Facile synthesis of acacetin and its derivatives

Article abstract of DOI:10.1016/j.bmcl.2016.06.018

Acacetin, a O-methylated bioflavonoid isolated from the traditional Chinese medicine Xuelianhua (Saussurea tridactyla), is a promising orally effective atrium-selective antiarrhythmic agent for the treatment of atrial fibrillation (AF). Here we describe an efficient two-component method for the synthesis of acacetin and its derivatives.

Full text of DOI:10.1016/j.bmcl.2016.06.018

Accepted Manuscript
Facile Synthesis of Acacetin and Its Derivatives
Yuan Zhao, Li Cai, Qiang Sui, Feng Lin, Wen Jiang, Jianli Chen, Weigeng Lu,
Qi Gao
PII:
S0960-894X(16)30629-1
DOI:
http://dx.doi.org/10.1016/j.bmcl.2016.06.018
BMCL 23971
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
11 May 2016
1 June 2016
8 June 2016
Please cite this article as: Zhao, Y., Cai, L., Sui, Q., Lin, F., Jiang, W., Chen, J., Lu, W., Gao, Q., Facile Synthesis
of Acacetin and Its Derivatives, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://dx.doi.org/10.1016/
j.bmcl.2016.06.018
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Facile Synthesis of Acacetin and Its Derivatives
b,
Yuan Zhao a, Li Cai *, Qiang Sui a, Feng Lin a, Wen Jiang c, Jianli Chen a, Weigeng Lu a,*, Qi
Gao a,*
a China State Institute of Pharmaceutical Industry, 285 Gebaini Road, Shanghai, 200120, China
b
Division of Mathematics and Science, University of South Carolina Salkehatchie, Walterboro,
SC 29488, United States
c
China National Pharmaceutical Industry Information Center, 1320 West Beijing Road,
Shanghai, 200040, China
* Corresponding author
Tel: +1 843-7828681 (L. Cai); +86 21-20572000 ext. 6045 (Q. Gao)
Email address: CAILI@mailbox.sc.edu (L. Cai); sipiluwg@163.com (W. Lu);
gao.qi@sipi.com.cn (Q. Gao)

Abstract: Acacetin, a O-methylated bioflavonoid isolated from the traditional Chinese medicine
Xuelianhua (Saussurea tridactyla), is a promising orally effective atrium-selective
antiarrhythmic agent for the treatment of atrial fibrillation (AF). Here we describe an efficient
two-component method for the synthesis of acacetin and its derivatives.
Graphic Abstract
Key words: Flavones, acacetin, synthesis, Fries rearrangement, atrial fibrillation, catalysis

Flavones are a class of flavonoids based on the backbone of 2-phenyl-1-benzopyran-4-
one. These compounds have been shown to possess antioxidant1, anti-proliferative, anti-tumor,
anti-microbial2, estrogenic3, acetylcholinesterase inhibitory, and anti-inflammatory activities4
and thus been used in cancer, cardiovascular disease, neurodegenerative disorders, etc.5
Therefore, wide attention has been attracted to the design and optimization of polyfunctional
flavone derivatives for the development of new therapeutic agents.6-7 Atrial fibrillation (AF) is
the most common form of sustained cardiac dysrhythmia and a major cause of morbidity and
mortality because it increases the risk of death, congestive heart failure, and embolic phenomena
including stroke.8 Acacetin (4'-methoxy-5,7-dihydroxyflavone, Scheme 1) is a natural flavone
compound extracted from the Chinese medicine Xuelianhua that selectively inhibits ultrarapid
delayed rectifier potassium current (IKur, a major target for the treatment of AF) in human atria
and effectively prevents AF in anesthetized dogs after intraduodenal administration.9 Further
studies revealed that acacetin mainly blocks hKv1.5 channels (coding IKur) in a use- and
frequency-dependent manner by binding to the S6-domain.10 These results indicate that oral
acacetin is a promising atrium-selective agent for the treatment of AF. In addition, other novel
pharmacological properties of acacetin, including antinociceptive/anti-inflammatory11 and anti-
proliferative/anti-cancer12-14 activities, have also been discovered recently.
Scheme 1. Previous methods for the preparation of acacetin and its associated derivatives.
While many pathways on the synthesis of various flavones have been reported15, less
attention has been drawn to the preparation of acacetin. Thus the main source of acacetin is
usually through extraction and isolation from plant materials.16-17 Traditional methods for
acacetin synthesis were mainly based on the Baker-Venkataraman rearrangement in which a base
promoted intramolecular acyl transfer in 2-acetoxyacetophenone leads to a 1,3-diketone
intermediate (a quick cyclization-dehydration step is thus followed to afford the acacetin

backbone) (Scheme 1, routes I-III). For example, Chatterjee et al. reported a low-yield route
utilizing phloroglucinol to synthesize 2,4,6-trihydroxyacetophenone which was then coupled
with 4-methoxybenzoyl chloride to give the rearrangement precursor (Scheme 1, route I).16
Costantino et al. treated the 2,4,6-trihydroxyacetophenone with adequate amount of LiHMDS to
deprotonate all of the phenols and generate the lithium enolate before the acid chloride was
added; and the 1,3-diketone intermediate was afforded directly (Scheme 1, route II). However,
the 28% yield of acacetin was still not satisfying.18 Gao improved the overall yield to 45% by
treating the 2,4,6-trihydroxyacetophenone with aroyl chloride in the presence of an excess
amount of potassium carbonate in a one-step fashion (Scheme 1, route III).19-20 In a different way
(Scheme 1, route IV), Mentzer reported a two-component thermal cyclocondensation between
phenols and β-ketoesters to yield a wide number of acacetin related flavones under high
temperature.21-22 Later, Seijas et al. enhanced this two-component synthesis by applying
microwave irradiation.23 Although with microwave heating the relatively high temperature
o
reached (~225 C) was similar to the one required under Mentzer’s approach22, the reaction time
was greatly reduced to minutes and thus yields were significantly improved (>80% vs. ~20%).23
Given that Seijas’s study reported consistently higher yields for flavones with different
substituted patterns on the B ring, a more broadly applicable method for flavones with
substitution on both aromatic rings is still needed, specifically to those researchers who do not
have ready access to microwave reactors. After studying the proposed mechanism of the two-
component synthesis that involves several key steps (transesterification, Fries rearrangement, and
cyclization)23-24, we hypothesize that adding a catalyst that promotes these key steps will enhance
and favor the product formation (Scheme 2).
As shown in Table 1, we started with several copper (II) salts as it was previously
reported that CuCl2 could greatly facilitate the dehydrative cyclization step under microwave
conditions.25 However, from the outset we did not set high expectations for these reactions to
offer significant improvement given that the particular step involved (cyclization) is non-rate-
determining for this two-component conversion. Just as we anticipated, no product was observed
for CuCl2 while the other two copper (II) salts were relatively ineffective (Table 1, entries 1-3).
We thus decided to switch to an acylation catalyst such as DMAP and tributylphosphine26 that
could act as a weak base and facilitate the transesterification and Fries rearrangement steps. In
fact, DMAP-mediated Fries rearrangement has already been reported recently and successfully
used in the synthesis of complex structures.27-29 As expected, the model reactions catalyzed by
DMAP, PPY (a DMAP derivative) and Bu3P all offered greatly improved yields with best result
obtained with DMAP (Table 1, entries 4-6). With the addition of the acylation catalyst, this
thermal cyclocondensation proceeded in an enhanced fashion and we speculate that the reason
for the enhancement can be seen in the mechanism of this reaction (Scheme 2). The acylation
promoter acted as a base that activated the β-ketoester starting material to yield an α-oxo ketene
intermediate.23,30 Addition of the phenol to the α-oxo ketene yielded a phenol ester intermediate,
completing the “transesterification” step. Then the phenol ester (O-acylation product) underwent
a base-promoted ortho-Fries rearrangement, followed by cyclization of the resulting 1,3-diaryl
diketone (formation of the hemiketal). Final proton transfer and dehydration drove the reaction
sequence forward.24 As a result, we successfully extended this method for the synthesis of a
series of acacetin derivatives using DMAP as the catalyst and the results were summarized in
Table 2. In order to investigate the substrate scope of this reaction, several substituted
phloroglucinol type phenols (e.g. Table 2, entries 1-4) and β-ketoesters (e.g. Table 2, entries 1, 5,
6 and 9) were tried. These results demonstrate that the reaction can tolerate a variety of

substitution patterns and electron donating/withdrawing groups on the aromatic rings. It also
provided a series of acacetin derivatives for prospective structure-activity relationship analysis.
Table 1. Catalyst screening for two-component synthesis of acacetin.
cat. (5%)
CuCl2
reaction time (h) yield
entry
5
5
5
0
1
2
3
4
5
6
CuSO4
23%
15%
56%
50%
53%
Cu(OTf)2
4-dimethylaminopyridine (DMAP) 2.5
4-pyrrolidinopyridine (PPY)
Bu3P
2.5
2.5
Scheme 2. Proposed mechanism for base catalyzed two-component acacetin synthesis.

Table 2. Substrate scope of DMAP-catalyzed cyclocondensation
entry substrate 1
substrate 2
Time (h)
product
comp. yield
1
3
71%
49%
73%
16%
1a
1b
1c
1d
2
3
4
3
3
3
5
6
7
8
3
3
3
3
53%
86%
80%
23%
2a
3a
3b
3c
9
3
67%
4a
In conclusion, we report herein an efficient one-pot synthesis of acacetin and its
derivatives, where DMAP acted as an effective catalyst for the two-component condensation
reaction. Based on the previously reported mechanism of this reaction, we speculate that the
weak base catalyst facilitated the formation of the α-oxo ketene intermediate and especially the
ortho-Fries rearrangement. This method is valid for flavones with different substitution patterns
on the aromatic rings. The pharmacological and pharmacokinetics studies of the obtained
molecules are ongoing.
Acknowledgements. L.C. acknowledges the support through an RISE (Research Initiative for
Summer Engagement) grant from the Office of the Vice President for Research at the University
of South Carolina.

Supplementary data: Supplementary data (experimental procedures and characterization of
compounds) associated with this article can be found, in the online version, at doi:
References and notes
1. Yokozawa, T.; Dong, E.; Liu, Z. W.; Shimizu, M. Phytother. Res. 1997, 11, 446.
2. Brinkworth, R. I.; Stoermer, M. J.; Fairlie, D. P. Biochem. Biophys. Res. Commun. 1992, 188, 631.
3. de Oliveira, A. P. S.; de Sousa, J. F.; da Silva, M. A.; Hilário, F.; Resende, F. A.; de Camargo, M. S.; Vilegas, W.;
dos Santos, L. C.; Varanda, E. A. Steroids 2013, 78, 1053.
4. Choi, J.-S.; Choi, Y.-J.; Park, S.-H.; Kang, J.-S.; Kang, Y.-H. J. Nutr. 2004, 134, 1013.
5. Singh, M.; Kaur, M.; Silakari, O. Eur. J. Med. Chem. 2014, 84, 206.
6. Li, R. W.; Theriault, A. G.; Au, K.; Douglas, T. D.; Casaschi, A.; Kurowska, E. M.; Mukherjee, R. Life Sci. 2006,
79, 365.
7. Guo, L.; Hu, W.-R.; Lian, J.-H.; Ji, W.; Deng, T.; Qian, M.; Gong, B.-Q. Eur. J. Pharmacol. 2006, 551, 80.
8. Stewart, S.; Hart, C. L.; Hole, D. J.; McMurray, J. J. V. Am. J. Med. 2002, 113, 359.
9. Li, G.-R.; Wang, H.-B.; Qin, G.-W.; Jin, M.-W.; Tang, Q.; Sun, H.-Y.; Du, X.-L.; Deng, X.-L.; Zhang, X.-H.;
Chen, J.-B.; Chen, L.; Xu, X.-H.; Cheng, L.-C.; Chiu, S.-W.; Tse, H.-F.; Vanhoutte, P. M.; Lau, C.-P. Circulation
2008, 117, 2449.
10. Wu, H.-J.; Wu, W.; Sun, H.-Y.; Qin, G.-W.; Wang, H.-B.; Wang, P.; Yalamanchili, H. K.; Wang, J.; Tse, H.-F.;
Lau, C.-P.; Vanhoutte, P. M.; Li, G.-R. J. Mol. Cell. Cardiol. 2011, 51, 966.
11. Carballo-Villalobos, A. I.; González-Trujano, M. E.; López-Muñoz, F. J. Eur. J. Pain (Oxford, U. K.) 2014, 18,
396.
12. Hsu, Y.-L.; Kuo, P.-L.; Lin, C.-C. Biochem. Pharmacol. (Amsterdam, Neth.) 2004, 67, 823.
13. Singh, R. P.; Agrawal, P.; Yim, D.; Agarwal, C.; Agarwal, R. Carcinogenesis 2005, 26, 845.
14. Hsu, Y.-L.; Kuo, P.-L.; Liu, C.-F.; Lin, C.-C. Cancer Lett. (N. Y., NY, U. S.), 2004, 212, 53.
15. Varma, R. S.; Saini, R. K.; Kumar, D. J. Chem. Res. (S) 1998, 348.
16. Chatterjee, A.; Sarkar, S.; Saha, S. K. Phytochemistry (Elsevier) 1981, 20, 1760.
17. Yang, W.-J.; Liu, C.; Gu, Z.-Y.; Zhang, X.-Y.; Cheng, B.; Mao, Y.; Xue, G.-P. Chin. Med. (London, U. K.) 2014,
9, 28.
18. Costantino, L.; Rastelli, G.; Albasini, A. Eur. J. Med. Chem. 1996, 31, 693.
19. Gao, H.; Kawabata, J. Biosci., Biotechnol., Biochem. 2004, 68, 1858.
20. Bois, F.; Beney, C.; Mariotte, A.-M.; Boumendjel, A. Synlett 1999, 1999, 1480.
21. Mentzer, C.; Molho, D.; Vercier, P. Compt. Rend. 1951, 232, 1488.
22. Mentzer, C.; Pillon, D. Compt. Rend. 1952, 234, 444.
23. Seijas, J. A.; Vázquez-Tato, M. P.; Carballido-Reboredo, R. J. Org. Chem. 2005, 70, 2855.
24. Molho, D.; Aknin, J. Compt. Rend. Acad. Sc. 1964, 259, 1645.
25. Kabalka, G. W.; Mereddy, A. R. Tetrahedron Lett. 2005, 46, 6315.
26. Vedejs, E.; Diver, S. T. J. Am. Chem. Soc. 1993, 115, 3358.
27. Ghobril, C.; Kister, J.; Baati, R. Eur. J. Org. Chem. 2011, 2011, 3416.
28. Adrian, J.; Stark, C. B. W. Org. Lett. 2014, 16, 5886.
29. Jeong, Y.-C.; Moloney, M. G. J. Org. Chem. 2011, 76, 1342.
30. Freiermuth, B.; Wentrup, C. J. Org. Chem. 1991, 56, 2286.