Synthesis and structural determination of 3,5-bis(2-fluorobenzylidene)-4-piperidone analogs of curcumin

Article abstract of DOI:10.1016/j.molstruc.2009.07.016

Analogs of cytotoxic compound 3,5-bis(2-fluorobenzylidene)-4-piperidone (1) were synthesized and their molecular structures were characterized by ¹H, ¹³C NMR, ESI-MS, IR spectra and X-ray crystallography. The central ring of the pipe

Full text of DOI:10.1016/j.molstruc.2009.07.016

Journal of Molecular Structure 936 (2009) 23–28
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and structural determination
of 3,5-bis(2-fluorobenzylidene)-4-piperidone analogs of curcumin
Pallavi Lagisetty a, Douglas R. Powell b, Vibhudutta Awasthi a,
*
a Department of Pharmaceutical Sciences, University of Oklahoma Health Sciences Center, 1110 N. Stonewall Avenue, Oklahoma City, OK 73117, USA
b Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, OK 73019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 May 2009
Received in revised form 7 July 2009
Accepted 16 July 2009
Available online 25 July 2009
Analogs of cytotoxic compound 3,5-bis(2-fluorobenzylidene)-4-piperidone (1) were synthesized and
their molecular structures were characterized by 1H, 13C NMR, ESI–MS, IR spectra and X-ray crystallog-
raphy. The central ring of the piperidin-4-one ring assumes a sofa conformation with two benzylidene
rings connected through E,E oriented groups. The compound 1 crystallized in triclinic space group P1.
In order to obtain potentially more efficacious compounds, three dimers of 3,5-bis(2-fluorobenzylid-
ꢀ
ene)-4-piperidone were synthesized. The two molecules of 1 were conjugated together via ANAoxa-
Keywords:
lylANA (3), ANAfumarylANA (4) or ANADTPAANA (5) linkers. Compound
monoclinic space group P21/n.
3 crystallized in
Bis(2-fluorobenzylidene)-4-piperidone
X-ray crystallography
Curcumin
Ó 2009 Elsevier B.V. All rights reserved.
Cancer
1. Introduction
2. Experimental
We synthesized a curcumin analog, 3,5-bis(2-fluorobenzylid-
ene)-4-piperidone (1) by acid catalyzed Claisen–Schmidt conden-
sation of 4-piperidone and 2-fluorobenzaldehyde [1]. Compound
1 is an antiproliferative compound, and is also known as EF24
[2,3]. As compared to curcumin, it possesses significantly more po-
tent biological activity and better bioavailability profile [4]. The
compound has been investigated in vitro and in preclinical models
of cancer, including those of breast cancer [5], colon cancer [3] and
pancreatic cancer (unpublished results in our laboratory).
Although the exact molecular target(s) for its action is not clearly
2.1. Materials and methods
All reagents were obtained from commercial sources and used
directly without further purification. The reactions were monitored
by thin layer chromatography (TLC) on 250 l
m silica plates. 1H
NMR spectra and 13C NMR spectra (DMSO-d6 and CDCl3) were re-
corded at 300 MHz, and 75 MHz on Mercury-VX 300, 4 nuclei auto-
switchable PFG probe. The E,E-geometry of benzylidene double
bonds was confirmed by conducting Nuclear Overhauser Enhance-
ment (NOE) experiments using 1D-NOESY pulse sequence in V
NMR software (Varian Inc., Palo Alto, CA). Spectra were referenced
to the residual protonated solvents. Chemical shifts and coupling
constants were reported in d parts per million (ppm) and Hertz
(Hz), respectively. Mass spectra were recorded by Finnigon Mat
LCQ mass spectrometer (San Jose, CA). Samples for IR spectroscopic
measurements were finely ground and prepared as KBr pellets in a
Bruker IFS 66v spectrometer with a KBr beam splitter. Sixty-four
scans at a spectral resolution of 1 cmꢀ1 were averaged for each
spectrum. The reported melting points (degree Celsius) are
uncorrected.
understood, the role of NFjB has been proposed as pivotal to the
compound 1’s antiproliferative effects [3–6]. The exhaustive struc-
ture activity relationship of compound 1 has not been reported, but
it is possible to obtain better understanding about stereochemical
features of molecular targets by elucidating molecular and crystal
structure of compound 1. In this article, we report a detailed crys-
tallographic analysis of compound 1. In addition, we report for the
first time dimers of compound 1 for potentially enhanced activity
against proliferating cells.
The crystal structures of 3,5-bis(2-fluorobenzylidene)-4-piperi-
done (1) and its dimer (3) were determined by X-ray diffraction,
measured with a CCD Bruker APEX diffractometer [7]. The struc-
ture was solved by direct methods using SHELXL [8] and refined
on F2 by full-matrix least-squares with the SHELXL [9]. Details of
data collection and refinement parameters of compounds 1 and 3
are listed in Table 1. The selected bond distances and angles of
* Corresponding author. Tel.: +1 405 271 6593x47331; fax: +1 405 271 7505.
E-mail address: vawasthi@ouhsc.edu (V. Awasthi).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.07.016

24
P. Lagisetty et al. / Journal of Molecular Structure 936 (2009) 23–28
Table 1
Table 2a
Crystal data and structure refinement for 1 and 3.
Selected bond lengths, angles and torsion angles for compound 1.
Compound
1
3
Atoms
O1AC4
N1AC6
N1AC2
C2AC3
C3AC7
C3AC4
C4AC5
Bond lengths (Å)
1.227 (16)
1.490 (19)
1.486 (2)
1.504 (19)
1.343 (2)
1.490 (19)
1.494 (2)
1.348 (2)
Atoms
C5AC6
C7AC8
C8AC13
C8AC9
Bond lengths (Å)
1.505 (19)
1.461 (2)
1.395 (2)
1.404 (2)
1.465 (2)
1.390 (2)
1.376 (2)
1.401 (2)
Empirical formula
C21 H20 Cl F2
O3
407.83
Triclinic
P1
N
C40 H28 F4 N2 O4
Formula mass
Crystal system
Space group
Unit cell dimensions
a (Å)
676.64
Monoclinic
P21/n
C
C
C
C
14AC15
15AC16
16AC17
15AC20
ꢀ
8.8238 (9)
10.6781 (11)
11.9079 (12)
973.06 (17)
2
1.392
424
0.237
1.85–28.34°
13658
4837/3/278
wR2 = 0.1036
R1 = 0.0391
0.998
1.038
4165
14.560 (4)
11.287 (4)
19.410 (5)
3183.9 (16)
4
1.412
1400
0.107
2.09–26.02
25705
5860/6/471
wR2 = 0.1112
R1 = 0.0423
0.935
1.003
4597
C5AC14
Atoms
b (Å)
c (Å)
Angles (°)
Atoms
14AC5AC6
Angles (°)
C2AN1AC6
N1AC2AC3
C7AC3AC4
C7AC3AC2
C4AC3AC2
O1AC4AC3
O1AC4AC5
C3AC4AC5
113.39 (11)
110.40 (12)
116.76 (12)
124.69 (13)
118.55 (12)
120.41 (13)
120.58 (13)
119.01 (12)
115.81 (12)
120.1 (5)
122.34 (15)
Torsion angles (°)
33.0 (2)
ꢀ31.5 (3)
C
125.84 (13)
118.34 (12)
110.20 (12)
129.68 (13)
118.79 (13)
124.81 (13)
132.63 (14)
117.43 (14)
118.87 (14)
117.1 (15)
116.25 (13)
Torsion angles (°)
ꢀ33.36 (17)
34.09 (16)
Volume (Å3)
Z
C4AC5AC6
N1AC6AC5
C3AC7AC8
Dcalc (Mg/m3)
F(0 0 0)
C
13AC8AC7
Absorption coefficient
l
(mmꢀ1
)
C9AC8AC7
C5AC14AC15
F2AC16AC17
F2AC16AC15
0
Theta range for data collection
Reflections collected
Data/restraints/parameters
wR(F2 all data)
C14AC5AC4
0
F
2 AC20AC19
F
2 AC20AC15
R(Fobsd. data)
C19AC20AC15
C13AC8AC9
Atoms
N1AC2AC3AC4
C4AC5AC6AN1
The data completeness
Atoms
C3AC7AC8AC9
C5AC14AC15AC16
Goodness-of-fit on F2
Observed data [I > 2
Largest diff. peak and hole (e/Å3)
r
(I)]
0.515 and
ꢀ0.394
0.246 and
ꢀ0.266
The minimum and maximum transmission
factors
0.901 and 0.952 0.954 and 0.979
ride monohydrate and (2.03 ml, 17.50 mmol) of benzaldehyde,
the title compound of 2.05 gm, 85% yield was obtained as hydro-
chloride salt (m.p of 242–243 °C). 1H NMR (300 MHz, DMSO-d6):
d 9.45 (s, 1H, NH, D2O exchangeable), 7.90 (s, 2H, C@CH), 7.50–
7.40 (m, 10H, ArAH), 4.50 (s, 4H). 13C NMR (75 MHz, DMSO-d6):
d 165.43, 139.92, 130.76, 130.39, 123.52, 114.15, 44.12. ESI HRMS
calculated for C19H18NO (M+ + H) 276.13, found 276.12.
compounds 1 and 3 were listed in Tables 2a and b. The complete
set of structural parameters in CIF format is available as an Elec-
tronic Supplementary Publication from the Cambridge Crystallo-
graphic Data Centre (CCDC Nos. 730847 and 730848).
2.2. Synthesis of 3,5-bis(benzylidene)-4-piperidones (1–5)
2.2.4. 1,2-Bis-[3,5-bis(2-fluorobenzylidene)-4-oxo-piperidin-1-yl]-
ethan-1,2-dione (3)
To a solution of compound 1 (300 mg, 0.96 mmol) in 1,2 dichlo-
roethane, triethylamine (0.4 ml, 2.85 mmol) was added. The reac-
2.2.1. [3,5-Bis(2-fluorobenzylidene)-4-piperidone acetate] (1)
Hydrochloric acid gas (generated in situ) was bubbled into a
solution of 4-piperidone hydrochloride monohydrate (3 gm,
19.5 mmol) in glacial acetic acid (80 ml) until a clear solution
was obtained (about 15 min). 2-Fluorobenzaldehyde (6 ml,
56.5 mmol) was added and the reaction mixture was left at room
temperature for 48 h. The crystals formed were filtered on a Buch-
ner funnel, washed with absolute ethanol (50 ml) and conse-
quently with ether (50 ml), and then dried to get compound 1 as
a yellow crystalline solid (6.81 gm, 94% yield). Rf (60:40 ethyl ace-
tate:hexanes) = 0.46. It has m.p. of 185–186 °C. 1H NMR (300 MHz,
DMSO-d6): d 9.85 (br s, 1H, NH), 7.88 (s, 2H, C@CH), 7.54 (q, 2H,
ArAH, J = 8.5), 7.48 (t, 2H, ArAH, J = 7.8), 7.37 (q, 4H, ArAH,
J = 7.0), 4.35 (s, 4H), 1.91 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-
d6): d 181.98, 172.09, 160.37 (d, J = 249.1), 132.55 (d, J = 8.5),
131.79 (d, J = 3.8), 131.03 (d, J = 1.5), 129.89, 124.95 (d, J = 3.2),
121.52 (d, J = 13.1), 116.09 (d, J = 21.0), 43.85 (d, J = 3.1), 21.16.
FT-IR (cmꢀ1): 797, 1201, 1243, 1453, 1483, 1616, 1715, 2977,
3025. ESI HRMS calculated for C19H16F2NO (M+ + HACH3COOH)
312.12, found 312.13.
tion mixture was cooled to 0 °C on ice and oxalyl chloride (50 ll,
0.57 mmol) in 1,2 dichloroethane was added drop-wise. The reac-
tion mixture was stirred at room temperature for 16 h. To this
reaction mixture was added 10% potassium carbonate solution
(10 ml) and stirring was continued for another 3 h. The organic
phase was separated, dried (Na2SO4) and concentrated to obtain
crude syrup. The residue was chromatographed on silica and pure
compound 3 was eluted with 60:40 ethyl acetate in hexanes. Com-
pound 3 was obtained as a yellow crystalline solid (244 mg, 63%
yield), and was recrystallized from chloroform. Rf (60:40 ethyl ace-
tate:hexanes) = 0.50. It has m.p. of 113–115 °C. 1H NMR (300 MHz,
CDCl3): d 7.83 (s, 2H, C@CH), 7.71 (s, 2H, C@CH), 7.50–7.30 (m, 4H,
ArAH), 7.25–7.05 (m, 12H, ArAH), 4.46 (s, 4H), 4.33 (s, 4H). 13C
NMR (75 MHz, CDCl3): d 183.93, 162.35, 161.89 (d, J = 55.3),
159.38 (d, J = 55.3), 131.95 (d, J = 3.9), 131.78, 131.66, 131.19,
130.52, 124.48, 124.31, 122.13 (d, J = 14.0), 121.52 (d, J = 13.2),
116.32 (d, J = 21.8), 116.05 (d, J = 21.8), 46.37, 41.94. FT-IR
(cmꢀ1): 1156, 1176, 1225, 1239, 1254, 1273, 1453, 1484, 1621,
1652. ESI HRMS calculated for C40H28F4N2NaO4 (M+ + Na) 699.19,
found 699.13.
2.2.2. [3,5-Bis(2-fluorobenzylidene)-4-piperidone] (1)
Free base of compound 1 was generated by treating the acetate
salt with 10% potassium carbonate solution. 1H NMR (300 MHz,
CDCl3): 9.94 (s, 1H, NH), 7.90 (s, 2H, C@CH), 7.61–7.54 (m, 2H,
ArAH), 7.51 (t, 4H, ArAH, J = 7.8), 7.39 (q, 2H, ArAH, J = 7.2), 4.37
(s, 4H). ESI HRMS calculated for C19H16F2NO (M+ + H) 312.12, found
312.13.
2.2.5. 1,4-Bis-[3,5-bis(2-fluorobenzylidene)-4-oxo-piperidin-1-yl]-2E-
butene-1,4-dione (4)
Dimer 4 was prepared in a manner similar to that of compound
3. Briefly, compound 1 (500 mg, 1.60 mmol) and fumaryl chloride
(85 ll, 0.80 mmol) were reacted to obtain compound 4 as a yellow
2.2.3. [3,5-Bis(benzylidene)-4-piperidone hydrochloride] (2)
Compound 2 was synthesized in a manner similar to that of
compound 1. From 1.35 g, 8.79 mmol of 4-piperidonehydrochlo-
solid (395 mg, 70% yield) after usual work up and purification. Rf
(60:40 ethyl acetate:hexanes) = 0.50. The compound showed m.p.
of 145–146 °C. 1H NMR (300 MHz, CDCl3): d 7.91 (s, 2H, C@CH),

P. Lagisetty et al. / Journal of Molecular Structure 936 (2009) 23–28
25
Table 2b
J = 8.6), 128.90, 126.31, 122.70, 119.86 (d, J = 11.5), 113.81 (d,
J = 21.6), 53.70, 52.52, 49.57, 48.23, 43.72, 40.39. ESI HRMS calcu-
lated for C52H49F4N5NaO10 (M+ + Na) 1002.33, found 1002.21.
Selected bond lengths, angles and torsion angles for compound 3.
Atoms
Bond lengths (Å)
1.225 (2)
1.460 (2)
1.461 (2)
1.506 (3)
1.342 (3)
1.500 (3)
1.497 (3)
1.346 (3)
1.507 (3)
1.472 (3)
1.388 (3)
1.396 (3)
1.457 (3)
1.396 (3)
1.402 (3)
Atoms
Bond lengths (Å)
1.223 (2)
1.462 (2)
1.464 (2)
1.504 (3)
1.343 (3)
1.501 (3)
1.501 (3)
1.343 (3)
1.515 (3)
1.472 (3)
1.390 (3)
1.405 (3)
1.467 (3)
1.392 (3)
1.397 (3)
O
N
N
1AAC4A
1AAC6A
1AAC2A
O
N
N
1BAC4B
1BAC6B
1BAC2B
2.3. X-ray crystallography
C
C
C
C
C
C
C
C
C
C
C
C
2AAC3A
3AAC7A
3AAC4A
4AAC5A
5AAC14A
5AAC6A
7AAC8A
8AAC13A
8AAC9A
14AAC15A
15AAC20A
15AAC16A
C2BAC3B
C3BAC7B
C3BAC4B
C4BAC5B
C5BAC14B
C5BAC6B
C7BAC8B
C8BAC13B
C8BAC9B
C14BAC15B
C15BAC20B
C15BAC16B
Yellow prism-shaped crystals, of dimensions (1) 0.44 ꢁ 0.30 ꢁ
0.22 mm and (3) 0.41 ꢁ 0.30 ꢁ 0.20 mm were selected for struc-
tural analysis. Intensity data for the compounds were collected
using a diffractometer with a Bruker APEX ccd area detector (1)
and graphite-monochromated Mo K
a radiation (k = 0.71073 Å).
Both samples were cooled to 100(2) K.
3. Results and discussion
3.1. Synthesis
Atoms
Angles (°)
Atoms
2AAC20AAC15A
N1BAC2BAC3B
Angles (°)
N1AAC1AC3
114.99 (15)
124.83 (17)
113.40 (14)
116.96 (17)
124.80 (17)
118.17 (16)
121.34 (16)
120.93 (17)
117.62 (16)
117.59 (17)
124.42 (17)
117.97 (15)
109.88 (15)
129.08 (18)
119.97 (17)
123.92 (16)
121.42 (18)
128.59 (18)
114.94 (17)
119.73 (17)
125.31 (17)
F
117.93 (17)
108.95 (15)
118.38 (18)
124.76 (17)
116.75 (16)
121.16 (17)
120.65 (17)
118.17 (17)
116.88 (18)
123.38 (17)
119.73 (16)
109.15 (15)
127.47 (19)
115.96 (17)
124.91 (17)
119.13 (18)
121.6 (2)
O4AC3AN1B
Acid-catalyzed Claisen–Schmidt condensation of 4-piperidone
hydrochloride and substituted aromatic aldehydes afforded 3,5-
bis(benzylidene)-4-piperidones 1 and 2 as their hydrochloride
salts in good yields (Scheme 1). There was no significant difference
in chemical shifts of these protons in 1H NMR. The 13C NMR of
compounds 1, 3, 4 and 5 show the typical coupling pattern with
the fluorine atoms. These compounds contain piperidine nitrogen
atom which can be protonated during the electrospray ionization
to give [M + H]+ as base peaks in the mass spectra.
It should be noted that these compounds are potential antitu-
mor agents, and even small differences in the structures may cause
significant changes in cytotoxicity. The piperidine nitrogen is reac-
tive towards anhydrides and acid chlorides. Several N-acyl deriva-
tives of 3,5-bis(benzylidene)-4-piperidones were known in
literature [10–13]. We modified secondary N-atom in piperidone
ring of compound 1 to prepare dimers with a goal to enhance anti-
proliferative activity (not reported). First, we prepared ANAoxa-
lylAN-linked dimer (3) (Scheme 2). The dimer obtained showed a
characteristic molecular ion peak in the ESI–MS. The second dimer
(4) of compound 1 was prepared via ANAfumarylANA linker
(Scheme 2). The third dimer (5) was obtained by treating com-
pound 1 with DTPA dianhydride (Scheme 2). Compound 5 and
was characterized by 1H, 13C NMR spectroscopy and mass
spectrometry.
C
C
C
C
6AAN1AAC2A
7AAC3AAC4A
7AAC3AAC2A
4AAC3AAC2A
C7BAC3BAC4B
C7BAC3BAC2B
C4BAC3BAC2B
O
O
1BAC4BAC5B
1BAC4BAC3B
O
O
1AAC4AAC5A
1AAC4AAC3A
C5BAC4BAC3B
C14BAC5BAC4B
C14BAC5BAC6B
C4BAC5BAC6B
C
C
C
C
5AAC4AAC3A
14AAC5AAC4A
14AAC5AAC6A
4AAC5AAC6A
N1BAC6BAC5B
N1AAC6AAC5A
C
C
C
C
C
C
C
C
C
3BAC7BAC8B
C
C
C
C
C
C
C
C
3AAC7AAC8A
13AAC8AAC7A
9AAC8AAC7A
10AAC9AAC8A
5AAC14AAC15A
20AAC15AAC16A
20AAC15AAC14A
16AAC15AAC14A
13BAC8BAC9B
13BAC8BAC7B
9BAC8BAC7B
10BAC9BAC8B
5BAC14BAC15B
20BAC15BAC14B
16BAC15BAC14B
19BAC20BAC15B
127.98 (19)
120.14 (17)
123.78 (17)
123.72 (18)
Atoms
Torsion angles (°) Atoms
26.7 (3)
Torsion angles (°)
38.9 (3)
C
C
3AAC7AAC8AAC9A
5AAC14AAC15AAC16A ꢀ23.3 (3)
C3BAC7BAC8BAC13B
C5BAC14BAC15BAC16B ꢀ39.9 (3)
7.86 (s, 2H, C@CH), 7.42–7.00 (m, 16H, ArAH), 6.95 (s, 2H, fumaryl
CH@CH), 4.78 (s, 4H), 4.54 (s, 4H). 13C NMR (75 MHz, CDCl3): d
160.03, 169.17, 161.55 (d, J = 51.5), 159.10 (d, J = 51.5), 133.40 (d,
J = 15.5), 131.43 (d, J = 9.2), 130.73, 130.29, 124.25 (d, J = 5.2),
124.58 (d, J = 14.1), 122.23 (d, J = 14.1), 116.32 (d, J = 21.9),
115.95 (d, J = 21.9), 47.22, 43.38. ESI HRMS calculated for
C42H31F4N2O4 (M+ + H) 703.22, found 703.20.
3.2. Spectroscopy
The 1H NMR spectrum of these 3,5-bis(benzylidene)-4-piperi-
dones were simple in appearance. The vinyl protons appeared at
2.2.6. 1,7-Bis-[3,5-bis(2-fluorobenzylidene)-4-oxo-piperidin-1-yl]-
1,4,7-triazaheptane-1,7-dione-1,4,7-triacetic acid (5)
To a solution of compound 1 (622 mg, 2 mmol) in dry dimethyl-
sulfoxide (DMSO) (12 ml), was added bis-[2-(2,6-dioxo-morphol-
in-4-yl)ethyl]-aminoacetic
acid
(357 mg,
1 mmol)
and
triethylamine (1.4 ml, 10 mmol). The reaction mixture was stirred
at room temperature for 2 h. Reaction was monitored by TLC, and
upon consumption of the starting material, the reaction mixture
was evaporated to dryness to remove triethylamine. The solid
was separated by pouring reaction mixture into water, followed
by centrifugation. The solid was recrystallized from methanol
and water to get the title compound as a yellow solid (890 mg,
91% yield). Rf (30:70 methanol:chloroform) = 0.30. It has m.p. of
90–92 °C. 1H NMR (300 MHz, DMSO-d6): d 7.70 (s, 2H, C@CH),
7.67 (s, 2H, C@CH), 7.60–7.45 (m, 8H, ArAH), 7.40–7.20 (m, 8H,
ArAH), 4.73 (s, 4H), 4.69 (s, 4H), 3.44–3.40 [(m, 4H, (NCOCH2)2],
3.22–3.20 (m, 2H, CH2COOH), 3.10–3.01(m, 4H, CH2COOH), 2.48–
2.45 (m, 8H, CH2). 13C NMR (75 MHz, DMSO-d6): d 183.41,
170.18, 168.14, 166.86, 158.39 (d, J = 249.9), 131.93, 129.87 (d,
Scheme 1. Synthesis of compounds 1, 2 and NOE showing E,E-geometry of double
bonds in compound 2.

26
P. Lagisetty et al. / Journal of Molecular Structure 936 (2009) 23–28
Scheme 2. Synthesis of compounds 3, 4 and 5.
d 7.80–7.90 ppm, typical of E geometry [14,15]. This is due to the
anisotropic effect of the carbonyl group resulting in the downfield
shifting of vinyl protons. The appearance of vinyl protons at d
6.20 ppm is an indication of Z conformation [16,17]. The E,E-geom-
etry of double bonds was further confirmed from NOE experi-
ments. The piperidone methylene protons were observed at d
4.30–4.50 ppm as a singlet. The dimer (3) showed a pattern almost
similar to that of monomer (1) in 1H NMR except that all protons
appeared as double. Compound 4 showed fumaric acid vinyl
protons merging with aromatic protons in the region of
d
7.42–6.95 ppm. The dimer (5) showed aromatic peaks at
7.60–7.20 ppm and aliphatic CH2 peaks at d 3.50–2.50 ppm. All
compounds (1–5) showed characteristic molecular ion peaks in
ESI–MS spectrometry.
The infrared spectra of compounds 1 and 3 showed absorptions
at 1715, 1652 and 1621 cmꢀ1 indicative of the carbonyl functional-
ity. The NH absorption at 3025 cmꢀ1 was present in compound 1.
The NH absorption was absent in compound 3, suggestive of N-
acylation.
3.3. NOE experiments
The E,E-geometry of benzylidene double bonds was further
confirmed by conducting NOE experiments of 3,5-bis(benzyli-
dene)-4-piperidone (2). The vinyl protons at d 7.90 ppm in 3,
5-bis(benzylidene)-4-piperidone (2) when irradiated, showed
NOE with aromatic ortho protons at d 7.47 ppm and did not show
any NOE with piperdine methylene protons at d 4.50 ppm. The pro-
tons at d 4.50 ppm demonstrated NOE with d 7.44 aromatic ortho
protons which are in close proximity (Scheme 1).
Fig. 1. The molecular structure of compounds (a)
ellipsoids were drawn at the 50% probability level.
1 and (b) 3. Displacement
tion and benzylidene double bonds were in E,E-geometry. The E-
conformation is consistent with the previous literature reports
for 3,5-bis(benzylidene)-4-piperidones as well as 2,6-bis(arylid-
ene)cyclohexanones [18–20]. The dihedral angles between the
planes passing through the benzene rings in curcumin is 19.1(3)°
[21] and in compound 1, the dihedral angles passing through the
plane of C(3)AC(7)AC(8)AC(9) and C(5)AC(14)AC(15)AC(16) are
33.0(2)° and ꢀ31.5(2)°, respectively. Compared to curcumin, these
compounds possess more rigid structures due to the presence of a
3.4. Crystal structures
Compound 1 crystallizes in triclinic crystal system (space group
P1) and compound 3 crystallizes in monoclinic crystal system
ꢀ
(space group P21/n). The molecular structures and atomic number-
ing of both compounds 1 and 3 are shown in Fig. 1. In the X-ray
crystal structure, the central piperidone ring was in sofa conforma-

P. Lagisetty et al. / Journal of Molecular Structure 936 (2009) 23–28
27
Fig. 2. Perspective view of the crystal packing of compounds (a) 1 and (b) 3. H atoms and the lesser occupied F atoms in the structure are removed for clarity.
piperidone ring. This structural anomaly may have implications in
apparently more potent antiproliferative actions of 3,5-bis(2-fluo-
robenzylidene)-4-piperidone analogs. As reported for similar com-
pounds in the literature [18], the two aryl groups in 1 and 3 are
twisted away from the central ring due to steric repulsions be-
tween the H atoms on C2/C6 and the phenyl rings. This is demon-
strated by the angles of C(3)AC(7) AC(8), C(5)AC(14)AC(15),
C(7)AC(8)AC(9) and C(14)AC(15)AC(16) which are greater than
the 120° expected of sp2 carbon atoms (Tables 2a and b). To reduce
the interactions further, the two phenyl rings rotate about the
C(7)AC(8) and C(14)AC(15) bonds in (1) and C(7B)AC(8B),
C(7A)AC(8A), C(14A)AC(15A) and C(14B)AC(15B) bonds in (3).
Hence one of the fluorine atoms on benzene ring seems disordered
in the monomer (1) and appears in both ortho positions. On the
other hand, in case of the dimer (3), both the fluorine atoms appear
disordered resulting in a crystal data showing the two ortho posi-
tions of both benzene rings occupied by fluorine.
The 2-fluorobenzylidene group with full occupancy orientates
fluorine toward the carbonyl of the central ring. The fluorine in
the other aryl group is distributed between the two ortho positions
of the ring, with the predominantly occupied site [0.910 (3), 0.090
(3)] oriented away from the carbonyl of the central ring. The two
fluoro atoms on the dimer were disordered. The occupancies for
F1B were refined to 0.804(3) and 0.196(3) for primed and un-
primed atoms. The occupancies for F2B were refined to 0.852(3)
and 0.148(3) for the primed and unprimed atoms.
Packing of atoms in compounds 1 and 3 is shown in Fig. 1. Sin-
gle-crystal-X-ray analysis of 1 reveals that there are two molecules
in one unit cell joined through hydrogen bonds (Fig. 2a). Com-
pound 3 packed in layers parallel with the (101) plane (Fig. 2b).
In case of compounds 4 and 5, we were unable to obtain crystal-
line products amenable to X-ray diffraction.
4. Conclusions
In summary, we synthesized and characterized bis(2-fluoroben-
zylidene)-4-piperidone (1) and its three dimers (3, 4 and 5). From
literature [4] and our own in vitro experiments (not reported), we
know that compound 1 is a highly potent cytotoxic agent. The
rigidity of these molecules apparent in the crystal structures pro-
vide a clue about the enhanced antiproliferative activity of 3,5-
bis(2-fluorobenzylidene)-4-piperidones as compared to curcumin.
Acknowledgements
The authors thank the National Science Foundation (CHE-
0130835) and the University of Oklahoma for funds to acquire

28
P. Lagisetty et al. / Journal of Molecular Structure 936 (2009) 23–28
[9] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112.
[10] U. Das, J. Alcorn, A. Shrivastav, R.K. Sharma, E. De Clerq, J. Balzarini, J.R.
Dimmock, Eur. J. Med. Chem. 42 (2007) 71.
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the diffractometer and computers used in this work. The authors
would also like to thank to Dr. Susan L. Nimmo (NMR manager)
at OU, Norman for assistance in NOE spectra of bis(benzylidene)-
4-piperidone. The Mercury VX-300 NMR Spectrometer with a
4-nuclei auto-switchable PFG probe was purchased in 2000 by
the multiuser NSF Grant CHE-0077707.
[14] J.R. Dimmock, N.M. Kandepu, A.J. Nazarali, T.P. Kowalchuk, N. Motaganahalli,
J.W. Quail, P.A. Mykytiuk, G.F. Audette, L. Prasad, P. Perjesi, T.M. Allen, C.L.
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