A concise synthesis of licochalcone E and its Regio-isomer, licochalcone F

Article abstract of DOI:10.1248/cpb.57.607

Licochalone E is one of the retrochalcones isolated from Glycyrrhiza inflata which shows potent cytotoxicty against human tumor cell lines. Biological studies suggested that topoisomerase I inhibition correlates with cytotoxic properties. Other research r

Full text of DOI:10.1248/cpb.57.607

June 2009
Notes
Chem. Pharm. Bull. 57(6) 607—609 (2009)
607
A Concise Synthesis of Licochalcone E and Its Regio-Isomer,
Licochalcone F
d
Younghwa NA, ,a Jeong-Heon CHA,b Ho-Geun YOON,c and Youngjoo KWON
*
a College of Pharmacy, Catholic University of Daegu; Gyeongsan, Gyeongbuk 712–702, Korea: b Department of Oral
Biology, BK21 Project, Oral Science Research Center, College of Dentistry, Yonsei University; c Department of
Biochemistry and Molecular Biology, Center for Chronic Metabolic Disease Research, College of Medicine, Yonsei
University; Seoul 120–752, Korea: and d College of Pharmacy, Ewha Womans University; Seoul 120–750, Korea.
Received November 14, 2008; accepted February 27, 2009
Licochalone E is one of the retrochalcones isolated from Glycyrrhiza inflata which shows potent cytotoxicty
against human tumor cell lines. Biological studies suggested that topoisomerase I inhibition correlates with cyto-
toxic properties. Other research revealed that licochalcone E modulats the nuclear factor (NF)-kB and Bcl-2
families to induce endothelial cell apoptosis. Since licochalcone E has been isolated recently, synthetic informa-
tion on this compound has not been reported yet. Therefore we report the concise synthesis of licochalcone E and
its regioisomer, tentatively called licochalcone F, by employing Claisen rearrangement for key intermediate syn-
thesis.
Key words retrochalcone; licochalcone E; licochalcone F; Claisen rearrangement
Retrochalcones,1,2) which lack 2ꢀ- and 6ꢀ-hydroxy groups
Since there are no reports on the synthesis of licochalcone
in the normal chalcone core, are an unusual phenolic com- E, we report here a concise synthetic method for licochal-
pound family. Glycyrrhiza inflata is the major source of these cone E and its regio-isomer, tentatively called licochalcone F
compounds. Six retrochalcone compounds, licochalcone A— (1b), prepared as by-product in the course of synthesis.
E and echinatin, have been isolated from Glycyrrhiza inflata.
They have shown various pharmacological profiles including Results and Discussion
anticancer,3,4) antiparasitic,5) antibacterial,2) antioxidative and
The key step employed for the synthesis was Claisen re-
superoxide-scavenging activities.6) Among these compounds, arrangement in an N,N-dimethylaniline solvent system9) at
licochalcone E7) has recently been isolated and its biological high temperature using n-butyric anhydride as the protective
study is in the initial stage. According to the biological activ- group of phenolic OH generated as a rearrangement product.
ity reports, licochalone E has potent cytotoxic activity and
The synthetic methods are depicted in Chart 1. First, we
this might be correlated with topoisomerase I inhibitory prepared a key intermediate, 5-(1,2-dimethyl-2-propenyl)-4-
properties. It has also been reported that licochalcone E mod- hydroxy-2-methoxy benzaldehyde (4a) from 4-hydroxy-2-
ulated the nuclear factor (NF)-kB and Bcl-2 families to in- methoxybenzaldehyde (1). 4-Hydroxy-2-methoxybenzalde-
duce endothelial cell apoptosis.8) We, however, suspect that hyde was coupled with 1-bromo-E-2-methyl-2-butene10)
this compound needs more intense biological investigation to under K2CO3 basic condition in acetone to give an O-alkeny-
1
verify the exact biological targets and its mechanism of ac- lated compound (2). In the H-NMR spectrum, we observed
tion. But, like other natural compounds isolated from plants,
the isolation yield of licochalcone E is very low (5 mg from
1 kg of powdered Glycyrrhiza inflata)4) and this is another
bottleneck for the further biological study of this compound.
Chart 1. Synthetic Method for Licochalcones E and F
∗ To whom correspondence should be addressed. e-mail: yna7315@cu.ac.kr
© 2009 Pharmaceutical Society of Japan

608
Vol. 57, No. 6
a quartet peak of a C-3ꢀ methine proton at d 5.66 coupled cone F is reported for the first time. This result will provide a
with a C-4ꢀ methyl group at d 1.68 and a singlet peak of C-1ꢀ tool to secure sufficient quantities of licochalcones E and F
methylene protons at d 4.45. To achieve the rearrangement necessary for further biological study to elucidate the mecha-
of the alkenyl group to the ortho-position, we employed two- nism of action of these compounds. Pharmacological activity
step methods modified from those reported in the literature9) studies of these compounds are ongoing, and the results will
including Claisen rearrangement as key process in N,N- be reported in future.
dimethylaniline with n-butyric anhydride at 220—240 °C
under nitrogen. We separated two compounds, 3a and 3b, in
Experimental
The solvents and reactants were of the best commercial grade available
the first step. In both compounds, the methine proton signal and were used without further purification unless noted. TLC plates were
Kieselgel 60 F254 (art A715, Merck), and for column chromatography Silica
of compound 2 at d 5.66 disappeared and a new quartet (3a)
gel 60 (0.040—0.063 mm ASTM, Merck) was used. 1H- and 13C-NMR spec-
or multiplet (3b) signal coupled with a methyl signal at d
tra were recorded on Varian NMR AS 400 MHz instrument. Chemical shifts
(d) are in parts per million (ppm) relative to tetramethylsilane as an internal
standard, and coupling constants (J values) are in Hertz. Mass spectral inves-
1.43 was generated. We also observed two sets of singlet sig-
nals corresponding to the protons attached to the terminal sp2
tigations were performed on a GC:7890A MS:5975C MSD (Agilent,
U.S.A.) mass spectrometer equipped with an electron ionization (EI) source
carbon at d 4.83 and 4.86 for 3a and at d 4.89 and 4.90 for
3b. These observations confirmed that the double bond loca-
at the Catholic University of Daegu, Gyeongsan, Korea. Melting points were
tion between C-2ꢀ and C-3ꢀ of the butenyl group in com-
measured on a Barnstead International MEL-TEMP 1202D instrument with-
pound 2 was shifted to the terminus of the propenyl group by
out correction.
the rearrangement process. Finally, compound 3a was con-
firmed by observing two singlet protons at d 6.77 and 7.66
corresponding to the C-3 and C-6 protons and other signals
Synthesis of Compound 2 1-Bromo-E-2-methyl-2-butene in the ace-
tone (10 ml) was added to the reaction mixture of 4-hydroxy-2-methoxyben-
zaldehyde (0.40 g, 2.63 mmol) and K2CO3 (0.57 g, 4.14 mmol) in the acetone
(20 ml). The reaction mixture was refluxed for 8 h and cooled to room tem-
perature. After filtration of solids, the solvent was evaporated under reduced
pressure. The residue was purified by silica gel column chromatography
(eluent: EtOAc : n-hexaneꢁ1 : 5) to give a colorless oil (0.4 g, 66.1%).
Compound 2: Rf: 0.60 (EtOAc : n-hexaneꢁ1 : 3), 1H-NMR (400 MHz,
CDCl3) d: 1.68 (d, Jꢁ6.8 Hz, 3H), 1.74 (s, 3H), 3.89 (s, 3H), 4.45 (s, 2H),
5.66 (q, Jꢁ6.8 Hz, 1H), 6.48 (d, Jꢁ2.0 Hz, 1H), 6.54 (dd, Jꢁ2.0, 8.8 Hz,
1H), 7.78 (d, Jꢁ8.8 Hz, 1H), 10.28 (s, 1H); 13C-NMR (100 MHz, CDCl3) d:
13.5, 13.8, 55.8, 74.7, 98.8, 106.7, 19.1, 124.7, 130.9, 131.2, 163.8, 165.9,
1
derived from the butyroyl group in the H-NMR spectrum.
This compound was isolated together with starting com-
pound 2. The other regio-isomer 3b showed two doublet sig-
nals at d 6.69 and 7.74 with coupling constant Jꢁ8.4 Hz,
which is the conventional coupling pattern of ortho protons
in the benzene ring system. Other 1H- and 13C-NMR data are
similar to those of compound 3a. Based on the spectral data,
we assigned compound 3b as a regio-isomer of compound 188.6 ppm; EI-MS (m/z) [M]ꢂ 220.1.
3a. The isolation ratio of these two regio-isomers was ap-
proximately 2 : 1 (3a : 3b).
Synthesis of Compounds 3a and 3b The reaction mixture of com-
pound 2 (0.76 g, 3.28 mmol) in N,N-dimethylaniline (7 ml) and n-butyric an-
hydride (3 ml) was kept at 220—240 °C for 3 h under nitrogen. After cooling
to room temperature, the reaction mixture was poured into d-HCl and ex-
tracted with ethyl acetate. The organic layer was separated, washed with sat-
urated NaCl and dried with anhydrous Na2SO4. The solvent was evaporated
under reduced pressure and the residue was purified by silica gel column
chromatography (eluent: EtOAc : n-hexaneꢁ1 : 5) to give compounds 3a
(0.35 g, 35.5%) and 3b (0.16 g, 15.7%) as oil, respectively.
After hydrolysis of 3a and 3b in 10% NaOH/EtOH, we ob-
tained key intermediates 4a and 4b in 99% and 84% yields,
respectively. Before coupling 4a and 4b with 4-tetrahydropy-
ranyloxyacetophenone (6), we protected the 4-hydroxyl
group of 4a and 4b to increase the product formation. Ac-
cording to the literature9) and our laboratory experience, per-
forming the condensation reaction without protection of the
hydroxyl group reduces the reaction yield. Compounds 5a
and 5b were prepared after protection of 4a and 4b with THP
and PTSA in CH2Cl2. Even 72 h reaction time at 50 °C was
not sufficient to complete this reaction, and we separated the
protected and starting compounds. We suspected that this
slow protection process was generated by the 1,2-dimethyl-2-
propenyl group located at the ortho-position, leading to the
steric hindrance of the OH group in compounds 4a and 4b.
The final condensation of 5a or 5b with 4-tetrahydropyrany-
Compound 3a: Rf: 0.56 (EtOAc : n-hexaneꢁ1 : 3), 1H-NMR (400 MHz,
CDCl3) d: 1.05 (t, Jꢁ7.6 Hz, 3H), 1.33 (d, Jꢁ7.2 Hz, 3H), 1.57 (s, 3H), 1.87
(hexet, Jꢁ7.6 Hz, 2H), 2.57 (t, Jꢁ7.6 Hz, 2H), 3.47 (q, Jꢁ7.2 Hz, 1H), 3.89
(s, 3H), 4.83 (s, 1H), 4.86 (s, 1H), 6.69 (s, 1H), 7.74 (s, 1H), 10.39 (s, 1H);
EI-MS (m/z) [M]ꢂ 290.1.
Compound 3b: Rf: 0.74 (EtOAc : n-hexaneꢁ1 : 3), 1H-NMR (400 MHz,
CDCl3) d: 1.03 (t, Jꢁ7.6 Hz, 3H), 1.46 (d, Jꢁ7.2 Hz, 3H), 1.61 (s, 3H), 1.74
(hexet, Jꢁ7.6 Hz, 2H), 2.49 (dt, Jꢁ2.8, 7.6 Hz, 2H), 3.87—3.90 (m, 1H),
3.91 (s, 3H), 4.89 (s, 1H), 4.90 (s, 1H), 6.92 (d, Jꢁ8.4 Hz, 1H), 7.76 (d,
Jꢁ8.4 Hz, 1H), 10.31 (s, 1H); 13C-NMR (100 MHz, CDCl3) d: 13.9, 17.7,
18.3 (*2), 22.9, 36.2, 65.3, 109.7, 120.7, 127.3, 128.0, 131.9, 147.1, 155.5,
163.1, 171.3, 189.4 ppm; EI-MS (m/z) [M]ꢂ 290.1.
General Synthetic Method for Compounds 4a and 4b The reaction
loxyacetophenone (6) was conducted with NaOH in EtOH to mixture in 10% NaOH and EtOH was stirred at 110 °C for 3 h. The solvent
was reduced under reduced pressure, and water was added to the residue.
The aqueous layer was washed with ether and acidified with 2 N HCl. The re-
sulting cloudy aqueous solution was extracted with ether and the organic
layer was washed with saturated NaHCO3. After drying with anhydrous
Na2SO4, the solvent was removed and dried under a vacuum to give the de-
sired products.
give the desired compounds licochalcone E or F, respectively.
The spectral data of licochalcone E were consistent with
those in the literature.7) All the spectral data for licochalcone
1
F were similar to those of licochalcone E. In the H-NMR
spectrum, two doublet signals were coupled with each other
with coupling constant Jꢁ15.6 Hz, which confirms the trans-
structure of licochalcone F.11)
Compound 4a (Orange Solid, 99%): mp 126 °C; Rf: 0.21 (EtOAc : n-
hexaneꢁ1 : 3), 1H-NMR (400 MHz, CDCl3) d: 1.43 (d, Jꢁ7.2 Hz, 3H), 1.65
(s, 3H), 3.55 (q, Jꢁ7.2 Hz, 1H), 3.87 (s, 3H), 5.06 (s, 1H), 5.12 (s, 1H), 6.77
(s, 1H), 7.66 (s, 1H), 10.28 (s, 1H); 13C-NMR (100 MHz, CDCl3) d: 18.6,
20.8, 41.9, 55.9, 99.9, 112.3, 118.7, 122.5, 129.6, 149.9, 162.5, 163.0,
188.9 ppm; EI-MS (m/z) [M]ꢂ 220.1.
Conclusion
In summary, we successfully synthesized licochalcones
E and F using 5-(1,2-dimethyl-2-propenyl)-4-hydroxy-2-
methoxy benzaldehyde (4a) as the key intermediate in five
steps. This is an efficient synthesis for licochalcone E of nat-
ural resources and its regio-isomer, licochalcone F. Licochal-
1
Compound 4b (Semi-solid, 84%): Rf: 0.38 (EtOAc : n-hexaneꢁ1 : 3), H-
NMR (400 MHz, CDCl3) d: 1.43 (d, Jꢁ6.8 Hz, 3H), 1.73 (s, 3H), 3.90 (s,
3H), 3.97 (q, Jꢁ6.8 Hz, 1H), 5.20 (dd, Jꢁ1.2, 3.2 Hz, 1H), 5.21 (s, 1H),
6.67 (d, Jꢁ8.4 Hz, 1H), 7.76 (d, Jꢁ8.4 Hz, 1H), 10.28 (s, 1H); 13C-NMR
(100 MHz, CDCl3) d: 17.5, 23.1, 36.0, 65.0, 112.6, 114.2, 122.8, 123.3,

June 2009
609
129.7, 149.7, 162.8, 163.0, 189.3 ppm; EI-MS (m/z) [M]ꢂ 220.1.
Jꢁ7.2 Hz, 2H), 7.51 (s, 1H), 7.63 (d, Jꢁ15.6 Hz, 1H), 7.98 (d, Jꢁ7.2 Hz,
2H), 8.00 (d, Jꢁ15.6 Hz, 1H); 13C-NMR (100 MHz, acetone-d6) d: 18.9,
21.6, 38.0, 55.3, 99.1, 109.5, 115.4 (*2), 115.9, 118.9, 124.3, 128.7, 130.8
(*2), 131.1, 139.1, 149.2, 158.6, 159.1, 161.6, 187.7 ppm; EI-MS (m/z)
[M]ꢂ 338.2, [MꢃCH3O]ꢂ 307.2.
General Synthetic Method for Compounds 5a and 5b A solution
of compound 4a or 4b (1 eq), pyridinium p-toluenesulfonate (catalytic
amount), and 3,4-dihydro-2H-pyran (2 eq) in CH2Cl2 was stirred at 30 °C for
72 h. The reaction mixture was diluted with CH2Cl2. The solution was
washed with 1 M Na2CO3, dried with anhydrous Na2SO4, and concentrated
under reduced pressure. The residue was applied to silica gel column chro-
matography (eluent: EtOAc : n-hexaneꢁ1 : 5) to give the desired products as
oil in diastereomeric mixture.
Licochalcone
F (42.1%): mp 165—167 °C; Rf: 0.28 (EtOAc : n-
1
hexaneꢁ1 : 1), H-NMR (400 MHz, acetone-d6) d: 1.49 (d, Jꢁ7.2 Hz, 3H),
1.68 (s, 3H), 3.74 (s, 3H), 3.98 (q, Jꢁ7.2 Hz, 1H), 4.85 (d, Jꢁ1.4 Hz, 1H),
4.86 (d, Jꢁ1.4 Hz, 1H), 6.71 (d, Jꢁ8.7 Hz, 1H), 6.96 (d, Jꢁ8.8 Hz, 2H),
7.68 (d, Jꢁ15.6 Hz, 1H), 7.69 (d, Jꢁ8.7 Hz, 1H), 7.99 (d, Jꢁ15.6 Hz, 1H),
8.05 (d, Jꢁ8.8 Hz, 2H); 13C-NMR (100 MHz, acetone-d6) d: 16.8, 22.2,
36.3, 62.5, 109.0, 112.8, 115.5 (*2), 119.9, 120.7, 125.1, 127.2, 130.9, 131.0
(*2), 138.7, 148.1, 159.5, 160.1, 161.8, 187.6 ppm; EI-MS (m/z) [M]ꢂ
338.2, [MꢃCH3O]ꢂ 307.2.
Compound 5a (82.0%): Rf: 0.42 (EtOAc : n-hexaneꢁ1 : 3), 1H-NMR
(400 MHz, CDCl3) d: 1.33 (1.35) (d, Jꢁ7.2 Hz, 3H), 1.59 (1.62) (s, 3H),
1.69 (1.74) (m, 2H), 1.88—1.92 (m, 2H), 1.96—2.02 (m, 2H), 3.66—3.74
(m, 1H), 3.75—3.79 (m, 1H), 3.80—3.86 (m, 1H), 3.89 (s, 3H), 4.80 (s,
1H), 4.83 (4.84) (s, 1H), 5.58 (d, Jꢁ10.8 Hz, 1H), 6.77 (s, 1H), 7.66 (7.67)
(s, 1H), 10.31 (s, 1H); 13C-NMR (100 MHz, CDCl3) d: 18.4 (18.5), 19.3
(19.4), 21.9 (22.1), 25.3, 30.3 (30.4), 38.7 (38.8), 55.8 (55.9), 61.8 (62.0),
96.1 (96.4), 97.7 (97.8), 110.2 (110.3), 118.7 (118.8), 126.7 (126.8), 127.7
(127.8), 148.7 (148.8), 161.3 (161.4), 162.5, 188.8 ppm; EI-MS (m/z)
[MꢃDHP]ꢂ 220.1.
References
1) Kajiyama K., Demizu S., Hiraga Y., Kinoshita K., Koyama K., Taka-
hashi K., Tamura Y., Okada K., Kinoshita T., Phytochemistry, 31,
3229—3232 (1992).
Compound 5b (45.8%): Rf: 0.58 (EtOAc : n-hexaneꢁ1 : 3), 1H-NMR
(400 MHz, CDCl3) d: 1.44 (1.48) (d, Jꢁ7.2 Hz, 3H), 1.61 (1.65) (s, 3H),
1.82—1.92 (m, 4H), 1.96—2.02 (m, 2H), 3.61—3.66 (m, 1H), 3.76—3.88
(m, 1H), 3.87 (3.88) (s, 3H), 3.95—4.03 (m, 1H), 4.81 (4.83) (s, 1H), 4.84
(4.95) (s, 1H), 5.43—5.44 (m, 1H), 7.01 (7.03) (d, Jꢁ8.4 Hz, 1H), 7.73 (d,
Jꢁ8.4 Hz, 1H), 10.22 (10.23) (s, 1H); 13C-NMR (100 MHz, CDCl3) d: 17.3
(17.5), 18.4 (18.6), 22.9, 25.3, 30.1 (30.3), 35.8 (35.9), 61.9 (62.0), 64.8
(64.9), 96.1 (96.7), 108.8 (109.0), 110.8 (111.0), 123.4 (123.5), 127.3
(127.4), 129.0 (129.3), 147.6 (147.7), 162.2 (162.5), 162.8 (162.9),
189.5 ppm; EI-MS (m/z) [MꢃDHP]ꢂ 220.1.
2) Haraguchi H., Tanimoto K., Tamura Y., Mizutani K., Kinoshita T.,
Phytochemistry, 48, 125—129 (1998).
3) Park E. J., Park H. R., Lee J. S., Kim J. W., Planta Med., 64, 464—466
(1998).
4) Yoon G., Kang B. Y., Cheon S. H., Arch. Pharm. Res., 30, 313—316
(2007).
5) Nielsen S. F., Chen M., Theander T. G., Kharazmi A., Christensen S.
B., Bioorg. Med. Chem. Lett., 5, 449—452 (1995).
6) Haraguchi H., Ishikawa H., Mizutani K., Tamura Y., Kinoshita T.,
Bioorg. Med. Chem., 6, 339—347 (1998).
General Synthetic Method for Licochalcones E and F A solution of 6
(1 eq), compound 5a or 5b (1 eq), and NaOH (1.5 eq) in EtOH was stirred at
50 °C for 17 h. To this solution was added 4 M HCl and this mixture was
stirred for an additional 20 min. After adding water, the reaction mixture was
extracted with ethyl acetate, and the organic layer was washed with water
and saturated NaCl successively. After drying with anhydrous Na2SO4, the
solvent was removed under reduced pressure. The residue was applied to sil-
ica gel column chromatography (eluent: EtOAc : n-hexaneꢁ1 : 3 to 3 : 2) to
give desired products.
7) Yoon G., Jung Y. D., Cheon S. H., Chem. Pharm. Bull., 53, 694—695
(2005).
8) Chang H. J., Yoon G., Park J. S., Kim M. H., Baek M. K., Kim N. H.,
Shin B. A., Ahn B. W., Cheon S. H., Jung Y. D., Biol. Pharm. Bull.,
30, 2290—2293 (2007).
9) Xu R.-S., Wen K.-L., Jiang S. F., Wang C. G., Jiang F. X., Xie Y. Y.,
Gao Y. S., Acta Chim. Sinica, 37, 289—297 (1979).
10) Trost B. M., Papillon J. P. N., Nussbaumer T., J. Am. Chem. Soc., 127,
17921—17937 (2005).
11) Jeong B. S., Wang Q., Son J. K., Jahng Y., Eur. J. Org. Chem., 2007,
1338—1344 (2007).
Licochalcone E (77.4%): Rf: 0.28 (EtOAc : n-hexaneꢁ1 : 1), 1H-NMR
(400 MHz, acetone-d6) d: 1.34 (d, Jꢁ7.0 Hz, 3H), 1.66 (s, 3H), 3.79 (q,
Jꢁ7.0 Hz, 1H), 3.87 (s, 3H), 4.87 (s, 1H), 4.89 (s, 1H), 6.59 (s, 1H), 6.94 (d,