One-pot synthesis of new carbamate-type molecular tweezers derived from deoxycholic acid under microwave irradiation

Article abstract of DOI:10.3184/030823410X12812894581758

A rapid, safe, and efficient method for the preparation of new carbamate type molecular tweezers based on deoxycholic acid was reported. Eleven new molecular tweezers have been synthesised in good yield(91-96%). The structures of these receptors were conf

Full text of DOI:10.3184/030823410X12812894581758

JOURNAL OF CHEMICAL RESEARCH 2010
RESEARCH PAPER 481
SEPTEMBER, 481–484
One-pot synthesis of new carbamate-type molecular tweezers derived
from deoxycholic acid under microwave irradiation
Zhi-Gang Zhao*, Xing-Li Liu, Yu Chen and Zhi-Chuan Shi
College of Chemistry and Environmental Protection Engineering, Southwest University for Nationnalities, Chengdu 610041, P. R. China
Arapid, safe, andefficientmethodforthepreparationofnewcarbamatetypemoleculartweezersbasedondeoxycholic
acid was reported. Eleven new molecular tweezers have been synthesised in good yield(91–96%). The structures of
1
these receptors were confirmed by H NMR, IR, MS spectra and elemental analysis. These molecular tweezers not
only recognised neutral molecules, but also showed good enantioselectivity for D-amino acid methyl esters.
Keywords: molecular tweezers, microwave irradiation, deoxycholic acid, molecular recognition
Molecular recognition plays a pivotal role in life processes.1
Ove the past two decades a number of artificial receptors for
neutral organic molecules, chiral molecules, and anions have
been successfully synthesised.2–5 Among the various types of
artificial receptors that have been synthesised, a special class
of receptors called molecular tweezers are currently attracting
great interest. The natural rigid concave structure and inherent
asymmetry of cholic acid suggest it is an ideal building block
for the construction of molecular tweezers. In recent years,
some molecular tweezers based on steroidal cholic acid have
been synthesised.6–9
The application of microwave techniques for organic syn-
thesis has attracted considerable interest in recent years.10–13
The reason is that this technology can enhance the selectivity
and reactivity, increase the chemical yields and greatly shorten
the reaction time. In our previous work,14–15 it was found that
by using microwave irradiation, the ester and carbamate
molecular tweezers based on α-hyodeoxycholic acid can be
successfully synthesised in less time, higher yields compared
to the conventional method. Furthermore, these molecular
tweezers have a good enantioselectivity for L-amino acid
methyl esters. We have continued our programme to synthesise
molecular tweezer artificial receptors using microwave meth-
ods16–18 inordertoinvestigatetheeffectofthemicroenvironment
and cleft size on molecular recognition ability. We report here
a facile and rapid synthetic method of carbamate type molecular
tweezer derived from deoxycholic acid under microwave
irradiation which, to the best of our knowledge, has not been
attempted previously. We have made some preliminarily stu-
dies of the recognition properties of this kind of molecular
tweezer for neutral molecules and chiral molecules. The
synthetic route is depicted in Scheme 1.
As the guest molecules were being added, the absorbance rose
in a regular pattern. It showed that these molecular tweezers
possessed the ability to form complex with guests molecules
examined. The titration data were analysed by using the
Hildebrand-Benesi equation.19 Based on the equation (1), when
[G]0>>[H]0, the plots of 1/[G]0 versus 1/∆A were measured.
The plot gave a straight line . From the intercept and the slope
of the line, we calculated with the association constants (Ka).
The free energy change (–∆G0) was obtained according to
equation (2). Association constants (Ka) and free energy
changes (–∆G0) for the inclusion complexes of aromatic
amines and D/L –amino acid methyl esters with molecular
tweezers 4a, 4b and 4e are listed in Tables 2 and 3. As shown
in Tables 2 and 3, these molecular tweezers possessed the
ability to form complex as with the guest molecules that
were examined. The supramolecular complexes consisted of
1:1 host and guest molecules. The association constants of
molecular tweezer 4b, for example, is 5191.20, 1756.30,
3832.40 L·mol–1 for p-nitroaniline, aniline, p-methoxyaniline,
respectively. At the same time, these receptors showed good
chiral recognition for D-amino acid methyl esters, the maxi-
mum KD/KL of molecular tweezer 4e reaches 4.92 for D/L-
Tyr-OMe. The details of the molecular recognition of these
molecular tweezers are under further study.
1
1
1
=
+
(1)
(2)
ΔA aKa G
[ ]0
a
ΔG0 = −RTLnKa
Experimental
Melting points were determined on a micro-melting point apparatus
and the thermometer was uncorrected. IR spectra were obtained on
1
Results and discussion
1700 Perkin-Elmer FTIR using KBr disks. H NMR spectra were
recorded on a Varian INORA 400MHz spectrometer in CDCl3 as
solvent and TMS as internal standard. Mass spectra were determined
on Finnigan LCQDECA instrument. Elemental analyses were performed
on a Car10-Erba-1106 auto analyser. Optical rotation was measured
on a Wzz-2B polarimeter. Microwave irradiation was carried out with
a MCL-3 microwave oven at full power (700 W). The oven was
modified from a domestic microwave oven and tested to conform to
the performance index before use. All the solvents were purified
before use. Deoxycholic acid was converted to deoxycholic acid
methyl ester following a reported procedure.20
As illustrated in Table 1, microwave-enhanced method
compared with the conventional solvent method has the
following advantages: (1) the reaction avoids the use of very
toxic phosgene and reduces the pollution of the environment;
(2) the reaction rate increased 26–31 times, and greatly
shortened the reaction time; and (3) the yields of molecular
tweezers have increased from 65–73% to 91–96%. The
microwave assisted procedure is a safe, fast, efficient and green
synthetic method for carbamate type molecular tweezers based
on deoxycholic acid.
Microwave method for the preparation of molecular tweezers 4a–k
Triphosgene (0.11 g, 0.37 mmol)was added to a solution of methyl
deoxycholate 2 (0.2 g, 0.5 mmol) in dry CH2Cl2 (10 mL) and dry pyri-
dine (0.2 mL) at room temperature. The solution was placed in the
microwave oven and irradiated for 10 min at 100 W. Intermediate 3
was formed and without separation, the aromatic amines (1.5 mmol)
and dry pyridine (0.2 mL) were added directly to the mixture and the
irradiation was continued for 10–15 min at the same power. The sol-
vent was removed and the residue was diluted with ethyl acetate
(30 mL) and washed with 10% NaHCO3 (15 mL×3), brine (15 mL×3),
and finally dried over anhydrous Na2SO4. The solvent was evaporated
The recognition of molecular tweezers 4a, 4b and 4e for
aromatic amines and D/L –amino acid methyl esters has been
investigated by UV-visible spectra titration in CHCl3 at 25 °C.
We added aromatic amines and D/L-amino acid methyl esters
solution of different concentrations to 4a, 4b and 4e of fixed
concentration of 1×10–4–10×10–4 mol L–1, and measured the
characteristic absorbance value of the host–guest complexes.
* Correspondent. E-mail: zzg63129@yahoo.com.cn

482 JOURNAL OF CHEMICAL RESEARCH 2010
Scheme 1 Reagents and conditions: (i) CH3COCl, CH3OH; (ii) CH2Cl2, CO(OCCl3)2, pyridine; (iii) pyridine, ArNH2.
to give the crude product. The crude product was purified by column
chromatography on silica gel H with dichloromethane/ethyl acetate as
eluant. The physical and spectra data of the compounds 4a–k are as
follows.
4c: White solid, yield 94%, m.p. 88–90 °C, [α]2D0+79.9 (c 0.14,
CH2Cl2); IR (KBr)(cm–1): 3344, 2941, 2854, 1724, 1600, 1518, 1458,
1220, 1082; 1H NMR (400 MHz, CDCl3) δ: 7.29–7.26 (m, 4H, ArH),
6.89–6.83 (m, 4H, ArH), 6.58 (br, 2H, CONH), 5.12 (s, 1H, 12β–H),
4.64 (br, s, 1H, 3β–H), 3.80 (s, 3H, ArOCH3), 3.78 (s, 3H, ArOCH3),
3.65 (s, 3H, COOCH3), 0.96 (s, 3H, 19–CH3), 0.91 (d, J = 6.4 Hz, 3H,
21–CH3), 0.75 (s, 3H, 18–CH3); ESI–MS m/z (%): 727.5 [(M+Na)+,
100]. Anal. Calcd for C41H56N2O8: C, 69.86; H, 8.01; N, 3.97. Found
C, 69.70; H, 8.03; N 3.99%.
4a: White solid, yield 93%, m.p. 98–100 °C, [α]2D0+95.2 (c 0.13,
CH2Cl2); IR (KBr)(cm–1): 3346, 2946, 2882, 1726, 1534, 1442, 1220;
1H NMR (400 MHz, CDCl3) δ: 7.46 (d, J = 8.0 Hz, 2H, ArH), 7.35–
7.28 (m, 6H, ArH), 7.10–7.02 (m, 2H, ArH), 6.73 (s, 1H, CONH),
6.48 (s, 1H, CONH), 5.11 (s, 1H, 12β–H), 4.70–4.65 (m, 1H, 3β–H),
3.64 (s, 3H, COOCH3), 0.93 (s, 3H, 19–CH3), 0.89 (d, J = 6.4 Hz, 3H,
21–CH3), 0.76 (s, 3H, 18–CH3); ESI–MS m/z (%): 643.4 [(M-H)–,
100]. Anal. Calcd for C39H52 N2O6: C, 72.64; H, 8.13; N, 4.34. Found
C, 72.41; H, 8.10; N 4.36%.
4d: White solid, yield 92%, m.p. 101–103 °C, [α]2D0+105.3 (c 0.13,
CH2Cl2); IR (KBr)(cm–1): 3348, 2947, 2845, 1726, 1596, 1507, 1459,
1
1224, 1080; H NMR (400 MHz, CDCl3) δ: 7.33 (d, J = 8.0 Hz, 2H,
ArH), 7.23 (d, J = 8.0 Hz, 2H, ArH), 7.14 (d, J = 8.0 Hz, 2H, ArH),
7.09 (d, J = 8.0 Hz, 2H, ArH), 6.64 (s, 1H, CONH), 6.41 (s, 1H,
CONH), 5.09 (s, 1H, 12β–H), 4.69–4.63 (m, 1H, 3β–H), 3.64 (s, 3H,
COOCH3), 2.31 (s, 3H, ArCH3), 2.29 (s, 3H, ArCH3), 0.93 (s, 3H,
19–CH3), 0.89 (d, J = 6.4 Hz, 3H, 21–CH3), 0.75 (s, 3H, 18–CH3);
ESI–MS m/z (%): 695.5 [(M+Na)+, 100]. Anal. Calcd for C41H56N2O6:
C, 73.18; H , 8.39; N, 4.16. Found C, 72.98; H, 8.41; N, 4.18%.
4e: Pale yellow solid, yield 95%, m.p. 105–106 °C, [α]2D0+95. 4 (c
0.17, CH2Cl2); IR (KBr)(cm–1): 3360, 2946, 2844, 1734, 1598, 1432,
1222, 1062; 1H NMR (400 MHz, CDCl3) δ: 8.36 (s, 1H, ArH), 8.31 (s,
1H, ArH), 7.94–7.88 (m, 3H, ArH), 7.67 (s, 1H, ArH), 7.52–7.43 (m,
4b: Pale yellow solid, yield 95%, m.p. 127–128 °C, [α]2D0+128.2 (c
0.12, CH2Cl2); IR (KBr)(cm–1): 3358, 2946, 2843, 1736, 1602, 1510,
1412, 1216, 1048; 1H NMR (400 MHz, CDCl3) δ: 8.25 (d, J = 9.2 Hz,
2H, ArH), 8.20 (d, J = 9.2 Hz, 2H, ArH), 7.61 (d, J = 9.2 Hz, 2H,
ArH), 7.54 (d, J = 9.2 Hz, 2H, ArH), 7.18 (s, 1H, CONH), 6.92 (s, 1H,
CONH), 5.18 (s, 1H, 12β–H), 4.71–4.67 (m, 1H, 3β–H), 3.65 (s, 3H,
COOCH3), 0.95 (s, 3H, 19–CH3), 0.90 (d, J = 6.4 Hz, 3H, 21–CH3),
0.79 (s, 3H, 18–CH3); ESI–MS m/z (%): 757.3 [(M+Na)+, 100]. Anal.
Calcd for C39H50N4O10: C, 63.75; H, 6.86; N, 7. 62. Found C, 63.50; H,
6.89; N, 7.64%.

JOURNAL OF CHEMICAL RESEARCH 2010 483
Table 1 Synthetic comparison of molecular tweezers 4a–k
between microwave irradiation and conventional heating
C41H56N2O8: C, 69.86; H, 8.01; N; 3. 97. Found C, 69.74; H, 7.98; N
3.94%.
4g: White solid, yield 94%, m.p. 90–92 °C, [α]2D0+123.1 (c 0.13,
CH2Cl2); IR (KBr)(cm–1): 3350, 2944, 2854, 1730, 1612, 1542, 1450,
1220, 1056; 1H NMR (400 MHz, CDCl3) δ: 7.34 (s, 1H, ArH), 7.23–
7.10 (m, 5H, ArH), 6.90–6.84 (m, 2H, ArH), 6.67 (s, 1H, CONH),
6.42 (s, 1H, CONH), 5.10 (s, 1H, 12β–H), 4.67–4.63 (m, 1H, 3β–H),
3.64 (s, 3H, COOCH3), 2.35 (s, 3H, ArCH3), 2.31 (s, 3H, ArCH3), 0.93
(s, 3H, 19–CH3), 0.89 (d, J = 6.4 Hz, 3H, 21–CH3), 0.76 (s, 3H, 18–
CH3); ESI–MS m/z (%): 695.6 [(M+Na)+, 100]. Anal. Calcd for
C41H56N2O6: C, 73.18; H, 8.39; N 4.16. Found C, 72.96; H 8.37; N
4.14%.
a
Compd.
Conventional
method
Microwave
method
tc/tw
Time/min Yield/% Time/min Yield/%
4a
4b
4c
4d
4e
4f
4g
4h
4i
540
600
660
660
600
660
660
720
720
720
720
65
73
70
67
71
69
66
72
71
68
65
20
23
21
24
23
21
24
25
25
25
25
93
95
94
92
95
91
94
95
93
96
94
27
26
31
28
26
31
28
29
29
29
29
4h: White solid, yield 95%, m.p. 101–102 °C, [α]2D0+100.0 (c 0.13,
CH2Cl2); IR (KBr)(cm–1): 3344, 2946, 2875, 1718, 1596, 1526, 1448,
1
1220, 1092; H NMR (400 MHz, CDCl3) δ: 7.40 (d, J = 8.8 Hz, 2H,
4j
4k
ArH), 7.30 (d, J = 8.8 Hz, 4H, ArH), 7.25 (d, J = 8.8 Hz, 2H, ArH),
6.76 (s, 1H, CONH), 6.53 (s, 1H, CONH), 5.11 (s, 1H, 12β–H), 4.68–
4.62 (m, 1H, 3β–H), 3.64 (s, 3H, COOCH3), 0.92 (s, 3H, 19–CH3),
0.88 (d, J = 6.0 Hz, 3H, 21–CH3), 0.76 (s, 3H, 18–CH3); ESI–MS m/z
(%): 735.4 [(M+Na)+, 100]. Anal. Calcd for C39H50Cl2N2O6: C, 65.63;
H, 7.06; N 3.92. Found C, 65.50; H, 7.04; N 3.90%.
a tc, Conventional method needs time; tw, microwave method
needs time.
4i: White solid, yield 93%, m.p. 95–97 °C, [α]2D0+125.7 (c 0.12,
CH2Cl2); IR (KBr)(cm–1): 3343, 2942, 2883, 1716, 1586, 1530, 1484,
1218, 1062; 1H NMR (400 MHz, CDCl3) δ: 7.57 (s, 1H, ArH), 7.48 (s,
1H, ArH), 7.31 (s, 1H, ArH), 7.24–7.18 (m, 3H, ArH), 7.06–7.00 (m,
2H, ArH), 6.87 (s, 1H, CONH), 6.57 (s, 1H, CONH), 5.11 (s, 1H,
12β–H), 4.69–4.63 (m, 1H, 3β–H), 3.65 (s, 3H, COOCH3), 0.94
(s, 3H, 19–CH3), 0.87 (d, J = 6.4 Hz, 3H, 21–CH3), 0.75 (s, 3H, 18–
CH3); ESI–MS m/z (%): 735.4 [(M+Na)+, 100]. Anal. Calcd for
C39H50Cl2N2O6: C, 65.63; H, 7.06; N 3.92. Found C, 65.72; H, 7.08; N
3.93%.
Table 2 Association constants (Ka) and Gibbs free energy
changes (–∆G°) for the inclusion complexes of guests with
molecular tweezers 4a, 4b and 4e in CHCl3 at 25 oC
Host
Guest
Ka/L mol–1
–∆G°/kJ mol–1
4a
p-Nitroaniline
Aniline
p-Methoxyaniline
p-Nitroaniline
Aniline
1148.3
546.5
368.5
5191.2
1756.3
3832.4
776.1
629.3
536.5
17.45
15.61
14.63
21.18
18.50
20.43
16.48
15.96
15.56
4b
4e
4j: White solid, yield 96%, m.p. 105–106 °C, [α]2D0+103.3 (c 0.15,
CH2Cl2); IR (KBr)(cm–1): 3350, 2944, 2875, 1720, 1584, 1522, 1477,
p-Methoxyaniline
p-Nitroaniline
Aniline
1
1218, 1048; H NMR (400 MHz, CDCl3) δ: 7.44 (d, J = 8.8 Hz, 2H,
ArH), 7.40 (d, J = 8.8 Hz, 2H, ArH), 7.36 (d, J = 8.8 Hz, 2H, ArH),
7.25 (d, J = 8.8 Hz, 2H, ArH), 6.79 (s, 1H, CONH), 6.56 (s, 1H,
CONH), 5.10 (s, 1H, 12β–H), 4.68–4.64 (m, 1H, 3β–H), 3.64 (s, 3H,
COOCH3), 0.91 (s, 3H, 19–CH3), 0.86 (d, J = 6.4 Hz, 3H, 21–CH3),
0.75 (s, 3H, 18–CH3); ESI–MS m/z (%): 825.5 [(M+Na)+, 100]. Anal.
Calcd for C39H50Br2N2O6: C, 58.36; H, 6.28; N, 3.49. Found C, 58.20;
H, 6.30; N 3.51%.
p-Methoxyaniline
Table 3 Association constants(Ka) and Gibbs free energy
changes (–∆G0) for the inclusion complexes of amino acid
methyl esters with molecular tweezers 4a, 4b, 4e in CHCl3 at
25 oC
4k: White solid, yield 94%, m.p. 97–98 °C, [α]2D0+82.8 (c 0.17,
CH2Cl2); IR (KBr)(cm–1): 3342, 2946, 1718, 1582, 1526, 1480, 1217,
1050; 1H NMR (400 MHz, CDCl3) δ: 7.72 (s, 1H, ArH), 7.62 (s, 1H,
ArH), 7.39 (s, 1H, ArH), 7.28 (s, 1H, ArH), 7.19–7.11 (m, 4H, ArH),
6.94 (s, 1H, CONH), 6.63 (s, 1H, CONH), 5.10 (s, 1H, 12β–H), 4.66–
4.64 (m, 1H, 3β–H), 3.65 (s, 3H, COOCH3), 0.89 (s, 3H, 19–CH3),
0.88 (d, J = 6.4 Hz, 3H, 21–CH3), 0.77 (s, 3H, 18–CH3); ESI–MS m/z
(%): 825.3 [(M+Na)+, 100]. Anal. Calcd for C39H50Br2N2O6: C, 58.36;
H, 6.28; N 3.49. Found C, 58.25; H, 6.26; N 3.47%.
Host
Guest
Ka/L mol–1 –∆G°/kJ mol–1
KD/KL
4a
D-Phe-OMe
L-Phe-OMe
D-Tyr-OMe
L-Tyr-OMe
D-Phe-OMe
L-Phe-OMe
D-Tyr-OMe
L-Tyr-OMe
D-Phe-OMe
L-Phe-OMe
D-Tyr-OMe
L-Tyr-OMe
328.23
126.41
483.15
287.36
242.63
143.39
335.92
216.41
123.15
53.54
14.35
11.98
15.30
14.02
13.60
12.30
14.40
13.32
11.92
9.86
2.60
1.68
1.69
1.55
2.30
4.92
4b
4e
Conventional method for the preparation of molecular tweezers 4a–k
Triphosgene (0.11 g, 0.37 mmol)was added to a solution of methyl
deoxycholate 2 (0.2 g, 0.5 mmol) in dry CH2Cl2 (20 mL) and dry pyri-
dine (0.2 mL) at room temperature. It was then refluxed for 5 h. Inter-
mediate 3 was formed and without separation, aromatic amines (1.5
mmol) and dry pyridine (0.2 mL) were added directly to the mixture
and the reflux was continued for 4–7 h. The solvent was removed and
the residue was diluted with ethyl acetate (30 mL) and washed with
10% NaHCO3 (15 mL×3), brine (15 mL×3), and finally dried over
anhydrous Na2SO4. The solvent was evaporated to give the crude
product. The crude product was purified by column chromatography
on silica gel H with dichloromethane/ethyl acetate as eluant.
298.26
60.63
14.11
10.17
2H, ArH), 7.23 (s, 1H, CONH), 6.93 (s, 1H, CONH), 5.16 (s, 1H,
12β–H), 4.70 (br, s, 1H, 3β–H), 3.65 (s, 3H, COOCH3), 0.93 (s, 3H,
19–CH3), 0.91 (d, J = 6.8 Hz, 3H, 21–CH3), 0.77 (s, 3H, 18–CH3);
ESI–MS m/z (%): 757.3 [(M+Na)+, 100]. Anal. Calcd for C39H50N4O10:
C, 63.75; H, 6.86; N, 7. 62. Found C, 63.51; H, 6.88; N, 7.65%.
4f: White solid, yield 91%, m.p. 83–85 °C, [α]2D0+121.6 (c 0.15,
CH2Cl2); IR (KBr)(cm–1): 3346, 2949, 2843, 1732, 1606, 1547, 1468,
1222, 1044; 1H NMR (400 MHz, CDCl3) δ: 7.24–7.15 (m, 5H, ArH),
6.99 (d, J = 8.0 Hz, 1H, ArH), 6.84 (d, J = 8.0 Hz, 2H, ArH), 6.82–
6.58 (m, 2H, CONH), 5.11 (s, 1H, 12β–H), 4.65 (br, s, 1H, 3β–H),
3.81 (s, 3H, ArOCH3), 3.78 (s, 3H, ArOCH3), 3.65 (s, 3H, COOCH3),
0.91 (s, 3H, 19–CH3), 0.89 (d, J = 6.4 Hz, 3H, 21–CH3), 0.75 (s, 3H,
18–CH3); ESI–MS m/z (%): 727.6 [(M+Na)+, 100]. Anal. Calcd for
We thank the Natural Science Foundation of the State Ethnic
Affairs Commission of P.R.China (Project No.09XN08) for
the financial support.
Received 28 June 2010; accepted 23 July 2010
Paper 1000229 doi: 10.3184/030823410X12812894581758
Published online: 7 October 2010

484 JOURNAL OF CHEMICAL RESEARCH 2010
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Chinese).

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