Zeolite-catalyzed Pechmann synthesis of coumarins
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Catalysis Letters Vol. 112, Nos. 1–2, November 2006 (Ó 2006) DOI: 10.1007/s10562-006-0171-2
Zeolite-catalyzed Pechmann synthesis of coumarins Adrienn Hegedu¨s and Zolta´n Hell* Department of Organic Chemical Technology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary
Received 2 August 2006; accepted 30 August 2006
The Pechmann condensation of phenols with ethyl acetoacetate in the presence of a modified small pore size zeolite E4a resulting in the formation of coumarin derivatives in good yields. KEY WORDS: coumarins; Pechmann reaction; zeolites; green chemistry.
The coumarins are important compounds in the synthetic organic and medicinal chemistry. They are widely used as additives in foods, perfumes, cosmetics, pharmaceuticals [1], and in the preparation of insecticides, optical brighteners [2] and dispersed fluorescent and laser dyes [3]. Coumarins have been synthesized by several methods including Pechmann [4], Perkin [5], Knoevenagel [6,7] Reformatsky [8] and Wittig [9,10] reactions. The Pechmann reaction is one of the most widely applied method for synthesizing coumarins. It involves the condensation of phenols with b-ketoesters in the presence of a variety of acidic condensing agents. Several acid catalyst have been used in the Pechmann reaction, e.g. sulfuric acid [4], aluminum chloride [11], phosphorus pentoxide [12,13] or trifluoroacetic acid [14]. However, these catalysts have to be used in large excess, for example 10–12 equiv. sulfuric acid [15], 3–4 equiv. trifluoroacetic acid [14], and they cannot be recovered or reused. The disposal of acidic waste leads to environmental pollution. Therefore, it is important to find a simple, cheap, recoverable and/or reusable catalyst for the synthesis of coumarins. Nowadays cleaner and safer methods have been developed, using microwave irradiation [16,17], ionic liquids [18] and InCl3 [19], Sm(NO3)3*6H2O [20] or ZrCl4 [21] as catalysts. Ersorb-4 (E4) is a weakly acidic clinoptylolite-type zeolite adsorbent with 4 A˚ pore size. It can adsorb only small molecules such as water, hydrochloric acid, ammonia, methanol. E4a is the more acidic modification of E4 (the pH of its aqueous suspension is about 3) [22,23]. Recently we reported, that E4 showed good activity in different condensation reactions such as synthesis of oxazoline derivatives from b-aminoalcohols and carboxylic acids [24], preparation of 2-arylimidazolines and 2-arylbenzoxazoles [25], and tetrahydropyranylation of *To whom correspondence should be addressed. E-mail: [email protected]
alcohols and phenols [26]. E4a has been found to be a good catalyst in the preparation of 1-substituted tetrahydroisoquinolines via the Pictet–Spengler reaction [27], in the oxa-Pictet–Spengler reaction [28], in the synthesis of 3,4-dihydropyrimidin-2(1H)-ones [29] or in the synthesis of 1,5-benzodiazepines [30]. In continuation of our work we examined the application of modified Ersorb adsorbents as catalysts in the Pechmann reaction under mild reaction conditions. The reaction of phenol and ethyl ace
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