Ferrocenyl chalcones versus organic chalcones: A comparative study of their nematocidal activity

Article abstract of DOI:10.1016/j.bmc.2011.01.048

A series of 30 organic chlacones and 33 ferrocenyl (Fc) chalcones were synthesized and characterized by melting point, elemental analysis, spectroscopy (¹H NMR and FTIR) and, in two cases, by X-ray crystallography. The biological activity of each compound (10^-4 M in DMSO) against the model nematode Caenorhabditis elegans was examined in terms of % mortality (percent nematodes that died) and % fecundity (percent nematodes that reproduced) and compared to that obtained for the control medium (1% DMSO) over a 14-day period. Detailed conformational analyses for two Fc-chalcones (studied also by X-ray crystallography) were performed via molecular modeling studies. In general, the organic chalcones were found to be less polar than their Fc analogs. Some structure-activity relationships (SARs) were determined: (a) The nematocidal activities of the organic chalcones in this series were found to be much greater than those of their ferrocenyl analogs. (b) The position of the carbonyl group played a central role in the biological activity of both classes of chalcones studied. (c) For both classes of chalcones, lipophilicity of a compound seemed to play a significant role in its nematocidal activity. (d) The planarity of a ferrocenyl-chlacone seems to play a role in its activity.

Full text of DOI:10.1016/j.bmc.2011.01.048

Bioorganic & Medicinal Chemistry 19 (2011) 2055–2073
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Ferrocenyl chalcones versus organic chalcones: A comparative study
of their nematocidal activity
Saeed Attar a, , Zachary O’Brien a, Hasan Alhaddad b, Melissa L. Golden a, Alejandro Calderón-Urrea b
⇑
a Department of Chemistry (M/S SB-70), California State University, Fresno, CA 93740, USA
b Department of Biology (M/S SB-73), California State University, Fresno, CA 93740, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 30 organic chlacones and 33 ferrocenyl (Fc) chalcones were synthesized and characterized by
melting point, elemental analysis, spectroscopy (1H NMR and FTIR) and, in two cases, by X-ray crystallog-
raphy. The biological activity of each compound (10À4 M in DMSO) against the model nematode
Caenorhabditis elegans was examined in terms of % mortality (percent nematodes that died) and % fecun-
dity (percent nematodes that reproduced) and compared to that obtained for the control medium (1%
DMSO) over a 14-day period. Detailed conformational analyses for two Fc-chalcones (studied also by
X-ray crystallography) were performed via molecular modeling studies. In general, the organic chalcones
were found to be less polar than their Fc analogs. Some structure–activity relationships (SARs) were
determined: (a) The nematocidal activities of the organic chalcones in this series were found to be much
greater than those of their ferrocenyl analogs. (b) The position of the carbonyl group played a central role
in the biological activity of both classes of chalcones studied. (c) For both classes of chalcones, lipophil-
icity of a compound seemed to play a significant role in its nematocidal activity. (d) The planarity of a
ferrocenyl-chlacone seems to play a role in its activity.
Received 7 October 2010
Revised 18 January 2011
Accepted 24 January 2011
Available online 2 February 2011
Keywords:
Ferrocenyl chalcone
Biological activity
Nematocidal
Caenorhabditis elegans
Published by Elsevier Ltd.
1. Introduction
rate study,3b Jaouen et al. reported that the use of a titanocenyl-
Tamoxifen conjugate led to an increased rate of growth of
One of the more recent developments in organometallic chem-
istry has occurred at its interface with the biological sciences,
hence appropriately called bioorganometallic chemistry.1 In many
of the reported examples, a phenyl group in an organic molecule
with well-known biological activity has been replaced with an
organometallic moiety in order to determine whether it can be
made more effective. In this regard, the ferrocene derivatives have
featured prominently due to their excellent stability in aqueous,
aerobic media, the easy accessibility of a large variety of deriva-
tives, and favorable electrochemical properties.2 Despite some re-
ported successes, the findings of such studies have been mixed.
In some cases, the new bioorganometallic conjugate is dramati-
cally more effective than the original organic analog, one notable
example being that reported by Jaouen and co-workers on Tamox-
ifen (the drug used most often to treat breast cancer) and its active
metabolite, hydroxytamoxifen.3a When one of the aromatic rings
was replaced with a ferrocenyl group, the resulting ‘ferrocifen’
was found to be a more effective drug, possibly due to the revers-
ible oxidation and reduction of the iron atom, which could produce
hydroxyl radicals that damage DNA. On the other hand, in a sepa-
estrogen-dependent breast tumors because it acted like estrogen.
In another set of studies, Biot and co-workers3c–e have shown that
ferroquine and other ferrocenyl analogs of chlorquine (an estab-
lished antimalarial compound) are very active, in vitro, against
both chloroquine-sensitive and chloroquine-resistant Plasmodium
falciparum across a variety of strains.
Chalcones are naturally occurring derivatives of the parent
compound 1,3-diphenyl-2-propen-1-one (1, Chart 1) and belong
to the flavonoid family of organic compounds.4a Chalcones have
been reported to have a broad range of biological activities such
as antimalarial, antibacterial, antitumor, antioxidant, antihypergly-
cemic, and anti-HIV.4a–l A ferrocenyl chalcone (Fc-chalcone) deriv-
ative is formed when either Ring A or B in 1 is replaced with a
ferrocenyl group (e.g., 31 or 43, Chart 2). As shown in Chart 2,
two types of Fc-chalcones are generally recognized: Type 1 where-
in the carbonyl group is directly attached to the ferrocenyl ring,
and Type 2 wherein the carbonyl group is attached to the phenyl
ring. For the purpose of comparing the structure–activity relation-
ships (SARs) of the organic chalcones with those of the ferrocenyl
analogs, the compounds in the organic series have also been di-
vided into two types (Chart 1), depending on whether the phenyl
ring attached to the carbonyl carbon is substituted (Type 2) or
not (Type 1).
⇑
Corresponding author. Tel.: +1 559 278 2639; fax: +1 559 278 4402.
E-mail address: sattar@csufresno.edu (S. Attar).
0968-0896/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.bmc.2011.01.048

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Chart 1. Structures of the organic chalcones (1–30) prepared for this study.
Despite the fact that Fc-chalcones have been known since the
investigated were found to be less active than their organic analogs
as previously reported by Go and co-workers as well.4a Finally, So-
har and co-workers6c have reported on the synthesis of some gly-
cocide derivatives of Fc-chalcones and Fc-pyrazolines and their
in vitro antitumor activity against human leukemia (HL-60) cells.
It was found that four of the chalcone derivatives containing either
a ferrocenyl moiety and a p-hydroxyphenolic ring or a size-inde-
pendent apolar substitution of the ferrocene ring showed promis-
ing antitumor properties.
Nematodes are ubiquitous throughout the world, populating
moist environments that include plant and animal tissues. Plant
pathogenic nematodes account for a loss of about $80 billion to
agriculture every year.7 The most common solution for the control
of plant pathogenic nematodes, as well as other soil pests, has been
the use of the common fumigant, methyl bromide (CH3Br).8a,b
However, this compound accumulates in the upper atmosphere
and can contribute to stratospheric ozone depletion when photo-
lyzed into free radicals by high-energy ultraviolet radiation.8a For
1960s,5 only two studies on their biological activity have been con-
ducted so far.6 In a preliminary paper,6a followed by a full ac-
count,6b Go and co-workers have reported on the synthesis of a
series of Fc-chalcones and the evaluation of their in vitro antima-
larial (antiplasmodial) activity against a chloroquine-resistant
strain of P. falciparam. These authors found that, in general, Type
1 Fc-chalcones were more active than their Type 2 isomers; for
example, 37 (Ring B = 4-nitrophenyl; IC50 = 5.1
be much more active than 52 (Ring A = 4-nitrophenyl;
IC50 = 61.7 M). However, if a heterocyclic ring was part of the
molecule, then the Type 2 compound was more active than its Type
1 isomer; for example, 60 (Ring A = 3-pyridinyl; IC50 = 4.5 M) was
more active than its Type 1 isomer, 18 (Ring B = 3-pyridinyl;
IC50 = 17.0 M) and the most active among all Fc-chalcones stud-
lM) was found to
l
l
l
ied. In addition, it was found that 31 (Ring B = Ph; IC50 = 19
l
M)
was much more active than 43 (Ring A = Ph, IC50 = 175
lM).
However, in terms of the antimalarial activity, the Fc-chalcones

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2057
Chart 2. Structures of the ferrocenyl chalcones prepared for this study.
that reason, as of 2005, the use of methyl bromide has been phased
out as an agricultural fumigant in developed countries according to
the guidelines of the Montreal Protocol.8b-d However, despite the
current ban on the use of methyl bromide, it is still widely used
throughout the agriculture industry by way of exemptions since
no effective alternative has been found as yet.8d
There have been a few published reports on the nematocidal
activity of organic chalcones. As early as 1967, Laliberté et al.9a re-
ported that (E)-chalcone (1, Chart 1) and other related compounds
showed high activity against zooparasitic nematodes Syphacia
obvelata and Notodiaptomus dubius (pinworms in mice). Gonzalez
et al.9b reported that 1, and other similar aromatic compounds re-
lated to the Shikimate pathway, showed activity against phytopar-
asitic species Globodera pallida and Globodera rostochiensis (potato-
cyst nematodes) and that the observed biological activity was due
to the presence of a conjugated carbonyl system in each of these
molecules. In a later report,9c the same authors found that not only
was compound 1 highly toxic for the two nematodes mentioned
(CL50 = 33 mM), but that it also acted as a potent inhibitor of nem-
atode hatch (HIC50 = 7 mM).
Considering the reported nematocidal properties of organic
chalcones,9 and the general dearth of information on the biological
activity of Fc-chalcones,6 we became interested in conducting a
study on their nematocidal activities as compared to those of their
organic analogs. As shown in Charts 1 and 2, a total of 63 chalcones
were synthesized and their biological activities against Caenorhab-
ditis elegans nematodes were determined. The free-living C. elegans
was chosen as a model for studying the nematocidal activities of
compounds 1–63 primarily due to the ease with which it can be
handled. This soil nematode, which is about 1 mm long, is found

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Scheme 1. Synthesis of organic and Fc-chalcones.
in temperate regions of the planet.10a It was first used in the 1960s
for studies on the genetics of development and neurobiology and
since then it has been used increasingly as a model system for par-
asitic nematodes and higher organisms where defining the func-
tion of genes of interest can be difficult due to lack of
appropriate knock-out approaches or suitable functional assays.10b
These nematodes pose no threat to the researcher, can thrive at
room temperature, reproduce quickly, and have short life spans
(ꢀ15 days). In addition, as Holden-Dye and Walker10c have pointed
out, C. elegans is no more dissimilar to parasitic nematodes than
each individual species of parasite is to another. Thus, all of the
mentioned factors made C. elegans an ideal organism for our study.
Herein, we wish to report on the results of our combined chemical
and biological investigations on the two sets of chalcone deriva-
tives shown in Charts 1 and 2.
acetophenone derivative (64 or 66) and benzaldehyde derivative
(65 or 67), chalcones 1–62 have been prepared, with percent yields
generally in the 70–80% range after chromatography and/or crys-
tallization. In addition, 1,3-diferrocenyl-2-propen-1-one (63),
wherein both rings A and B are ferrocenyl groups, was also synthe-
sized from the combination of 66 and 67 in order to make a direct
comparison to its diphenyl analog (1). The purity of each com-
pound was confirmed by 1H NMR spectroscopy, melting point
determination, and TLC analysis, in addition to elemental analysis
for thirteen new Fc-chalcones prepared for this study, as described
in Section 4.
2.1.2. Structural characterization by spectroscopy
Some of the structural features of chalcones, as determined via
their spectral properties, may aid us in explaining certain aspects
of their observed biological activity. The 1H NMR and IR spectral
data for the organic and Fc-chalcones are presented in Table 1A
and B, respectively, and their salient features are discussed below.
The synthesis of chalcones via the Claisen–Schmidt condensa-
tion leads to the exclusive formation of the (E)-stereoisomer, which
is a result of the more favorable anti periplanar elimination of the
hydroxide group from the carbanion intermediate in this reaction.
The predominance of the (E) configuration at the C@C bond of chal-
cones has been established via 1H NMR spectroscopy.4a,11a The
2. Results and discussion
2.1. Chemistry
2.1.1. Synthesis
As shown in Scheme 1, chalcones can be readily synthesized via
the classical base-catalyzed Claisen–Schmidt condensation
reaction4a in ethanol solvent. Thus, starting from the appropriate

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2059
Table 1A
Table 1B
Spectral data for organic chalcones 1–30
Spectral data for Fc-chalcones 31–63
Compd
NMR
dHb
FT-IR
Compd
NMR
dHb
FT-IR
dHa
3JH
(Hz)
mC@O (cmÀ1
)
mC@C (cmÀ1
)
dH
a
3JH
(Hz)
aHb
mC@O (cmÀ1
)
mC@C (cmÀ1
)
a
Hb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
7.52
7.80
8.04
8.20
7.76
7.80
7.82
8.14
7.99
8.12
7.80
8.03
7.89
7.85
7.83
7.91
8.02
7.61
7.81
7.91
7.81
7.84
7.84
7.87
7.90
7.88
8.04
8.33
7.86
7.40
7.26
15.8
15.4
15.4
15.6
15.8
16.0
15.9
16.0
15.4
15.8
15.7
15.4
15.4
15.4
15.7
15.8
15.8
16.0
16.0
15.7
15.9
16.0
15.4
15.4
15.6
15.7
16.2
15.8
15.6
15.6
1661
1662
1652
1660
1668
1667
1658
1682
1669
1667
1683
1667
1657
1654
1646
1662
1680
1663
1651
1661
1659
1654
1655
1678
1672
1669
1668
1683
1652
1650
1607
1597
1589
1592
1605
1610
1592
1578
1610
1603
1602
1575
1592
1586
1596
1602
1592
1605
1603
1585
1600
1585
1594
1583
1607
1605
1607
1652
1590
1583
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
7.14
7.14
7.24
7.25
7.07
6.74
7.20
7.07
7.02
7.46
7.18
7.20
7.13
7.04
6.89
7.29
6.70
7.06
6.67
7.11
7.21
7.08
7.08
7.06
7.15
7.07
7.15
6.61
7.74
7.08
7.02
7.03
6.75
7.81
7.78
8.69
7.98
8.10
7.71
7.82
8.10
7.68
7.72
7.78
7.77
7.76
7.50
7.47
7.83
7.33
7.81
7.34
7.78
7.84
7.83
7.74
7.56
7.74
7.75
7.75
7.63
8.19
7.83
7.79
7.80
7.71
15.8
15.6
15.3
15.3
15.8
15.1
15.4
15.5
15.1
15.4
15.6
15.6
15.1
15.6
15.8
15.6
15.4
15.2
15.8
15.1
15.9
15.4
15.8
15.8
15.4
15.1
15.2
15.2
15.8
15.8
15.6
15.4
15.5
1652
1652
1650
1650
1643
1661
1654
1656
1650
1654
1656
1662
1655
1657
1658
1652
1639
1656
1644
1655
1655
1650
1650
1649
1652
1649
1651
1675
1661
1656
1644
1642
1646
1595
1592
1596
1586
1604
1603
1605
1581
1588
1605
1607
1608
1586
1589
1591
1584
1591
1582
1584
1600
1595
1573
1600
1603
1575
1570
1588
1602
1592
1589
1582
1575
1577
a
a
7.34
a
7.66
7.37
a
7.77
7.62
7.54
7.35
7.47
7.29
7.33
7.47
7.43
7.26
7.65
7.47
7.36
7.34
7.27
7.27
7.48
7.56
7.95
7.52
7.4
7.26
a
The NMR signal for this proton was obscured by the overlapping signals in the
aromatic region.
magnitude of the coupling constant (3JHH ꢀ 15–16 Hz)11a for the
ortho position of the phenyl group (Scheme 2B).4a,11 However,
the position of the equilibrium shown in Scheme 2B can be shifted
by the appropriate substitution of the enone system. Thus, an alkyl
substituent at C2 favors the s-trans conformer, while the existence
of such a substitutent at C3 favors the s-cis conformer. In addition,
an increase in the steric bulk of the phenyl group attached to the
carbonyl carbon favors the s-cis confomer regardless of other sub-
stituents at C2 or C3. Furthermore, increasing the solvent’s polarity
is generally expected to favor (stabilize) the s-trans conformer
since it is more polar than its s-cis counterpart.
The IR data (Tables 1A and 1B) show that while the electron-
withdrawing or -donating properties of the two rings in a chal-
cone may vary, the magnitudes of the stretching frequencies of
the C@O or C@C bonds are remarkably comparable. For example,
among Type 1 organic chalcones (1–11), wherein the nature of
the substituent on the ‘A’ ring changes from the weakly elec-
tron-donating CH3 group in 2 to the highly electron-withdrawing
NO2 group in 6, the magnitude (cmÀ1) of mC@O varies as follows: 2
(1662), 3 (1652), 4 (1660), 5 (1668), and 6 (1667). In addition, the
location of the carbonyl group in the chalcone molecule does not
seem to greatly affect the position of the stretching frequency
band assigned to this group. For example, the magnitude of mC@O
among the eight pairs of isomeric organic chalcones listed in
Chart 1 varies as follows: 2 (1662) and 12 (1667), 3 (1652) and
13 (1657), 4 (1660) and 14 (1654), 5 (1668) and 15 (1646), 6
(1667) and 20 (1661), 7 (1658) and 21 1659), 9 (1669) and 27
(1668), 10 (1667) and 28 (1683). The mC@C values also show little
variation among the organic chalcones. Similar observations are
made regarding the magnitudes of both mC@O and mC@C among
the Fc-chalcones 31–63. The general conclusion one can draw
from the aforementioned discussion on the NMR and IR
properties of these chalcones is that the basic structure of these
two vinylic protons (those attached to C
a and Cb; Scheme 2A)
has been used to verify the (E)-configuration. As seen in Table 1A
and B, all chalcones prepared in this study exhibit the expected
(E)-stereochemistry at their C@C bonds, with the magnitude of
the coupling constant (3JH Hb) being in the expected range. It is
a
noted that the chemical shift of Hb is greater than that of H
a
throughout both series of chalcones. This is explained by the exis-
tence of a formal positive charge on Cb (Scheme 2A), formed as a
result of resonance distribution of p-electron density in going from
one canonical form (1a) to another (1b), which causes the shift of
Hb signal to lower fields. There is no other discernable trend in the
magnitude of either dH or dHb as a function of the nature of the
a
substituent on either phenyl ring in Type 1 and Type 2 chalcones
of both series. It should also be noted that the configuration at
the C@C bond of a chalcone can play a significant role in its biolog-
ical activity.11a For example, Larsen et al.11b have prepared a series
of chalcones analogues, locked in either the (Z) or the (E) form, and
evaluated their antiplasmodial activity. It was shown that the (Z)-
locked analogues were nearly inactive, whereas the (E)-locked ana-
logues were equipotent to the parent chalcones, indicating that the
(E)-form is the active stereoisomer.
In addition to the configuration at the C@C bond of the enone
systems represented by chalcones, there exist two limiting con-
formers, s-cis and s-trans that result from free rotation about the
C1–C2 bond in such molecules (Scheme 2B).4a,11b This has been
confirmed through X-ray crystallography,4a IR spectroscopy,11g
and computational methods.11g
An un-substituted enone exists predominantly in the s-cis con-
formation (both in solution and in the solid state) because the s-
trans conformer is disfavored thermodynamically due to the steric
interaction between the H atom attached to C3 and that on the

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Scheme 2. For (E)-chalcone (1): (A) two canonical forms; (B) two limiting conformations.
Figure 1. (A) Molecular structures of 1-(2,4-difluorophenyl)-3-ferrocenyl-2-propen-1-one (51); (B) molecular structure of 1-(2-furanyl)-3-ferrocenyl-2-propen-1-one (61).
chalcones is remarkably similar despite changes in the steric and
electronic nature of the phenyl rings.
2.1.3. Structural characterization by X-ray crsytallography
The solid-state structures of two Fc-chalcones prepared in our
work (51 and 61) have unequivocally been determined by X-ray
crystallography. The structures are shown in Figure 1A and B,
respectively, and their selected structural parameters are pre-
sented in Table 2.
As shown in Table 2, all the bond lengths and angles are within
the normal range found for such molecules in previous studies.4a
For example, the C@O bond length is 1.244 Å in 51 and 1.266 Å
in 61. The bond angle around the carbonyl carbon, that is, C(12)–
C(13)–C(14), is 119.49° in 51 and 115.3° for 61, both very close
to the ideal value of 120°. Each structure in Figure 1 shows the ex-
pected (E)-stereochemistry of the C@C bond and the s-cis confor-
mation of the enone portion of each molecule. The extent of
Finally, it should be noted that despite attempts to precisely
determine the s-cis/s-trans equilibrium ratio using IR spectro-
scopic data, obtaining reliable information and drawing definitive
conclusions from such data has proven difficult.11d What is clear
from all published studies on this structural aspect of chalcones
is that while the s-cis conformer predominates in solution for
most chalcones studies to date, the equilibrium concentration
of the s-trans confomer is substantial.
A discussion of the
s-cis/s-trans ratio of a chalcone in solution becomes relevant in
light of recent studies that have revealed the significant role that
such conformational changes can play in a chalcone’s biological
activity.12a,c,4l

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2061
Table 2
delocalization of the pi electrons of the C@O and C@C bonds
(Scheme 2A) which, in turn, reduces the double-bond character
of the carbon–oxygen bond leading to a lower magnitude of mC@O
for 61. The reduction in the pi-electron density of the C@O bond
in going from 51 to 61 is accompanied by a concomitant increase
in the length of this bond, (51, 1.244 Å; 61, 1.266 Å). The greater
planarity of 61 may play a significant role in its higher nematocidal
activity as compared to that of 51 (vide infra). A search of the con-
tents of Chemical Abstracts via the SciFinder Scholar™ service13 re-
vealed that seven other crystal structures of Fc-chalcones have
been reported so far. These include structures of compounds
34,14a 37,6b 43,6b 58,6b two anthracenyl derivatives,14b and a dipy-
renyl derivative.14c It is also noteworthy that the structure of 61
(Fig. 1B) represents the first such determination for a heterocyclic
derivative of an Fc-chalcone. The structures shown in Figure 1A
and B confirm the (E)-stereochemistry at the C@C bond, as well
as the s-cis conformation of the enone portion, in each of these
molecules. In addition, the structure of 51 shows that it has
adopted the anti conformation with respect to the position of the
ortho-F atom on the phenyl ring relative to that of the C@O group,
hence the anti-s-cis designation (Scheme 3A). This can be explained
in terms of the steric repulsion between these two groups in the
syn conformer versus that in the anti conformer. As for compound
61, the preferred solid-state conformation is one in which the oxy-
gen atom of the furanyl ring is in a syn position relative to that of
the C@O group, hence the syn-s-cis designation (Scheme 3B). A
similar observation has been made in a thiophenyl derivative of
chalcone and has been explained in terms of an attractive electro-
static interaction between the sulfur atom of the thiophene ring
and the oxygen atom of the carbonyl group.15 More will be dis-
cussed about this structural aspect of 51 and 61 in Section 2.3.
Selected structural parameters for Fc-chalcones 51 and 61
Compound
51
61
Bond lengths (Å)
C(11)–C(12)
C(12)–C(13)
C(13)–O(1)
1.334(3)
1.463(2)
1.244(2)
1.494(3)
1.316(16)
1.477(15)
1.266(12)
1.491(16)
C(13)–C(14)
Bond angles (°)
C(12)–C(11)–C(6)
C(12)–C(11)–C(1)
C(11)–C(12)–C(13)
O(1)–C(13)–C(12)
O(1)–C(13)–C(14)
C(12)–C(13)–C(14)
125.36(17)
127.9(9)
120.73(17)
121.97(17)
118.52(16)
119.49(16)
124.3(10)
122.7(10)
122.0(10)
115.3(9)
Torsion angles
C(10)–C(6)–C(11)–C(12)
C(2)–C(1)–C(11)–C(12)
C(6)–C(11)–C(12)–C(13)
C(1)–C(11)–C(12)–C(13)
C(11)–C(12)–C(13)–O(1)
C(11)–C(12)–C(13)–C(14)
C(12)–C(13)–C(14)–C(15)
C(12)–C(13)–C(14)–C(19)
C(12)–C(13)–C(14)–O(2)
À4.4
À11.2
174.60
178.7
À0.6
À21.7
159.3
À32.6
150.6
À179.0
À16.3
166.3
planarity of the enone portion of each molecule is different. As
shown in Table 2, the magnitude of the derived torsion angle
C11–C12–C13–O(1) in 51 (À21.7°) is substantially different from
that in 61 (À0.6°). This indicates that the carbonyl group is signif-
icantly twisted out of the plane of the adjacent 2,4-difluorophenyl
ring in 51, but that it is essentially in the same plane as that of the
furanyl ring in 61. In fact, the difference between these two partic-
ular torsion angles is the largest among those listed in Table 2. The
larger magnitude of the carbonyl stretching frequency of 51
(1655 cmÀ1), as compared to that of 61 (1644 cmÀ1; Table 1B), is
indicative of the greater deviation from planarity of the enone por-
tion of 51 relative to that of 61. This is due to the fact that the
greater planarity of the enone portion allows for more extended
2.2. Biology
2.2.1. Experimental setup
Prior to starting the studies on the biological activity of the
chalcones 1–63, the testing solution had to be optimized. As ex-
Scheme 3. (A) Conformations of the substituted phenyl ring in 51; (B) conformations of the furanyl ring in 61.
Figure 2. Appearance of C. elegans nematodes at different stages of the 14-day test under a dissecting microscope: (A) alive nematodes; (B) alive and reproduced nematodes;
(C) dead nematodes; (D) dead and reproduced nematodes; (E) yeast-contaminated well.

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Figure 3. Comaprison between the effect of (A) organic- and (B) Fc-chalcones on nematodes on day 3 of their exposure. Black bars: % mortality (% nematodes dead); white
bars: % fecundity (% nematodes reproduced); compound zero is the DMSO control; absence of any bar for a given compound indicates no detectable activity.
pected, the Fc-chalcones (31–63) were found to be water-insoluble
due to the presence of a hydrophobic fragment in each. Dimethyl
sulfoxide (DMSO) was therefore chosen since, as a polar aprotic
solvent, it dissolves most organic compounds and is miscible with
water. Before any testing of the Fc-chalcones could commence, a
negative control solution had to be prepared. This required the
determination of a minimum concentration of DMSO in the liquid
medium that did not cause death or inhibit the reproduction of the
C. elegans nematodes in our study. Thus, ten repetitions of each of
the four different DMSO solutions (50%, 10%, 2%, and 1% by

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2063
O
O
DMSO control
O
O
Fe
29
61
Figure 4. Comparison between the biological activity against C. elegans nematodes of an organic chalcone (29) and a ferrocenyl chalcone (61), both relative to that of a control
solution (DMSO); (A) % mortality, (B) % fecundity; average values from triplicate sets of 96-well microtiter plates over 14 days; absence of any bar indicates no activity.

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
volume), placed in liquid nematode medium, were tested. It was
found that 1% was the maximum concentration of DMSO in liquid
nematode medium that did not have any adverse biological effects
on the nematodes. Each test was run for 14 days since the nema-
todes were placed into the testing solution 1 day after synchroni-
zation and their average life span is 15 days. Results obtained
from leaving the nematodes in the testing solution any longer than
two weeks would not be particularly meaningful because most of
them would be dead by then regardless of the presence of a
nematocidal agent.16
In order to determine the minimum inhibitory concentration
(MIC) of the Fc-chalcones 31–63, they were initially tested at
10À6 and 10À5 M concentrations but no effect was observed in each
case. However, when the concentration was increased to 10À4 M,
biological activity was observed. A concentration of 10À3 M was
also attempted but the results were very similar to the those ob-
tained at 10À4 M, in addition to the fact that the 10À3 M solution
had a layer of crystals (presumably of Fc-chalcones) on the bottom
of the vessel. This suggested to us that a 10À4 M concentration is
near the saturation point of Fc-chalcones in DMSO. Subsequent
to the determination of MIC for Fc-chalcones, all compounds (1–
63) were tested at a concentration of 10À4 M in order to make a va-
lid comparison between the nematocidal activity of ferrocenyl
chalcones and their organic analogs.
A bioassay was constructed to test the ability of the compounds
to kill the worms (% mortality) and to inhibit their reproduction (%
fecundity). Each compound was screened for effectiveness by first
testing it on 10 nematodes at a time in parallel with a set of ten
worms in a control solution (1% DMSO + 99% liquid worm medium
without any test compound). Each test well (experiment or con-
trol) was monitored over a 14-day period (days 1, 3, 5, 7, 9, 11,
and 14 after incubation). Figure 2 shows the typical appearance,
under a dissecting microscope, of C. elegans nematodes at different
stages of the 14-day test.
structure–activity relationships (SARs), as discussed later in this
report.
Since investigating the nematocidal activity of Fc-chalcones was
the focus of this study, we decided to take a closer look at those
compounds that had shown significant activity in the preliminary
screening (i.e., 37, 43, 50, 51, 56, 61, and 62). Thus, in order to ob-
tain statistically relevant data, each of these seven compounds was
tested (in triplicate) using 96 nematodes, that is, one nematode in
each well of a 96-well microtiter plate, in parallel to a set of 96
controls as described before. Although compounds 37, 43, 51,
and 56 had exhibited promising biological activity in the prelimin-
ary tests (Fig. 3B), each yielded results very similar to the DMSO
control in the larger-scale tests, that is, many nematodes repro-
duced quickly and almost all survived. Thus, only Fc-chalcones
50, 61, and 62 were found to show significant activity relative to
the DMSO control. For comparison, the activities of the organic
analogs of these three compounds, that is, 18, 29, and 30 were also
determined. Plots of % mortality (A) or % fecundity (B) versus test
day (1–14) for compounds 29 and 61, shown in Figure 4A and B,
are representative of all three pairs of compounds.
Comparing the activities of the three pairs of compounds spec-
ified above, as represented by Figure 4A and B, reveals a few trends.
First, with respect to % mortality of nematodes, the organic chal-
cones were found to be more active than their ferrocenyl analogs
and seemed to exert their nematocidal influence at a much faster
rate (100% mortality for 18 and 30 on the first day of incubation
versus 92.0% for 62 on the 14th day).
Second, with respect to % fecundity (Figs. 5B, 6B and 7B), the six
chalcones studied resulted in significantly lower numbers of repro-
duced nematodes compared to the control and this activity, when
exhibited, remained fairly constant throughout the 14-day testing
period.
2.3. Molecular modeling studies
During a given test, the viability of each nematode was assessed
under a dissecting microscope by disturbing it with a probe and
looking for movement; percent mortality was then calculated for
each chalcone. In the same fashion, percent fecundity for each
chalcone was determined by counting the number of reproduced
nematodes.
Molecular modeling studies can provide useful information that
could be used in elucidating the structure–activity relationships
(SARs). There have been many reports involving molecular model-
ing studies of organic chalcones,12c,17 but only one on their ferroce-
nyl analogs.6b We chose compounds 51 and 61 for this purpose for
two reasons. First, we were able to use the structural parameters
(especially the torsion angles) obtained through X-ray crystallogra-
phy as starting points to build their molecular models. Second,
although both 51 and 61 are Type 2 Fc-chalcones, they exhibited
significantly different biological activities in the preliminary
screening as shown in Figure 3B (51: 23.2% mortality, 15.7% fecun-
dity; 61: 50.8% mortality, 0% fecundity). Therefore, a conforma-
tional analysis of these two compounds could reveal structural
features that play a role leading to such varied activity. Thus,
molecular models were built and optimized via the semi-empirical
PM-3 method (using software Spatan ‘08™).18 The conformational
profile of each compound was then determined by varying each of
the three torsion angles (h1, h2, h3; Table 3) in steps of 15°, with en-
ergy minimization in each step, while constraining the other two
angles. The results of these studies on the conformational energy
changes, along with the corresponding variation in the magnitude
2.2.2. Results of the biological studies
The results (% mortality and % fecundity) of the preliminary
screening of compounds 1–63, obtained on the third day of incuba-
tion, are presented in Figure 3A and B.
Day 3 was chosen for this purpose because that seemed to be
the appropriate time after incubation when the biological effects
of a given compound on C. elegans nematodes could easily be ob-
served. In these tests, we looked for compounds with significant
biological activity, that is, those compounds that caused high %
mortality (nematode death) and low % fecundity (nematode repro-
duction) relative to the control (DMSO) solution.
At the first glance of Figure 3, it becomes immediately obvious
that the organic chalcones are, as a class, more active than their
ferrocenyl analogs. For example, whereas eight organic chalcones
(1, 16, 17, 18, 19, 22, 25, and 30) showed 100% mortality (hence
0% fecundity) on the third day, none of the Fc-chalcones showed
that type of activity relative to the results obtained for the control
solution (compound ‘0’ in Fig. 3A and B; 1.9% mortality, 52.5%
fecundity). Other organic chalcones with significant nematocidal
activity were 15, 26, and 29. Among Fc-chalcones, compounds
37, 43, 50, 51, 56, 61, and 62 showed significant biological activity,
with 61 showing the highest activity (52.3% mortality, 0.0%
fecundity). Another trend that is revealed in both Figure 4A and
B is that, for both classes of chalcones, Type 2 compounds appear
to be more active than Type 1 analogs. This points towards certain
of dipole moment (l) for each conformer, are numerically shown
in Table 3 and graphically presented in Figures 5 and 6.
2.3.1. Conformational analysis of compound 51
Rotation around the C13–C14 bond in 51 generates the torsion
angle h1 (defined by C12–C13–C14–C15 atoms). As seen in Fig-
ure 5A (solid line with circles), this rotation produces two minima,
one at h1 = 240° (À120°) and the other at h1 = 315° (À45°). (Note:
The positive and negative signs denote the clockwise and counter-
clockwise rotations of the torsion angle, respectively, and can be

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2065
Table 3
Results of the conformational analysisa–d studies for 51 and 61
h (°)
Compound 51
E (h2) (h2)
Compound 61
E (h2) (h2)
D
E (h1)
l
(h1)
D
l
D
E (h3)
l
(h3)
D
E (h1)
l
(h1)
D
l
D
E (h3)
l (h3)
0
15
30
45
60
13.39
13.57
7.20
2.02
0.54
0.89
1.20
1.39
1.51
1.83
4.60
10.36
9.70
7.38
6.33
2.73
0.00
0.13
0.59
0.60
0.26
0.09
1.97
7.21
13.36
3.97
4.00
4.01
4.01
3.97
3.86
3.63
3.35
3.01
2.66
2.36
2.04
1.71
1.74
1.89
2.44
2.69
3.00
3.34
3.61
3.81
3.94
3.97
3.97
3.97
2.38
3.01
3.06
5.10
6.75
6.04
6.03
7.43
9.89
11.40
12.23
15.87
23.52
24.69
25.39
23.63
22.05
14.24
9.85
2.60
2.49
2.55
2.58
2.40
2.34
2.30
2.30
2.35
2.45
2.54
2.58
2.52
2.27
1.98
1.75
1.60
1.46
1.41
1.47
1.62
1.84
2.10
2.43
2.60
15.62
15.57
15.65
16.46
18.09
19.91
21.81
20.96
19.49
17.78
18.23
15.40
15.47
14.81
11.84
2.90
0.00
3.59
7.40
10.67
13.35
13.63
14.42
15.43
15.62
2.02
2.00
1.88
1.72
1.54
1.42
1.36
1.43
1.55
1.72
1.94
2.12
2.23
2.23
2.22
2.20
2.14
2.07
2.02
2.00
1.98
1.98
2.00
2.00
2.02
0.39
0.21
0.31
1.64
5.53
7.03
5.72
5.38
4.13
2.46
1.06
0.28
0.05
0.25
0.96
2.28
3.91
5.15
5.51
4.88
5.37
1.49
0.09
0.00
0.39
3.26
3.27
3.29
3.31
3.34
3.33
3.28
3.23
3.14
3.04
2.94
2.85
2.80
2.79
2.83
2.90
2.99
3.09
3.17
3.23
3.29
3.29
3.28
3.26
3.26
0.50
3.29
4.23
3.57
4.85
4.56
5.14
4.58
5.20
4.44
4.86
10.23
19.07
23.65
15.75
9.77
7.01
6.49
5.95
4.68
3.16
1.49
0.28
0.00
0.50
3.25
3.40
3.52
3.59
3.69
3.80
3.83
3.90
4.03
4.16
4.21
4.09
3.94
3.73
3.58
3.43
3.28
3.13
3.01
2.95
2.94
2.97
3.03
3.13
3.25
17.34
10.73
3.06
1.66
2.54
5.54
5.07
3.29
0.00
3.32
3.52
3.47
3.46
3.44
3.45
3.47
3.49
3.47
3.46
3.42
3.36
3.28
3.15
3.00
2.84
2.74
2.67
2.68
2.72
2.80
2.94
3.12
3.25
3.32
75
90
105
120
135
150
165
180
195
210
225
240
255
270
285
300
315
330
345
360
3.34
11.72
17.42
17.61
18.63
20.01
21.21
21.97
22.31
21.28
21.26
19.43
17.85
19.06
19.21
17.34
7.10
4.11
1.38
0.00
0.95
2.38
a
Conformational analysis was performed using the semi-empirical PM-3 method available through SPARTAN ’08.18
Torsion angles (h1, h2, h3) are defined in the structures for 51 and 61 (above) wherein the numbering system is based on X-ray crystal structures shown in in Figure 1A and
.
b
B, respectively.
c
D
E (kJ/mol) is the difference between the energy of a conformer and that of the lowest-energy conformer.
d
Dipole moment (l) in units of Debye.
defined either way depending on the position of the viewer relative
to the molecule.) In addition, the formation of other low-energy
conformers at h1 = 60°, 75°, 255°, 270°, 285°, and 300° indicates
the shallowness of the energy profile in this region of the molecule.
The most unstable conformers were formed at h1 = 0°, 15°, 165°,
to that of the syn conformer was attributed to the steric repulsion
between the ortho-F and the carbonyl oxygen atoms in the latter.
This was confirmed by X-ray crystallography whereby it was found
that the FÁ Á ÁO distance of 4.048 Å in the anti conformer is 4.048 Å,
as compared to 2.772 Å in the syn conformer (calculated for
h1 = 0°).
and 180° with
D = 13.4, 13.6, 10.4, and 9.4 kJ/mol, respectively, rel-
ative to the most stable conformer. The calculated energies of the
unstable conformers are explained based on the steric interaction
between the ortho-F atom and the carbonyl oxygen atom when
Next, we will consider the conformational analysis around the
torsion angle h2 in 51, which is generated by rotation around the
C12–C13 bond and is defined by C11–C12–C13–O1 (comprising
the enone portion of this molecule). As seen in Figure 5A (dashed
line with squares), the most stable conformer is formed at
h2 = 330°, with the next low-energy conformer formed at
h2 = 345° (less stable by 0.95 kJ/mol). Overall, the energy profile
around h2 is not as shallow as that around h1. In addition, as com-
pared to h1, the magnitude of h2 = 330° (or À30°) for the most sta-
ble conformer of 51 is in better agreement with that found in its
crystal structure (À21.7° or 338.3°; Table 2). Furthermore, the con-
formational analysis around h2 in 51 confirms the general finding
that the s-cis conformer is more stable relative to the s-trans con-
former, as shown for chalcone 1 in Scheme 2B. As seen in Table 3
for 51, at h2 = 0 (s-cis conformer) and 180° (s-trans conformer),
h1 = 0°, as well as that between the ortho-F atom and the a-hydro-
gen atom (attached to C12) when h1 = 180°. Although the magni-
tude of h1 (240°) for the most stable conformer of 51 is different
from that found in its crystal structure (À32.6° or 327.4°; Table 2),
the latter value corresponds to a low-energy (stable) conformation
on the solid line in Figure 5A, which is generally expected for a
structure found in the solid state.
The conformational analysis around h1 in 51 also predicts, in a
general way, the greater stability of the anti conformer, relative
to that of the syn conformer (shown in Scheme 3A). For example,
as seen in Table 3, at h1 = 0° and 15° (roughly corresponding to
the syn conformation),
DE(h1) = 13.39 and 13.57 kJ/mol, respec-
tively; both of the latter values correspond to the highest points
in the solid-line plot in Figure 5A. On the other hand, the h1 values
of 240° (calcd) and 324° (exptl.), representing anti conformers of
51, correspond to the lowest points on the same plot in Figure 5A.
In Section 2.1.3, the greater stability of the anti conformer relative
D
D
E(h2) = 2.38 and 23.52 kJ/mol, respectively. The higher value of
E(h2) for the s-trans conformer can be attributed to a repulsive
electrostatic interaction between the ortho-F and b-H atoms due
to a close FÁ Á ÁH distance of 2.320 Å as compared to 4.823 Å for
the s-cis conformer (both calculated by molecular modeling). This

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Figure 5. Conformational energy profiles as plots of relative energy (kJ/mol) versus torsion angles h1, h2, or h3 for compounds 51 (plot A) and 61 (plot B).
distance is found to be 4.490 Å in the actual solid-state structure of
51, which is very close to that found for the s-cis conformer, thus
verifying in a direct manner the validity of the molecular modeling
calculations presented here.
Moving on to the torsion angle h3 in 51, generated by rotation
around C6–C11 and defined by C10–C6–C11–C12, we find that
the most stable conformer occurs at h3 = 240°, which differs signif-
icantly from that obtained from crystallography (À4.4° or 355.6°;
Table 2). Similar to the conformational analysis of h2, the energy
profile around h3 is not shallow. Overall, based on the three relative
energy curves in Figure 5A, one can conclude that the energy pro-
file of compound 51 is affected the most by the variations in either
h2 or h3 as compared to those in h1.
51, the calculated h1 value of 345° in 61 (À15°) is very close to that
found in its crystal structure (À16.3°; Table 2).
Unlike the case of compound 51, the preference of the syn con-
formation in 61 (i.e., the co-planarity of carbonyl oxygen and fura-
nyl ring oxygen atoms; Scheme 3B) is not immediately obvious
from the conformational analysis around h1 in this compound. As
mentioned in Section 2.1.3, the observance of a comparable syn
conformation in a thiophenyl derivative of chalcone has been
attributed to an attractive electrostatic interaction between the
sulfur atom of the thiophene ring and the oxygen atom of the car-
bonyl group.15 However, such an argument would not be useful
here since the two atoms in the case of 61 are both oxygens
(OÁ Á ÁO distance = 2.778 Å by X-ray crystallography) and are thus
expected to result in a repulsive interaction. The conformational
2.3.2. Conformational analysis of compound 61
analysis around h1 in 61 (Table 3) shows that
mol at h1 = 0° (corresponding to the syn conformation), whereas
E(h1) = 0.05 kJ/mol for h1 = 180° (corresponding to the anti con-
former). Thus, the anti conformer is predicted (via molecular mod-
eling) to be more stable than the syn conformer by only 0.34 kJ/
mol.
DE(h1) = 0.39 kJ/
We will now consider the energy profile for compound 61. As
shown in Table 3 and Figure 5B (solid line with circles), rotation
around the C13–C14 bond (torsion angle h1, defined by C12–
C13–C14–C15) generates many low-energy conformers including
the three minima at h1 = 345°, 180°, and 330°. As was found for
D

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2067
Figure 6. Plots of dipole moment versus torsion angles h1, h2, or h3 for compounds 51 (plot A) and 61 (plot B).
Scheme 4. Canonical forms of the syn and anti conformers of 61 showing the favorable electrostatic interaction in the former.
The X-ray crystallographic finding on the preference of the syn
conformation over anti, despite the calculated greater stability (al-
beit small) of the latter conformer, may be justified by a simple
explanation. As depicted in Scheme 4, three canonical forms
(among many) may be drawn for each of the two conformers of
61 under discussion. Subsequently, one may argue that the syn

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
conformation in 61 allows for a favorable (stabilizing) electrostatic
attraction between the furanyl ring oxygen and the carbonyl oxy-
gen atoms whereas this is not possible for the anti conformation.
Such interactions have a small effect on the overall stability of
the resonance hybrid for each conformer, hence their ready inter-
conversion via low-energy rotational barriers.
Finally, a comparison of Figures 5A and 6A on the one hand, and
of Figures 5B and 6B on the other, reveals an interesting trend
among the conformers of 51 and 61, respectively. For example, if
one compares the conformational energy changes for the rotation
around h1 in 51 (Fig. 5A, solid line with circles) with those in the
conformational dipole moments for the rotation around the same
torsion angle (Fig. 6A, solid line with circles), one notices that the
minimum in dipole moment curve (at h1 = 180°) coincides with a
maximum on the relative energy curve. Inspections of other curves
in Figures 5A and 6A leads one to the general conclusion that, for
compound 51, the more stable conformers are also more polar.
However, analyses of the six curves in Figures 5B and 6B reveals
an opposite trend for compound 61, that is, the more stable con-
formers are the less polar ones. These findings may have implica-
tions in the nematocidal activities of these two compounds
(discussed further in Section 2.4).
Rotation around the C12–C13 bond in 61 generates the torsion
angle h2 (defined by C11–C12–C13–O1). A plot of
DE versus h2
(Fig. 5B, dashed line with squares) shows two minima at
h3 = 345° and 0°, which are separated in energy by 0.5 kJ/mol.
The latter minimum occurs at a h1 value that is very close to that
found in the crystal structure of 61 (À0.60°; Table 2). Furthermore,
as was the case for 51, the conformational analysis around h2 in 61
confirms again the greater stability of the s-cis conformer relative
to that of the s-trans conformer. As seen in Table 3 for 61, at
h2 = 0 (s-cis conformer) and 180° (s-trans conformer),
0.50 and 19.07 kJ/mol, respectively. The higher value of
D
E(h2) =
E(h2)
D
for the s-trans conformer can be attributed to a repulsive electro-
static interaction between the ortho-H (on the furanyl ring) and
b-H atoms at a HÁ Á ÁH distance of 2.090 Å, as compared to 5.115 Å
for the s-cis conformer (both calculated by molecular modeling).
This distance is found to be 5.122 Å in the actual solid-state struc-
ture of 61, which is very close to that found for the s-cis conformer.
Finally, rotation around the C1–C11 bond in 61 generates the
torsion angle h3 (defined by C2–C1–C11–C12). As shown in Fig-
ure 5B (dashed line with triangles), there is only one minimum
at h3 = 120°. As was observed for 51, the conformational profile
2.4. Structure–activity relationships (SARs)
The apparent structural attributes that seem to be responsible
for the biological activity of chalcones against C. elegans nematodes
in this study will be discussed below.
(1) The placement of the carbonyl group has a profound effect
on the biological activity of both organic chalcones and their ferr-
ocenyl analogs, at least with respect to C. elegans nematodes. This
is reflected in the observation that Type 2 compounds are, in gen-
eral, more active that those of Type 1 (Fig. 3). This means that, the
most active compounds are those where the carbonyl group is at-
tached to the substituted phenyl (or heterocyclic) ring in the or-
ganic series, and those where the carbonyl group is not attached
to the to ferrocenyl group in the Fc-series. Our observations on
the nematocidal activities of Fc-chalcones are opposite to those re-
ported by Go and co-workers6a,b wherein Type 1 Fc-chalcones were
found to show greater antimalarial activity. However, our finding
that Fc-chalcones of Type 2 with a heterocylic ring attached the
carbonyl group (i.e., 61 and 62) are the most active coincides with
that of Go et al. since the most active antimalarial compound in
their study was found to be 60. Any observed differences between
our results and those obtained by Go et al. may simply be due to
the fact that our biological studies were on different species which
may point to difference mechanisms and modes of action for these
apparently comparable Fc-chalcones.
(2) The lipophilicity of the chalcone may play a significant role in
its nematocidal activity. In a study on the antileishmanial activity of
a series of organic chalcones, Go et al.21 found that this activity is
associated with less lipophilic organic chalcones while antimalarial
activity is associated with more lipophilic ones. In a later study on
the antimalarial activity of Fc-chalcones,6b Go and co-workers
determined the lipophilicity of these types of compounds experi-
mentally from capacity factors measured by reversed-phase HPLC
(log kw, pH 7.0). Thus, Fc-chalcones were found to be lipophilic with
log kw values in the range 2.7–5.1. In addition, Type 1 Fc-chalcones
were found to be less lipophilic (log kw = 3.63 0.56, n = 15) than
their Type 2 analogs (log kw = 3.87 0.46, n = 15). In addition, it
was shown that, in general, Type 1 compounds exhibited greater
antimalarial activity than their Type 2 analogs unless Ring B was
a substituted heterocycle such as 3-pyridinyl (60). This apparent
contradictory observation may mean that, at least in the Fc-chal-
cone series, the magnitude of a compound’s lipophilicity may not
be a significant predictor of its antimalarial activity. A comparison
of this SAR with the apparent SAR observed in our work with re-
spect to C. elegans suggests that lipophilicity may indeed be a signif-
icant factor in the nemaocidal activity of these Fc-chalcones since
Type 2 compounds showed higher activity than their Type 1 ana-
logs (Fig. 3A), including the three most effective ones (50, 61, and
62).
around h3 in 61 is not shallow (the lowest
DE is at 2.54 kJ/mol).
In addition, it is observed that among the three torsion angles in
61, h3 shows the largest discrepancy between the calculated value
of the lowest-energy conformer (at 120°) and that found in the
crystal structure (À11.2° or 348.8°). As was observed for 51, it is
noted that the conformational energy profile of 61 is also affected
the most by the variations in either h2 or h3 as compared to those
in h1.
2.3.3. Conformational dipole moment
Another factor that one may consider in a conformational anal-
ysis study is the change in the conformational dipole moment (l)
as a function of the three torsion angles mentioned. Dipole mo-
ment is an important physical attribute of a molecule since it is di-
rectly related to its polarity, a property that can affect both the
passage of a molecule through the membrane of a target cell as
well as its interaction with a specific enzyme. There have been re-
ports of both experimental19a,b and theoretical19c work regarding a
direct relationship between biological activity of a molecule and its
dipole moment. The data obtained on the variation of the confor-
mational dipole moments of compounds 51 and 61 as a function
of the three torsion angles are presented numerically in Table 3
and pictorially in Figure 6A and B. The change in the conforma-
tional dipole moments of compounds 51 and 61 as a function of
the three torsion angles will be discussed below and the possible
effect of the molecular polarity of chalcones upon their nematocid-
al activity will be discussed in Section 2.4.
Each Figure 6A and B is a composite of three plots of dipole mo-
ment (l) versus torsion angles (h1, h2, or h3) for compounds 51 and
61, respectively. For the most part, the six curves in Figure 6A and B
have different appearances with some occasional common features.
First, although the variation in m as a function of h1 is much more
drastic in 51 than that in 61, varying h1 in both compounds pro-
duces similar plots (solid line with circles) wherein the least polar
conformer occurs at around 200°. Second, varying either h2 or h3
in 51 and 61 produces very different curves, although the least polar
conformer for both compounds occurs at h2 ꢀ 275°. Third, the con-
formational dipole moments for 51 (1.41–4.01 D) encompass a
wider range as compared to those for 61 (2.67–4.21 D).

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2069
(3) The above discussion on the effect of a chalcone’s lipophilic-
ity on its biological activity, coupled with the findings on the
trends in conformational energy changes and their corresponding
dipole moments in compounds 51 and 61 (as discussed in Sec-
tion 2.3), prompted us to search for a possible connection between
the dipole moment (hence polarity) of a chalcone and its nemato-
cidal activity across the 63-compound series prepared for this
study. For this purpose, models of all organic chalcones (1–30)
and the remaining Fc-chalcones (31–50, 52–60, and 62–63) were
built using SPARTAN ‘0818 and their calculated dipole moments were
obtained. It was found that the organic chalcones prepared for this
3). A similar correlation between planarity of an Fc-chalcone and
its biological activity was reported by Go and co-workers6b where
they found that, for compounds 37, 58, and 43, the magnitudes of
the derived torsion angle C12–C11–C1–C2 (reflecting the planarity
of the enone moiety) increase in the order À3.4° < 163.0° < 170.6,
while their antimalarial IC50 values increase in the order
5.1 mM < 80 mM < 175 mM, respectively. Thus, the most planar
Fc-chalcone (among the three studied crystallographically) is also
the most active. Therefore, one conclusion that might be drawn
here is that, although it might appear that the enone moiety may
not play a major direct role in the observed nematocidal activity
of Fc-chalcones (as stated in #3 above), one might say that this
fragment plays an indirect role in such activity by keeping the
whole molecule planar.
study are generally less polar (average
l = 2.86 D) than their ferr-
ocenyl analogs (average = 3.34 D). This finding should be viewed
l
in light of an earlier statement (Section 2.2.2; Fig. 3) that the organ-
ic chalcones, as a class, exhibit higher nematocidal activity com-
pared to Fc-chalcones. In addition, it was mentioned above
(Section 2.3) that, for compound 61, more stable conformers corre-
spond to the less polar ones while the opposite trend is found for
51. Again, this finding should be viewed in light of the observation
that 61 showed much higher nematocidal activity compared to 51.
There are two complementary explanations for the above find-
ings on the apparent relationship between the molecular polarity
and the nematocidal activity of the chalcones. First, the Fc-
chalcones may have difficulty passing through the non-polar cell
membranes due to their higher polarities as compared to those
of the organic chalcones. This rationale may also be used to ac-
count for the much higher nematocidal activity of the less polar
61 compared to the more polar 51. Second, the solvent used for dis-
solving these chalcones in the biological media prepared to feed
the nematodes was DMSO. As a polar, aprotic solvent, DMSO would
be expected to stabilize the more polar species. Therefore, Fc-chal-
cones would then be stabilized in solution relative to their organic
counterparts, which translates into the lower reactivity of the
former.
(4) The observation that chalcones with aromatic heterocyclic
rings attached to the carbonyl group showed very high nematocid-
al activity in both sets of compounds may suggest that pKa values,
in addition to lipophilicity, may play an important role in their bio-
logical activity.
(5) The presence of the enone portion in the chalcone frame-
work may not be essential for its biological activity. This may be
inferred from the spectral data (1H NMR, IR) obtained for these
compounds (Table 1A and B), which remain relatively unchanged
regardless of the variation in their biological activity. Biological
activity data in support of this observation has been reported in
the literature. For example, Christensen and co-workers22 found
that reduction (or alkylation) of the C@C bond in a series of organic
chalcones affects their antimalarial activity only slightly as
compared to the potencies of the parent chalcones. On the other
hand, Go et al.6b found that reduction of the C@C bond in some
Fc-chalcones had varied effects upon their antiplasmodial activity,
(7) The observation that the placement of the ferrocenyl group
makes a difference in its biological activity (e.g., Type 1 vs Type 2
for both nematocidal activity and antimalarial activity6a,b) leads
one to the conclusion that the Fc group does not merely play a bio-
isosteric role here.1,2 However, the exact role that this moiety plays
in the overall biological activity of Fc-chalcones is poorly under-
stood. Although the incorporation of a ferrocenyl moiety into some
molecules with established biological activity (e.g., ferrocifen3a and
ferrochloroquine3c–e) has yielded fruitful results, such incorpora-
tion of the Fc group into other well-known classes of compounds
such as chalcones has been less successful in terms of increasing
the biological activity of the Fc-substituted analogs. It may be, as
Go et al.6b have also pointed out, that it is an interplay of factors
other than the redox properties of the ferrocenyl group (i.e., lipo-
philicity, pKa, and other physicochemical properties) that deter-
mines the biological activity of an Fc-chalcone.
2.5. Possible mode of action of Fc-chalcones
Although the underlying mechanism for the biological activity
of the current set of Fc-chalcones against C. elegans is not known,
some may be postulated. As suggested by González and Estévez-
Braun,9b the nematocidal activity of organic chalcones, in general,
may result from their role as uncouplers of the oxidative phosphor-
ylation processes in the mitochondria. It seems probable that the
ferrocenyl analogs could also act at the mitochondrial level inhib-
iting or uncoupling respiration.
The mode of action of the Fc-chalcones in nematodes could also
result from the electrochemical properties of the Fe2+ center in the
ferrocenyl moiety. In their work with ferrocifen, Jaouen et al.3a
have proposed that the iron atom, due to its ability to undergo
reversible oxidation and reduction, can lead to the formation of
ferrocifen-localized hydroxyl radicals as Fe2+ is oxidized to Fe3+
by molecular oxygen present in the body. This enables ferrocifen
to bind to estrogen receptor proteins, resulting in the subsequent
dimerization of these proteins. The attachment of such protein
dimmers to DNA can halt or reduce the growth of a tumor.
In regards to the DNA binding of Fc-chalcones, Qureshi and co-
workers20 have recently reported on their investigation of such
interaction between 1-ferrocenyl-3-(4-nitrophenyl)-2-propen-1-
one (37, Chart 2) and DNA extracted from chicken blood. Using a
combination of experimental techniques such as cyclic voltamme-
try, UV–vis spectroscopy, and viscometry these authors have deter-
mined the binding constant, binding site size, diffusion coefficients,
and radii of both free and DNA-bound Fc-chalcone. The results of
that study have revealed that (a) the binding interaction is sponta-
neous, and (b) the intercalation of Fc into DNA is the dominant
mode of this interaction. Thus, by analogy, it is quite possible that
the nematocidal activity observed for the Fc-chalcones prepared in
our work results from the binding of these molecules to the C.
elegans DNA, which could also explain reproductive inhibition in
the nematodes.
wherein three Type
1 compounds showed reduced activity
whereas their Type 2 isomers showed improved activity. These
observations may suggest that the enone fragment functions
mostly as a spacer between the two aromatic rings, which are
the main pharmacophores.
(6) The extent of planarity of the Fc-chalcone molecule seems to
play a role in its observed nematocidal activity. This is evident
from the results of the X-ray crystallographic study of compounds
51 and 61 obtained in our work, in addition to those obtained by
Go et al.6b for 37, 43, and 58. The derived torsion angle C12–
C13–C14–C15, which reflects the planarity of the enone moiety
has a magnitude of À32.6° and À16.3° for 51 and 61, respectively
(Fig. 1 and Table 2). The smaller deviation of 61 from planarity
can be associated with its higher nematocidal activity (50.8% mor-
tality on day 3) as compared to that of 51 (23.2% mortality on day

2070
S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
Table 4
Crystal data and structure refinement for Fc-chalcones 51 and 61
Compound 51
Compound 61
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C
19H14F2FeO
C17H14FeO2
306.13
208(2)
0.71073
Monoclinic
P2(1)
352.15
208(2)
0.71073
Orthorhombic
Pbca
Space group
Unit cell dimensions
a = 7.3656(13) Å,
a
= 90°
a = 5.828(7) Å, a = 90°
b = 11.439(2) Å, b = 90°
b = 11.025(12) Å, b = 102.699(15)°
c = 35.139(6) Å,
2960.6(9)
8
1.580
1.041
c
= 90°
c = 10.750(12) Å, c = 90°
673.8(13)
2
1.509
1.116
316
Volume (Å3)
Z
Density (calcd) (Mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
1440
Crystal size (mm3)
h range for data collection
Index ranges
0.40 Â 0.40 Â 0.20
0.15 Â 0.07 Â 0.05
2.32–28.52°
1.94–27.56°
À9 6 h 6 4, À15 6 k 6 14, À44 6 l 6 46
18736
3607 [R(int) = 0.0566]
99.9%
Semi-empirical from equivalents
0.8188 and 0.6809
Full-matrix least-squares on F2
3607/0/209
À5 6 h 6 7, À14 6 k 6 9, À13 6 l 6 11
3906
2398 [R(int) = 0.0551]
99.3%
Semi-empirical from equivalents
0.9463 and 0.8505
Full-matrix least-squares on F2
2398/1/182
Reflections collected
Independent reflections
Completeness to h = 25.00°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1.092
1.062
Final R indices [I > 2
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
r
(I)]
R1 = 0.0433, wR2 = 0.1072
R1 = 0.0463, wR2 = 0.1105
0.0133(9)
R1 = 0.0787, wR2 = 0.1992
R1 = 0.1033, wR2 = 0.2138
0.009(6)
0.605 and À0.400 (e ÅÀ3
)
2.235 and À0.735 (e ÅÀ3
)
means that these results do not necessarily lead to an effective way
of combating plant-pathogenic nematodes. Tests would have to be
carried out to determine whether the results obtained for C. elegans
carry over to other species of nematodes. (3) Concentrations of solu-
tion of Fc-chalcones used in our work could not exceed 10À4 M in or-
der to avoid precipitation of these compounds, thus precluding the
possibility of finding a concentration at which all nematodes were
killed. Further research is currently underway to determine whether
adding hydrophilic substituents to the existing Fc-chalcones can in-
crease their water solubility in an effort to increase the maximum
concentration and reduce the amount of DMSO needed. (4) The com-
pounds synthesized for this study do not constitute an exhaustive
list of Fc-chalcones by any means. Compounds with other substitu-
tion patterns and different heteroatoms could be synthesized and
their biological activities investigated in the same way as in this
study. Different metals could be used to make different metallocenyl
chalcones. Different oxidation/reduction and electronegativity
properties of the metalscould cause the compoundsto have very dif-
ferent biological activities. Also, the ferrocene moiety can be func-
tionalized as well to give very different properties that might be
interesting in this study. (5) An important investigation would be
to ascertain what environmental impact, if any, would these com-
pounds have. The deleterious effects these compounds have on fish,
if any, would need to be established in the event of water contami-
nation. The effects on humans or animals would need to be known
in case these compounds happen to be effective against human- or
animal-pathogenic nematodes.
3. Conclusion
The main goal of this project was to contribute to the database
on biological activity of ferrocenyl derivatives of chalcones through
a comparative study of a series of such compounds along with their
organic analogs. To that end, we have successfully synthesized and
characterized a series of 63 compounds, which included 30 organic
chalcones and 33 of their ferrocenyl analogs, and have determined
their nematocidal activities against the model organism C. elegans.
Some structure–activity relationships (SARs) have been deter-
mined through a combination of spectral (1H NMR, IR), X-ray crys-
tallographic, and molecular modeling studies. First, the less polar
organic chalcones were found to be much more active against C.
elegans than their more polar Fc analogs. This indicates that the
lipophilicity of a chalcone may play a significant role in its nemato-
cidal activity as it relates to the chalcone’s ability to pass through
the organism’s cell walls. Second, for both the organic and the ferr-
ocenyl series, Type 2 compounds (wherein substituted phenyl
group or heterocycle is directly attached to the carbonyl carbon
atom) were found to be more active than their Type 1 isomers.
Third, the planarity of a chalcone (at least in the Fc series) seems
to play a role in its nematocidal activity.
3.1. Future work
There are a few observations that point to possible future direc-
tions of this research. (1) One interesting note about this research
is that the nematodes treated with the active compounds, for the
most part, did not reproduce (i.e., failed to produce embryos). Nem-
atodes thatdid produce embryoswhile being treated often died soon
after. The nematodes that hatched from these embryos often died
soon after as well, suggesting that these particular Fc-chalcones
are good nematocides and can inhibit reproduction but are not able
to penetrate the protective cuticle on the embryos. Future tests
could be run on gravid adult nematodes instead of those in early lar-
val stages to verify that this is the case. (2) One should keep in mind
that the model nematode in this study is a free-living species, which
4. Experimental
4.1. Chemistry
4.1.1. General
All reagents used in syntheses were purchased from commercial
sources (Acros or Aldrich) and were used as received without any
further purification. 1H NMR spectra were obtained using a 200-
MHz Varian Gemini-2000 spectrometer with CDCl3 as the solvent.

S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
2071
Chemical shift values (d) are expressed in ppm and are relative to
that of TMS as the internal standard. The multiplicity of each NMR
signal is defined as one of the following: s (singlet), d (doublet), t
(triplet), q (quartet), dd (doublet of doublets), or m (multiplet). All
IR spectra were obtained as thin, dry films on polished NaCl plates
(after evaporation of CH2Cl2 or CDCl3 solvent) using a Nicolet Avatar
320 FT-IR spectrophotometer. All melting points were obtained
using an Electrothermal Mel-TempÒ apparatus and are uncorrected.
Elemental analyses (C and H only) for the new compounds were per-
formed by Galbraith Laboratories, Inc. (Knoxville, TN).
4.1.2.3.
(2E)-1-Ferrocenyl-3-(2-diphenylphosphinophenyl)-2-
propen-1-one (39).
Yield 69.4%; mp (dec with darkening
>100 °C); 1H NMR: 7.87 (d, 1H), 7.71–6.82 (m, 15H), 4.77 (s, 2H),
4.50 (s, 2H), 4.20 (s, 5H); IR: 1650, 1586; Anal. Calcd for
C31H25FeOP: C, 74.41; H, 5.05. Found: C, 74.17; H, 5.11.
4.1.2.4. (2E)-1-(2-Methylphenyl)-3-ferrocenyl-2-propen-1-one
(44).
Yield 92.3%; mp 134–135 °C; 1H NMR: 7.50 (d, 1H),
7.27–7.21 (m, 4H) 7.04 (d, 1H), 4.62 (dd, 2H), 4.52 (dd, 2H), 4.19
(s, 5H), 2.40 (s, 3H); IR: 1657, 1589; Anal. Calcd for C20H18FeO: C,
72.74; H, 5.51. Found: C, 72.63; H, 5.33.
4.1.2. Synthesis and spectroscopic characterization of chalcones
All syntheses were carried out in the same fashion. Each reaction
was monitored by TLC for 24 h to determine when the starting mate-
rials had been consumed. All TLC analyses were run on Selecto Scien-
4.1.2.5. (2E)-1-(2-Chlorophenyl)-3-ferrocenyl-2-propen-1-one
(47).
4H), 7.33 (d, 1H), 6.70 (d, 1H), 4.59 (dd, 2H), 4.51 (dd, 2H), 4.19
(s, 5H); IR: 1639, 1591; Anal. Calcd for C19H15ClFeO: C, 65.08; H,
4.32. Found: C, 65.29; H, 4.58.
Yield 84.7%; mp 121–123 °C; 1H NMR: 7.82–7.34 (m,
tific flexible silica gel-coated plates containing
a fluorescent
indicator and were developed using a hexanes–ethyl acetate (4:1)
solution as the eluent. The following procedure is representative of
the synthesis of all chalcones (see Charts 1 and 2 for structures): A
25-mL round-bottomed flask was charged with the appropriate
derivatives of both acetophenone (3 mmol) and benzaldehyde
(3 mmol), and mixed with 10 mL of 95% EtOH. The mixture was then
stirred magnetically while being gently heated (in a 30 °C water
bath) until both starting materials dissolved. In a separate flask,
NaOH (3.5 mmol) was added to 10-mL of an ethanol–water (1:1)
and the mixture was stirred magnetically until the solid dissolved.
The NaOH solution was then added dropwise (using a Pasteur pipet)
to the ethanolic solution of acetophenone and benzaldehyde de-
scribed above. In most cases, the reaction mixture turned yellow
and solidified within a few minutes. Ice water (2 mL) was added to
the flask and the mixture was stirred vigorously. The solid was col-
lected on a Hirsch funnel, washed with cold water, and air-dried
overnight. The purity of the crude product was determined at this
point using a combination of TLC analysis, melting point measure-
ment, and 1H NMR spectroscopy. In case of an oily product, the reac-
tion mixture was extracted with two 10-mL portions of CH2Cl2 and
the organic phase was collected, dried over Na2SO4, and removed
using a rotary evaporator. The purity of the oily product was then
determined as described above. All impure products (solid or oil)
were purified by column chromatography using Merck Silica gel
(grade 60, 230–400 mesh) and 4:1 hexanes–ethyl acetate as eluent.
In case of a solid, chromatographic separation was followed by
recrystallization from either methanol or ethanol–water mixture.
In all cases, the purity of the final product was checked again as de-
scribed above; the spectral characteristics were found to be in good
general agreement with those found in the literature.4 The organic
chalcones prepared for this study were either pale-yellow solids or
oils of the same color (as specified); the ferrocenyl analogs were red-
dish-orange solids or oils. For each of the reported compounds be-
low, the 1H NMR data is presented as d (multiplicity, integral
4.1.2.6. (2E)-1-(3-Chlorophenyl)-3-ferrocenyl-2-propen-1-one
(48).
7.81 (d, 1H), 7.58–7.39 (m, 3H), 7.06 (d, 1H), 4.62 (s, 2H), 4.52 (s,
2H), 4.20 (s, 5H); IR: 1656, 1582; Anal. Calcd for C19H15ClFeO: C,
65.08; H, 4.32. Found: C, 64.89; H 4.15.
Yield 72.3%; mp 139–141 °C; 1H NMR: 7.89 (d, 1H),
4.1.2.7. (2E)-1-(2,4-Dichlorophenyl)-3-ferrocenyl-2-propen-1-
one (49).
Yield 97.1%; mp 143–145 °C; 1H NMR: 7.89 (d,
1H), 7.47 (s, 1H), 7.34 (m, 2H), 7.30 (d, 1H), 6.67 (d, 1H), 4.53
(dd, 2H), 4.50 (s, 2H), 4.17 (s, 5H); IR: 1644, 1584; Anal. Calcd for
C19H14Cl2FeO: C, 59.26; H, 3.67. Found: C, 59.02; H, 3.53.
4.1.2.8. (2E)-1-(2,4-Difluorophenyl)-3-ferrocenyl-2-propen-1-
one (51).
Yield 63.4%; mp 105–107 °C; 1H NMR: 7.94–7.62
(m, 3H), 7.77 (d, 1H), 7.21 (d, 1H), 4.61 (dd, 2H), 4.50 (s, 2H),
4.19 (s, 5H); IR: 1655, 1595; Anal. Calcd for C19H14F2FeO: C,
64.79; H, 4.02. Found: C, 64.63; H, 4.13.
4.1.2.9. (2E)-1-(2-Methoxyphenyl)-3-ferrocenyl-2-propen-1-one
(53).
Yield 80.2%; mp 107–109 °C; 1H NMR: 7.78 (d, 1H),
7.25–7.19 (m, 4H) 7.08 (d, 1H), 4.64 (s, 2H), 4.52 (s, 2H), 4.19 (s,
5H), 3.90 (s, 3H); IR: 1652, 1595; Anal. Calcd for C20H18FeO2: C,
69.38; H, 5.25. Found: C, 69.05; H, 5.13.
4.1.2.10. (2E)-1-(3,4-Dimethoxyphenyl)-3-ferrocenyl-2-propen-
1-one (55).
Yield 66.4%; mp 93–95 °C; 1H NMR: 7.74 (d, 1H),
7.57 (d, 1H), 7.15 (d, 1H), 6.85–6.93 (dd, 2H), 4.59 (dd, 2H), 4.46
(dd, 2H), 4.17 (s, 5H), 3.98 (s, 3H), 3.95 (s, 3H); IR: 1652, 1575;
Anal. Calcd for C21H20FeO3: C, 67.03; H, 5.37. Found: C, 67.08; H,
5.23.
4.1.2.11. (2E)-1-(3,4,5-Trimethoxyphenyl)-3-ferrocenyl-2-pro-
ratio), and the IR data as mC@O
,
m
C@C. The following % yield and phys-
pen-1-one (56).
Yield 89.4%; mp 81–83 °C; 1H NMR: 7.76
ical data are for new chalcones prepared for this study. Data for other
chalcones (not given below) have been reported elsewhere4–6 and
are also available online as Supplementary data.
(d, 1H), 7.23 (s, 2H), 7.07 (d, 1H), 4.61 (dd, 2H), 4.49 (dd, 2H),
4.19 (s, 5H), 3.96 (s, 3H), 3.94 (s, 6H); IR: 1649, 1570; Anal. Calcd
for C22H22FeO4: C, 65.03; H, 5.47. Found: C, 64.94; H, 5.62.
4.1.2.1. (2E)-1-Ferrocenyl-3-(2-methylphenyl)-2-propen-1-one
(32).
4.1.2.12.
(61).
(2E)-1-(2-Furanyl)-3-ferrocenyl-2-propen-1-one
Yield 90.1%; mp 134–135 °C; 1H NMR: 7.77 (d, 1H),
Yield 95.1%; mp 107–108 °C; 1H NMR: 7.79 (d, 1H),
7.65–7.57 (m, 2H); 7.42–7.39 (m, 2H), 7.13 (d, 1H), 4.92 (dd, 2H),
4.62 (dd, 2H), 4.21 (s, 5H), 2.40 (s, 3H); IR: 1652, 1595; Anal. Calcd
for C20H18FeO: C, 72.74; H, 5.51. Found: C, 72.79; H, 5.45.
7.60 (s, 1H), 7.24 (s, 1H), 7.02 (d, 1H), 6.53 (s, 1H), 4.56 (dd, 2H),
4.45 (s, 2H), 4.13 (s, 5H); IR: 1644, 1582; Anal. Calcd for
C17H14FeO2: C, 66.69; H, 4.62. Found: C, 66.85; H, 4.43.
4.1.2.2.
one (38).
(2E)-1-Ferrocenyl-3-(2-methoxyphenyl)-2-propen-1-
Yield 89.2%; mp 112–114 °C; 1H NMR: 8.10 (d,
4.1.2.13.
(62).
(2E)-1-(2-Thiophenyl)-3-ferrocenyl-2-propen-1-one
Yield 84.9%; mp 116–118 °C; 1H NMR: 7.80 (d, 1H),
1H), 7.74–7.66 (m, 2H), 7.38–7.20 (m, 2H), 7.07 (d, 1H), 4.93 (dd,
2H), 4.60 (dd, 2H), 4.23 (s, 5H), 3.90 (s, 3H); IR: 1656, 1581; Anal.
Calcd for C20H18FeO2: C, 69.38; H, 5.25. Found: C, 69.20; H, 5.31.
7.52 (s, 1H) 7.18 (s, 1H), 7.03 (d, 1H), 6.49 (s, 1H), 4.61 (s, 2H),
4.50 (s, 2H), 4.19 (s, 5H); IR: 1642, 1575; Anal. Calcd for
C17H14FeOS: C, 63.36; H, 4.39. Found: C, 63.56; H, 4.24.

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S. Attar et al. / Bioorg. Med. Chem. 19 (2011) 2055–2073
4.1.3. X-ray crystallography
during which the percent fecundity (number of nematodes that
reproduced) and percent mortality (number of dead nematodes)
values were tabulated. Viability of the nematodes was tested under
the dissecting microscope by examining each for movement after
disturbing it with a probe. Data was collected after 1, 3, 5, 7, 9,
11, 14 days of incubation.
Suitable crystals of compounds 51 and 61 were obtained by
gradual layering of hexanes over a methylene chloride solution of
each. Single-crystal X-ray structure determination was carried out
at 208(2) K on a Bruker P4 Diffractometer equipped with a Bruker
APEX detector. Mo K
structure was solved by direct methods with SHELXS-9723a and re-
fined by full-matrix least-squares procedures utilizing SHELXL
a radiation (k = 0.71073 Å) was used, and the
-
4.3. Molecular modeling
97.23b Crystallographic data collection and refinement information
are listed in Table 4. In addition, crystallographic data (excluding
structure factors) for structures in this paper have been deposited
with the Cambridge Crystallographic Data Center as supplemen-
tary publication numbers CCDC 763129 and CCDC 763130 for com-
pounds 51 and 61, respectively.
Calculations of conformational energies and dipole moments
were performed using the molecular modeling software SPARTAN
’08,18 supported on a MacBook Pro laptop computer. For the de-
tailed conformational analyses of Fc-chalcones 51 and 61 pre-
sented in Section 2.3, the starting conformer of each was built
within Spartan’s interactive building mode with the C@C bond
having the trans geometry. In addition, the values of the three tor-
sion (dihedral) angles h1, h2, and h3 were constrained to those
found by X-ray crystallography for each compound (Table 2). Sub-
sequently, each of the mentioned torsion angles was varied be-
tween 0° and 360° (in steps of 15°) while keeping the value of
the other two constant (as those of the starting conformer). The
strain energy of each conformer so obtained was calculated using
the SYBYL force field. The equilibrium geometry of this minimized
structure was then recalculated (optimized) using the semi-empir-
ical PM3 method whereby the energy and the dipole moment of
each conformer (presented in Table 3) was obtained. The same pro-
cedure was followed to calculate the dipole moments of all other
chalcones (for the plots in Fig. 6), starting with the trans geometry
of the C@C bond and the s-cis conformation of the enone portion of
the molecule.
4.2. Biology
4.2.1. General
All centrifugations were carried out at 500 rpm and 4 °C using
an Eppendorf 5810R centrifuge. Media were prepared as follows:
M-9 buffer solution: Na2HPO4 (6.44 g, 0.0454 mol), KH2PO4 (1.5 g,
0.0110 mol), NaCl (0.263 g, 0.0045 mol), and NH4Cl (0.51 g,
0.0096 mol) were diluted with doubly distilled water to 500 mL.
The solution was then autoclaved. Liquid worm medium: yeast ex-
tract (3 g), peptone (3 g), and dextrose (1 g) were diluted to
100 mL with doubly distilled water. The solution was then
autoclaved. Hemoglobin stock (10 mL) and cholesterol (0.8 mL,
5 mg/mL in ethanol) were then added sterilely. Bleaching solution:
(0.26 M KOH, 1.44% NaOCl in water).
4.2.2. C. elegans husbandray
C. elegans nematodes were cultured on a plate of nematode
growth medium stocked with OP-50 bacteria as a food source.
Worms were passed from plate to plate by washing the old plates
with M-9 buffer and spotting the resulting suspension onto the
fresh plates. The plates were stored in a sterile plastic box in a
25 °C incubator.
Acknowledgements
(1) The superb technical assistance of Mr. Douglas Kliewer with
all aspects of spectroscopy is gratefully acknowledged. (2) Two
authors (S.A. and A.C.U.) would like to acknowledge the generous
support of Dr. Andrew Rogerson, the Dean of the College of Science
and Mathematics (CSM) at California State University, Fresno, in
terms of funds for release time as well laboratory supplies
throughout this project. (3) M.L.G. would like to thanks Arnold L.
Rheingold of the University of California, San Diego (UCSD), and
Charles Campana of Bruker AXS, Inc., for organizing the training
workshop, as well as providing useful instruction, through the
Crystallography Summer School (July 30–August 8, 2007) during
which the X-ray crystallographic data were obtained. She also ex-
presses her gratitude to other course instructors, Richard Staples,
Bruce Foxman, Scott Kassel, Kraig Wheeler, and James Golen.
4.2.3. Synchronization
The worms were washed in two 5-mL portions into a 15-mL
centrifuge tube and centrifuged. The supernatant fluid was re-
moved and 10 mL of a bleaching solution was added. The worms
were soaked for 8 minutes with occasional inversion of the tube.
The tube was then centrifuged for 5 min at 4 °C. The supernatant
fluid was removed and 10 mL of M-9 buffer was added. The tube
was centrifuged for 2 min. The supernatant was removed and the
M-9 rinse was repeated twice more. The resulting pellet of em-
bryos was then spotted onto a fresh plate stocked with bacteria
and allowed to dry.
Supplementary data
4.2.4. Preparation of testing solutions and the placement of
nematodes
Supplementary data ((a) atomic coordinates (51: Table S1; 61:
Table S5); (b) bond lengths and angles (51: Table S2; 61:
Table S6); (c) anisotropic displacement parameters (51: Table S3;
61: Table S7); (d) hydrogen coordinates and isotropic displacement
parameters (51: Table S4; 61: Table S8); (e) physical characteriza-
tion of previously reported chalcones (Table S9); (f) packing dia-
grams for 51 and 61 (Fig. S1); (g) a comparison of mortality and
fecundity data for compounds 18 and 50 (Fig. S2); (h) a comparison
of mortality and fecundity data for compounds 30 and 62 (Fig. S2))
associated with this article can be found, in the online version, at
doi:10.1016/j.bmc.2011.01.048.
The chalcone to be tested (1–63, 10À4 mol) was placed in a 10-
mL volumetric flask and dimethyl sulfoxide (DMSO) was added to
the mark. Solution (50
l
L) was mixed with 4.95 mL of Liquid
Worm Medium to give a 10À4 M solution. Solution (50
l
L) was
pipetted into each well of a 96-well plate and one worm was trans-
ferred to each well from the stock plate. Each experiment was per-
formed in triplicate.
4.2.5. Nematode test monitoring
The bioassay was constructed to test the ability of the com-
pounds to kill the worms (% mortality) and inhibit their reproduc-
tion (% fecundity). Each test was preformed (in triplicate) in a
96-well plate starting with one nematodes in each well and was
observed over a 14-day period (approximate life span C. elegans),
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