Total Synthesis of Amentoflavone

Hae-Il Park1, Chuan-Ling Si2* and Jianjun Chen3,4* 1College of Pharmacy, Kangwon National University, Chuncheon, Korea 2Tianjin Key Laboratory of Pulp and Paper, College of Materials Science and Chemical Engineering, Tianjin University of Science and Technology, Tianjin, P R China 3Department of Pharmaceutical Sciences, South College, Knoxville, TN, USA 4Department of Pharmaceutical Sciences, College of Pharmacy, Chicago State University, Chicago, IL, USA


Introduction
Amentoflavone, a biflavanoid, is ubiquitously found in plants such as Calophyllum inophyllum [1], Eucommia ulmoides, Selaginella doederleinii [2], Paulownia tomentosa var. tomentosa, Ginkgo biloba [3,4], Juglans sigillata, Hypericum perforatum [4,5]. A wide variety of bioactivities such as anti-viral, anti-inflammatory, anti-tumor, antidepressant, anti-oxidant, anti-microbial, analgesic, antiplasmodial, leishmanicidal, lowering blood lipid and hepatoprotective activities have been reported for amentoflavone and its derivatives [1][2][3][5][6][7][8][9]. Due to the limited natural abundance, the massive production of amentoflavone is not possible from natural resources. Therefore, total synthesis of amentoflavone would be significant as it will be able to solve the availability issue of amentoflavone. Although the synthesis of amentoflavone through Suzuki-reaction was reported [10,11] two decades ago, which was to link the flavonyl-8-boronic acid with the 3'-iodoflavone to produce amentoflavone, no synthetic effort has been made ever since to explore an alternative scheme such that the flavonyl-3'-boronic acid ester can be linked to the 8-iodoflavone through Suzuki coupling. It would be highly beneficial to the scientific community if this alternative scheme is successful, as this will provide a similar but different route for the synthesis of amentoflavone and other similar biflavonoids, because the preparation of flavonylboronic acid, the key intermediate for the synthesis of biflavonoids, from the corresponding halogenated flavone is sometimes problematic due to steric hindrance or unfavorable electronic effects from neighboring substituting groups in the aromatic ring. Therefore, the goal of this work is to provide an alternative synthetic scheme for the production of amentoflavone and other similar biflavonoids utilizing the coupling of flavonyl-3'-boronic acid ester and 8-iodoflavone, instead of the reported method which was based on the coupling of two different intermediates, the flavonyl-8boronic acid and the 3'-iodoflavone [10]. Here we describe an efficient synthetic pathway to generate amentoflavone.

Experimental
All reagents were purchased from Sigma-Aldrich Chemical Co., Fisher Scientific (Pittsburgh, PA), Alfa Aesar (Ward Hill, MA), and AK Scientific (Mountain View, CA) and were used without further purification. The solvents for moisture sensitive reactions were freshly distilled, and the reactions were carried out under an argon atmosphere. DMF were dried by Barium oxide overnight before refluxed and distilled under reduced pressure. CHCl 3 and CH 3 CN were refluxed and distilled from calcium hydride prior to use. Routine thin layer chromatography (TLC) was carried out on aluminumbacked Uniplates (Analtech, Newark, DE). Column chromatography was performed on Merck 230-400 mesh silica gel. 1 H-NMR spectra were recorded on a JEOL ECS instrument (400 MHz) spectrometer. 13 C-NMR spectra were recorded on a JEOL ECS instrument (100 MHz) spectrometer. Chemical shifts are reported in ppm downfield relative to tetramethylsilane as an internal standard. Mass spectra were collected on a Shimadzu LCMS 2020 mass spectrometer. All melting points were measured with Fisher-Johns melting point apparatus and are uncorrected. Glassware for reactions requiring anhydrous conditions was dried by flame prior to use.

3'-Iodo-4-methoxybenzaldehyde (2)
p-Anisaldehyde (1 eqv) was dissolved in glacial acetic acid, iodine monochloride/ICl (1 eqv) solution (1 M in CH 2 Cl 2 ) was dropped to the reaction mixture heated to 60ºC and stirred for 10 h, then saturated Na 2 S 2 O 4 (aqueous) was dropped to the reaction mixture until the solution was clear, evaporated to remove CH 2 Cl 2 and standed in icebath for 30 min during this period a white precipitate formed, filtered and got white solid, crystallized from MeOH resulted in the desired product. Yield: 55%. 1

2-Hydroxy-4,6-dimethoxy-acetophenone (7)
2,4,6-Trihydroxyacetophenone (1 eqv) was dissolved in anhydrous acetone with stirring and potassium carbonate (3 eqv) was added in portions to give a yellow solution, resulting solution was stirred for 5 min, then dimethylsulfate (2 eqv) was added, heated to 60ºC, stirred and refluxed for 8 h, monitored by TLC with solvent system of hexaneacetone (5:1), the solution was cooled to room temperature, filtered and concentrated in vacuum to get yellow solid, the solid was washed with water and crystallized from methanol to yield the as 2-hydroxy-4,6-dimethoxyacetophenone. Yield: 98%. 1

Discussion and Conclusion
We have developed an efficient total synthesis of amentoflavone, a valuable natural product, utilizing Suzuki coupling with flavonyl-3'boronic ester and 8-iodoflavone. This method is similar to but different from the reported procedure [10,11] that coupled the flavonyl-8boronic acid and 3'-iodoflavone to form amentoflavone. Mass, NMR and melting point confirmed compound 12 is amentoflavone, as compared with data in literatures [4]. The overall yield is about 17% which is highly efficient. The new method provides an alternative method for the synthesis of amentoflavone which can be applied to the generation of other natural and biologically active biflavonoids and biphenyls for which the extensive preclinical development is often hampered by the limited availability of these compounds from natural resources. Therefore, the current work could serve as an excellent solution to the availability of many bioactive biflavonoids.