UV-induced solvent free synthesis of truxillic acid-bile acid conjugates

Article abstract of DOI:10.1016/j.molstruc.2009.05.001

The solvent free UV-induced [2 + 2] intermolecular cycloaddition of two molecules of 3α-cinnamic acid ester of methyl lithocholate produced in 99% yield of α- and ε-truxillic acid-bis(methyl lithocholate) isomers, which possess two structurally different

Full text of DOI:10.1016/j.molstruc.2009.05.001

Journal of Molecular Structure 930 (2009) 116–120
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
UV-induced solvent free synthesis of truxillic acid–bile acid conjugates
*
Juha Koivukorpi , Erkki Kolehmainen
Department of Chemistry, P.O. Box 35, FI-40014, University of Jyväskylä, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2009
Received in revised form 27 April 2009
Accepted 1 May 2009
Available online 9 May 2009
The solvent free UV-induced [2 + 2] intermolecular cycloaddition of two molecules of 3a-cinnamic acid
ester of methyl lithocholate produced in 99% yield of a- and e-truxillic acid-bis(methyl lithocholate) iso-
mers, which possess two structurally different potential binding sites. A prerequisite for this effective
solid state reaction is a proper self-assembled crystal structure of the starting conjugate crystallized from
acetonitrile. The crystallization of cinnamic acid ester of methyl lithocholate from acetonitrile produces
two different crystalline forms (polymorphs), which is the reason for the solid state formation of two iso-
mers of truxillic acid-bis(methyl lithocholate).
Keywords:
Bile acid
Cinnamic acid
Truxillic acid
Cycloaddition
UV irradiation
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
been also demonstrated that a-truxillic acid derivatives are po-
tential anti-inflammatory drugs [7], and they can inhibit the spe-
cific binding of muscarinic receptor antagonist to muscarinic
receptors in membranes [8].
In their recent perspective article Maitra and Nonappa high-
lighted the potential of bile acids in synthesis, supramolecular/
materials chemistry and nanoscience [1]. Among many bile acid
based receptors catalogued in their article bile acid derived
molecular tweezers containing aryl moieties can be used to rec-
ognize electron deficient substrates [2]. On the other hand, bile
acid-porphyrin conjugates have shown recently to possess prom-
ising potentials from saccharide recognition in the solution to the
selective cancer cell fluorescence detection [3]. In this communi-
cation we report a straightforward solid state synthetic route to
dimeric structures where these two different receptor types viz.
bile acid and aryl exist in one molecule. This offers interesting
future perspectives in biochemistry and biology since bile acids
themselves and their derivatives are potential carriers for liver
specific drugs and they can be used as cholesterol lowering
agents [4]. Furthermore, they have also been used for treatment
of bile acid deficiency and liver diseases [5]. Cinnamic acid is an
inexpensive building block which is known to undergo [2 + 2]
cycloaddition reaction when irradiated with UV light [6]. By
the addition of this photoreactive moiety to the bile acid back-
bone, we have created a route to a new kind of potential bile
acid-based conjugates which can be used as receptors and sen-
sors [3]. The binding properties of this kind of double receptor
can be enhanced by changing lithocholic acid to a more hydro-
philic bile acid containing more hydroxy functionalities. It has
2. Experimental
2.1. Spectroscopy
Liquid state NMR experiments were run with Bruker Avance
DPX 250 and DRX 500 FT NMR spectrometers equipped with a
5 mm diameter inverse detection probehead. 1H and 13C chemical
shifts were referenced to the residual signal of chloroform d(1H)
CHCl3 = 7.26 ppm, and the center peak of solvent signal, d(13C)
CDCl3 = 77.00 ppm from internal TMS. The structures of 1 and 2
have been characterized by 1H, 13C, 13C DEPT-135, PFG DQF
1H–1H COSY, 1H–1H ROESY, PFG 1H–13C HMQC, and PFG 1H–13
C
HMBC experiments. 13C CP MAS NMR experiments were run with
a Bruker Avance 400 FT NMR spectrometer from samples packed
in 4 mm diameter zirconia rotors spun at 10 KHz rate. Detailed lists
of NMR acquisition and processing parameters are available by E.K.
on request. Mass spectrometric measurements were performed
using a Micromass LCT time of flight (TOF) mass spectrometer with
electrospray ionization (ESI). Elemental analysis was performed
using a VarioEL III elemental analyzer.
2.2. Synthesis
Bile acid methyl esters were synthesized according to the
* Corresponding author. Tel.: +358 14 260 2684; fax: +358 14 260 2501.
E-mail address: juha.koivukorpi@jyu.fi (J. Koivukorpi).
previously reported procedure [9].
a/e-Truxillic acid bis(methyl
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.05.001

J. Koivukorpi, E. Kolehmainen / Journal of Molecular Structure 930 (2009) 116–120
117
Fig. 1. Synthesis of 1, and the structures and numberings of 1 and 2.
Fig. 2. Selected 1H NMR (250 MHz) spectra during the formation of 1 (bottom) from 2 (top). Irradiation time (days) is presented on the right end of the spectra.
lithocholyl) ester (1) was synthesized by irradiating cinnamic acid
ester of methyl lithocholate (2) (crystallized from acetonitrile)
with UV light (Fig. 1). All reagents and solvents (purity > 98% or
better) were purchased from Sigma–Aldrich or Fluka and used
without further purification.
The progress of the formation of 1 from 2 was monitored by
1H NMR measurements. Selected spectra during the irradiation
experiment are presented in Fig. 2. In the formation of 1 the
intensity of doublets from double bond protons H27 and H28
in 2 are decreasing and they can be used for monitoring the pro-
gress of reaction. In this case the doublet of H27 (d = 6.42 ppm)
was compared to the protons in C23 (d = 2.18-2.40 ppm), which
were clearly separated from the other aliphatic protons during
the whole process and they could be integrated accurately. The
irradiation was continued for 9 days until the reaction did not
proceed any more. The percentage of 1 during the reaction pro-
gress is plotted in Fig. 3. For a reference we irradiated a CDCl3
solution of 2 in quartz tube with UV light, and found that no
dimerization was taking place in solution.
Fig. 3. The amount of 1 (wt%) in the function of time.

118
J. Koivukorpi, E. Kolehmainen / Journal of Molecular Structure 930 (2009) 116–120
m, Ar), 7.52 (2H, m, Ar), 7.67 (1H, d, 28-H). m/z (ESI) 521
[M + H]+, 543 [M + Na]+, 559 [M + K]+. M.W. (C34H48O4) = 520.75.
(Found: C, 78.74; H, 9.40. Calc. for C34H48O4: C, 78.42; H, 9.29%).
2.4. a/e-Truxillic acid-bis(methyl lithocholate) conjugates 1
Solid 2 was ground in mortar and spread as a thin layer on a
glass plate and irradiated with UV light (365 nm, 6 W) for 9 days
using Ultra-Lum UVAC-16 ultraviolet lamp. The compound was
mixed manually every day to ensure completeness of the reac-
tion. Temperature under the lamp was 28 °C. The yield of 1
was 99% based on 1H NMR spectrum. Mp. 211 °C. 1H NMR
(500 MHz; CDCl3): d(ppm) = 0.63 (6H, s, Me), 0.83 (3H, s, Me),
0.84 (3H, s, Me), 0.92 (3H, d, Me), 0.93 (3H, d, Me), 0.95–2.00
(52H), 2.18–2.40 (4H, 2m, 23-H), 3.67 (6H, 2s, COCH3), 3.90–
3.96 (2H, 2 overlapping douplets, 27-H), 4.37 (2H, 2m, O-CH),
4.45 (2H, 2m, 28-H), 7.19–7.24 (2H, Ar), 7.28–7.34 (8H, Ar).
m/z (ESI) 1063 [M + Na]+, 1079 [M + K]+. M.W. (C68H96O8) =
1041.50. (Found: C, 78.71; H, 9.47. Calc. for C68H96O8: C, 78.42;
H, 9.29%).
3. Results and discussion
The reaction produced a 1:1 mixture of a- and e-truxillic acid-
bis(methyl lithocolate) conjugates (Fig. 4) similarly as reported ear-
lier for the reaction of cinnamic acid itself [6]. Dimeric structure of 1
is also ascertained by ESI-TOF MS and 13C NMR, which spectrum
showed two resolved chemical shifts for every carbon. The chemical
shifts of the protons 27/270 and 28/280 of 1 were also compared to
Fig. 4. Orientation of the substituents on the cyclobutane ring in
(right) truxillic acid-bis-bile acid conjugate isomers of 1.
a- (left) and e-
2.3. Cinnamic acid ester of methyl lithocholate 2
the previously reported results [8]. The existence of a-truxillic acid
dimer with trans-phenyls was also confirmed by 1H–1H ROESY NMR
measurement which showed NOE between phenyl protons and
both H27 and H28 protons in cyclobutane ring. The crystallization
of 2 from acetonitrile produces two different crystallized forms
which can be seen in 13C CP MAS spectrum (Fig. 5). This also sup-
Methyl lithocholate (1.22 g, 3.13 mmol), cinnamoyl chloride
(0.70 g, 4.20 mmol) and pyridine (400 l, 4.95 mmol) were dis-
l
solved in 50 ml of CH2Cl2 and stirred at rt for 22 h. Product was
washed with dil. HCl, water, 2 ꢀ dil. NaHCO3 and water, respec-
tively. Organic layer was dried over anhydr. Na2SO4 and solvent
was removed in vacuo. Product was crystallized from acetonitrile
(1.25 g, 77%). Mp. 151 °C. 1H NMR (500 MHz; CDCl3):
d(ppm) = 0.66 (3H, s, Me), 0.92 (3H, d, Me), 0.95 (3H, s, Me),
1.00–1.65 (20H), 1.70–2.00 (6H), 2.18–2.40 (2H, m, 23-H), 3.66
(3H, s, COCH3), 4.86 (1H, m, O–CH), 6.42 (1H, d, 27-H), 7.38 (3H,
ports the formation of both a- and e-truxillic acid derivatives.
1H NMR spectrum of final product 1 is presented in Fig. 6. Struc-
tures and numberings of 1 and 2 are shown in Fig. 1. 13C NMR
chemical shifts for 1 and 2 are presented in Tables 1 and 2,
respectively.
Fig. 5. Above: 13C NMR spectrum of 1 in CDCl3. Below: 13C CP MAS spectrum of 1.

J. Koivukorpi, E. Kolehmainen / Journal of Molecular Structure 930 (2009) 116–120
119
Fig. 6. 1H NMR (500 MHz) spectrum of 1 without any purification after UV irradiation.
4. Conclusions
Table 2
13C NMR chemical shifts of 2.
In summary, we have demonstrated a straightforward solid
state synthetic route starting from 3 -cinnamic acid ester of
methyl lithocholate to - and -truxillic acid-bis(methyl lithocho-
Carbon
d (ppm)
Carbon
d (ppm)
a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
32.4
28.2
74.5
35.1
42.0
26.8
26.3
35.8
40.5
34.6
20.9
40.2
42.8
56.5
24.2
27.1
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
56.0
12.0
23.3
35.4
18.3
31.1
31.0
174.7
51.4
166.4
118.8
144.3
134.6
128.0
128.8
130.1
a
e
late) derivatives. The crystallization of 2 from acetonitrile produces
two different crystalline forms (polymorphs), which is the reason
for the solid state formation of two isomers of truxillic acid-bis(-
methyl lithocholate). Prompted by the success in this approach
we are now working with syntheses towards corresponding struc-
tures containing more hydrophilic bile acid (such as deoxycholic
and cholic acid) moieties. We are also attempting to separate the
formed structural isomers of these bile acid derivatives by adding
different compounds to their mixture as demonstrated by Miyata
et al. for several bile acid derivatives [10]. Especially in case of
e-
isomer a formation of -stacked inclusion compound could lead
p
its crystallization out-of-solution and resolution of these stereoiso-
mers. It might be also possible to control the crystallization of
starting compound 2 by using suitable templates [11].
Acknowledgements
The authors acknowledge Dr. Nonappa and Spec. Lab. Techni-
cian Reijo Kauppinen for their help in running the NMR spectra,
Spec. Lab. Tech. Mirja Lahtiperä for running ESI-TOF+ mass spectra,
and Lab. Tech. Elina Hautakangas for elemental analysis.
Table 1
13C NMR chemical shifts (ppm) of
be exchangeable.
a- and
e-isomers of 1. Adjacent assignments may
Carbon
a
or ea
a
or ea
Carbon
a
or ea
a
or ea
1/10
31.4
28.2
74.6
34.9
41.6
26.2
25.7
35.7
40.3
34.4
20.8
40.2
42.8
56.5
24.2
26.8
31.9
28.2
74.6
34.9
41.8
26.3
26.2
35.7
40.3
34.4
20.8
40.2
42.8
56.6
24.2
26.9
17/170
18/180
19/190
20/200
21/210
22/220
23/230
24/240
25/250
26/260
27/270
28/280
29/290
30/300
31/310
32/320
56.1
12.0
23.2
35.4
18.3
31.1
31.0
174.7
51.4
171.3
46.8
56.1
12.0
23.2
35.4
18.3
31.1
31.0
174.7
51.4
171.4
46.9
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