for the biological
activity. This conclusion does not rule out the possibility that
analogues of 7a/7b bearing other substituents on the phenyl
ring such as nitro and trifluoromethyl would present some
activity. All other derivatives with a benzylidene unit, except
10j, presented inhibitory activities with IC50 < 40 μM. The
most active compounds, 10b and 10e, both bear a nitro group
at the benzylidene ring. In this case the presence of fluorine at
the phenyl ring does not seem to have a great influence on the
inhibitory potential. Compounds 10f and 10c, both with an
electron-withdrawing group CF3 at the benzylidene ring, were
also remarkably active. In the case of this pair, and of pair 10d
and 10a as well, the presence of the fluorine atom seems to
exert a small promoting effect on the biological activity. As
observed from logP values (Table 1), compounds with nitro
and trifluoromethyl groups are more polar. However, we found
PNSA3 =
(q S )A ϵ (qA < 0)
A
∑
A
A
and qA = atomic charge in atom A, SA = weighted partial
negatively charged molecular surface area, and TMSA = total
molecular surface area.
The best four models are given in Figures 5−8, with the
corresponding calculated versus measured efficiency plots.
These models suggest a correlation between the energy of
the frontier orbitals and the inhibitory potential. A lower energy
of the frontier orbitals leads to compounds with higher
efficiency, confirming that these compounds may interfere with
the chloroplastic electron transport chain,11 acting by an
electron acceptor mechanism in the photosynthetic process.
The FNSA3 descriptor indicates that the greater the fraction of
negatively charged molecular surface, the greater the inhibitory
efficiency of a compound. Therefore, the most effective
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Journal of Agricultural and Food Chemistry
Article
Figure 8. Experimental versus calculated efficiency with the LUMO
descriptor.
Figure 5. Experimental versus calculated efficiency with the FNSA3
descriptor.
those having higher ability to accept electrons, for example, by
an electrophilic reaction mechanism. This is justified by the
models obtained with either the energies of the frontier orbitals
or the molecular descriptor FNSA3, which is associated with
the fraction of negatively charged molecular surface area.
ASSOCIATED CONTENT
* Supporting Information
■
S
Table containing values of the descriptors selected in the
different models (FNSA-3 in e−; HOMO, LUMO, and LUMO
+1 in eV). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(L.C.A.B.) Phone: +55 31 38993068. Fax: +55 31 38993065.
E-mail: lcab@ufv.br. (G.F.) Phone: +39 0532 455311. Fax: +39
0532 249761. E-mail: flg@unife.it.
■
Figure 6. Experimental versus calculated efficiency with the LUMO+1
descriptor.
Funding
We acknowledge the following Brazilian agencies: Conselho
́
Nacional de Desenvolvimento Cientifico e Tecnolo
(CNPq) for research fellowships (L.C.A.B., C.R.A.M.,
J.W.M.C.) and financial support; Fundacao de Amparo a
́
gico
̧
̀
̀
̃
Pesquisa de Minas Gerais (FAPEMIG), Fundaca̧ o de Amparo a
̃
Pesquisa do Estado do Rio de Janeiro (FAPERJ), and FINEP
for financial support. We also acknowledge support from the
University of Ferrara (Fondo di Ateneo per la Ricerca 2010).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Akshat Rathi from the University of Oxford
(UK) for the high-resolution mass spectra data.
■
Figure 7. Experimental versus calculated efficiency with the HOMO
descriptor.
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higher water solubility than the previously characterized
nostoclides, they could represent promising candidates for the
development of new active principles targeting photosynthesis.
QSAR results indicate that the most efficient compounds are
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