Indian Journal of Chemistry Vol. 54B, November 2015, pp 1321-1326 Alkaline earth metal catalyzed, one-pot, multi-component approach for the synthesis of dihydropyridine, acridine and xanthene derivatives in water Srinivasarao Yaragorla*, Garima Singh & Abhishek Pareek Department of Chemistry, Central University of Rajasthan, Bandersindri 305 817, NH-8, Distt. Ajmer, India E-mail: srinivasarao@curaj.ac.in Received 12 January 2015; accepted (revised) 25 August 2015 Ca(II) catalyzed one-pot multicomponent approach for the biologically important diverse heterocyclic compounds such as hexahydroxanthene diones, dihydropyridines and octahydroacridine diones has been described in water. Use of environ- mentally benign catalyst, water as the green solvent, high yields and substrate diversity are the highlights of the current method. Keywords: Alkaline earth metals, acridinediones, dihydropyridines, xanthenediones, multicomponent reactions Xanthenediones, acridinediones, and dihydropyridines or Hantzsch esters (Figure 1) are a rich class of bio- logically active heterocyclic compounds. They possess a spectrum of biological and pharmacological properties such as analgesic 1 , antiviral 2 , antibacterial 3 and anti- inflammatory 2 activities, etc. and also they have been employed as important building blocks for the synthesis of many organic molecules 4 . Some of these fused heterocyclic compounds have also been utilized as a fluorescent materials 5 , leuco-dyes 6 , as sensitizers in photodynamic therapy 7 and in laser technology 8 . Due to their broad range of applications, they received tremendous attention from the synthetic community, which resulted in a number of synthetic protocols for their synthesis 9-11 . Broadly, the synthesis involves the thermal-acidic/basic condensation of 1,3-diketones and aldehydes for xanthenes; 1,3-diketones with aldehyde and nitrogen nucleophiles for the dihydropyridine/acri- dine synthesis. Though a variety of catalytic systems have been developed for the synthesis of xanthenes 9 , acridines 10 and Hantzsch dihydropyridines 11 , some of these protocols suffer from drawbacks such as use of strong acid or base catalysts, large amount of catalyst loadings, poor yields and usage of halogenated solvents for the reactions. Results and Discussion Recently, alkaline earth metals proved as alternatives to transition metal and lanthanide based catalysts 12,14a due to their wide abundance, low cost and environ- mentally benign properties. Among these alkaline earth metal catalysts, calcium salts bearing a hard conjugate base such as triflate ion is highly stable to moisture and air, nevertheless only a few reports have been available about the application of Ca(OTf) 2 in organic synthesis 13 . Very recently, a green synthesis of amides was reported from alcohols and nitriles via Ritter mechanism using calcium triflate as an efficient catalyst 14a . In continuation of our interest 14 in exploration of more abundant, inexpensive, and stable alkaline earth metal salts as alternatives to transition metals and lanthanide-based catalysts, herein is reported the Ca(OTf) 2 catalysed multi-component synthesis of a diverse array of fused heterocycles such as xanthenes, acridines and dihydropyridines. The initial investigation was started with the conden- sation of 2.0 mmol of 1,3-cyclohexanedione 2a with 1.0 mmol of benzaldehyde 1a in presence of Ca(OTf) 2 (5 mol%) as a catalyst and Bu 4 NPF 6 (5 mol%) as an additive at 100°C in aqueous medium 15 . After refluxing the reaction for 4 h, the targeted compound 9-phenyl- 3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione 3a was isolated in 75% yield after recrystallization. After noticing the formation of the requisite product, we investigated for the optimum conditions for the trans- formation. When the same reaction was performed in the absence of catalyst and additive, even after refluxing for 24 h in water no trace of the product 3a could be detected, rather we noticed the complete conversion to the thermodynamically favored tetraketone intermediate 2A (uncyclised) 2,2′-(phenylmethylene)bis(cyclohexane- 1,3-dione) in more than 92% yield. Later we found that Wang et al. has reported a catalyst-free method for the synthesis of these tetraketones 16 and the same