An Expeditious Synthesis of Flavones on Montmorillonite K 10 Clay with Microwaves$% Rajender S. Varma,* Rajesh K. Saini and Dalip Kumar Department of Chemistry and Texas Regional Institute for Environmental Studies (TRIES), Sam Houston State University, Huntsville, TX 77341-2117, USA A manipulatively simple and rapid method for the synthesis of flavones is described via a solid-state dehydrative cyclization of o-hydroxydibenzoylmethanes on a clay surface using microwaves. Flavonoids are a group of naturally occurring phenolic compounds widely distributed in the plant kingdom, the most abundant being the �Pavones.Members of this class have been shown to display a wide variety of biological activities1 and have proven useful in the treatment of var- ious diseases.2 There are number of methods available for the synthesis of �Pavones and their analogues,3a�}e including the Allan�}Robinson synthesis,3c synthesis from chalcones4 and via an intramolecular Wittig strategy.5 The most com- mon method, however, involves the Baker�}Venkataraman rearrangement6,7 wherein o-hydroxyacetophenone is benzoy- lated to form the benzoyl ester which is treated with base (pyridine/KOH) to e€ect an acyl group migration, forming a 1,3-diketone.The ensuing diketone is then cyclized under strongly acidic conditions using acetic acid and sulfuric acid to deliver the �Pavone. Consequently, there is a need for the development of a milder protocol for the cyclization process.Clay-catalysed organic reactions have generated consider- able interest in recent years in view of their inexpensive nature and special catalytic attributes under heterogeneous reaction conditions.7 Microwave (MW) heating is used for a wide variety of organic reactions and has found application in rapid and cleaner synthesis of organic compounds.8�}11 More recently, the emphasis has shifted in favor of micro- wave-assisted methods under solvent-free conditions9�}11 which have special appeal as they provide an opportunity to work with open vessels, thus avoiding the risk of high pressure development.Further, this approach enhances the possibility of upscaling the reactions on a preparative scale. In continuation of our studies on microwave-accelerated solvent-free reactions on mineral solid supports,10 we now report a manipulatively simple and rapid microwave protocol for the solid-state cyclodehydration of o-hydroxy- dibenzoylmethanes to �Pavones on a clay surface.The method in its entirety involves the microwave irradiation of o-hydroxydibenzoylmethanes adsorbed on montmorillonite K 10 clay for 1�}1.5 min (bulk temperature of alumina bath reaches 80�}120 8C). The exclusive for- mation of cyclized �Pavones 2a�}g occurs which are easily extractable in good yields from the support (Table 1). The alumina bath, in addition to being a container for the reac- tion vessel, also serves as a heat sink for the microwaves in view of the small amount of reactants normally employed. Among the various other mineral supports explored, namely silica gel, neutral or basic alumina, the formation of �Pavone is ideally accomplished on K 10 clay; some reaction does occur on neutral alumina and silica surfaces but it is not complete.In conclusion, we have developed a simple and mild method for the solid-state cyclodehydration of o-hydroxy- dibenzoylmethanes to �Pavones on a clay surface using microwave irradiation.Experimental Mps are uncorrected. 1H NMR spectra were recorded in CDCl3 solutions at 60 MHz, using TMS as an internal standard. o-Hydroxydibenzoylmethanes 1a�}g were obtained by rearrange- ment of the corresponding o-benzoyloxyacetophenones.13 A Sears Kenmore household microwave oven operating at 2450 MHz was used at its full power, 900 W, for all the experiments. Products were identiRed by comparison of their mp, IR and NMR spectra with those of authentic samples.Typical Procedure: 4'-Methyl-6-methoxy�Pavone 2f.D1-(2-Hydroxy- 5-methoxyphenyl)-3-(4-methylphenyl)propane-1,3-dione 1f (0.2 g, 0.70 mmol) was dissolved in a small amount of dichloromethane (1 ml) and adsorbed on montmorillonite K 10 clay (1.0 g). The contents, in a test-tube, were placed in an alumina bath inside the microwave oven and irradiated for 1.5 min. The crude product was extracted in dichloromethane (215 ml) and then crystallized from methanol to a€ord 2f in 80% yield, mp 161�}162 8C; 1H NMR (CDCl3) 2.26 (3 H, s, C4'�}CH3), 3.36 (3 H, s, C6-OCH3), 6.26 (1 H, s, C3�}H), 6.53�}7.33 (7 H, m, aromatic H); m/z 266 (100%).That the e€ect may not be purely thermal14 is supported by the fact that the reaction could not be completed (65%) in 24 h at the same bulk temperature of 80 8C using an alternate mode of J. Chem. Research (S), 1998, 348�}349$ Scheme Table 1 Synthesis of flavones from o-hydroxydibenzoylmethanes on claya mp (8C) Entry R1 R2 Yield (%) Observed Reported 2a H H 75 96 953b 2b H CH3 77 108�}109 1103d 2c H OCH3 76 155�}157 156�}1573d 2d H NO2 78 244�}245 246�}2473d 2e OCH3 H 73 161 163�}1643e 2f OCH3 CH3 80 161�}162 �} 2g OCH3 OCH3 72 193�}194 19512 aMW irradiated for 1 min (the bulk temperature of the alumina bath reached 80 8C at full power of the MW oven, 900 W).$This is a Short Paper as deRned in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there- fore no corresponding material in J.Chem. Research (M). %Presented in part at the 6th International Conference on Microwave and High frequency Heating, Fermo, Italy, 9�}13th September, 1997. *To receive any correspondence (e-mail: chm_rsv@shsu.edu). 348 J. CHEM. RESEARCH (S), 1998heating (oil-bath). The temperature of the reaction mixture inside the alumina bath reached 180 8C after 1 min of irradiation in a MW oven operating at full power of 900 W.We are grateful to Texas Advanced Research Program (ARP) in chemistry (Grant No. 003606-023) and TRIES, Oce of Naval Research/SERDP (Grant No. N00014-96-1- 1067) for Rnancial support. Received, 22nd December 1997; Accepted, 26th February 1998 Paper E/7/09146J References 1 A. F. Welton, L. D. Tobias, C. Fiedler-Nagy, W. Anderson, W. Hope, K. Meyers and J. W. Co€ey, in Plant Flavonoids in Biology and Medicine, ed. V. Cody, E. Middelton Jr and J. B. Harborne, Alan R. Liss, New York, 1986, p. 231. 2 B. Havsteen, Biochem. Pharmacol., 1983, 32, 1141. 3 (a) H. Wagner and L. Farkas, in The Flavonoids, ed. J. Harborne, T. J. Mabry and H. Mabry, Academic Press, New York, 1975, p. 127; (b) A. Banerji and N. Goomer, Synthesis, 1980, 874; (c) J. Allan and R. Robinson, J. Chem. Soc., 1924, 20, 2192; (d ) M. S. Khanna, O. V. Singh, C. P. Garg and R. 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