Microwave-assisted, montmorillonite K-10 catalyzed three-component synthesis of 2H-indazolo[2,1-b]phthalazine-triones under solvent-free conditions Mudumala Veeranarayana Reddy a , Gangireddy Chandra Sekhar Reddy b , Yeon Tae Jeong a, * a Department of Image Science and Engineering, Pukyong National University, Busan 608-737, Republic of Korea b The WCU Center for Synthetic Polymer Bio-conjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea article info Article history: Received 9 May 2012 Received in revised form 6 June 2012 Accepted 9 June 2012 Available online 15 June 2012 Keywords: 2H-indazolo[2,1-b]phthalazine-triones Montmorillonite K-10 Microwave irradiation Solvent-free reaction abstract An efficient, rapid, and green synthesis of 2H-indazolo[2,1-b]phthalazine-triones has been accomplished under solvent-free conditions by the reaction of phthalhydrazide, aldehydes and 5,5- dimethylcyclohexane-1,3-dione. This approach exploits the synthetic potential of microwave irradia- tion and Montmorillonite K-10 combination and offers many advantages, such as excellent product yields, shorter reaction time, reusable catalyst, easy isolation of products, and environmentally benign reaction conditions. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Green chemistry approaches are significant due to the reduction in by-products, reaction waste, and reduction of energy cost. Or- ganic reactions under solvent-free conditions have attracted much interest from chemists particularly from the viewpoint of green chemistry. 1e3 In recent times, exploration of solvent-free reactions has gained importance due to several advantages, such as experi- mental simplicity, less energy requirement, and almost quantitative reactivity of the substrates due to intimacy of the reagents. Im- proving the substrates reactivity and product formation efficiency by energising the reactions with microwave irradiation is another development in organic green chemical synthesis. Microwave- assisted organic synthesis is characterized by spectacular acceler- ations in many reactions as a consequence of three dimensional heating of the reaction mass, which cannot be produced by classical heating. Simple workup, high yields, improved selectivity and clean reaction pathways are additional advantages with microwave- assisted preparation of organic compounds. Indeed, even re- actions that do not occur by conventional heating can be effectively performed using microwaves. In particular, microwave heating when coupled with solvent-free strategy, presents a powerful and green alternative to conventional synthesis. Theoretical calculations have also suggested that reactions with high activation energies can be performed under microwave irradiation without the use of harsh reaction conditions. Recently, numerous important heterocycles have been synthe- sized under solvent-free conditions accelerated by microwave irradiation. 4e9 Nitrogen-containing heterocyclic compounds are widespread in nature, and their applications to pharmaceuticals, agrochemi- cals, and functional materials are becoming more and more im- portant. 10 Among a large variety of N-containing heterocyclic compounds, those containing hydrazine moiety as ’fusion site’ have received considerable attention because of their pharmaco- logical properties and clinical applications. 11 Moreover, fused phthalazines were found to possess multiple biological activities, such as antimicrobial, 12 anticonvulsant, 13 antifungal, 14 antican- cer, 15 and anti-inflammatory activities. 16 Nevertheless the de- velopment of new synthetic methods for the efficient preparation of heterocycles containing phthalazine ring fragment is an in- teresting challenge. Recently, several elegant multicomponent strategies for the synthesis of 2H-indazolo[2,1-b]phthalazine-tri- ones by the cyclocondensation of phthalhydrazide, aldehydes, and 1,3-diketones utilizing different types of catalysts have been reported. 17e25 The reported methods show varying degrees of successes as well as limitations, such as harsh reaction conditions, expensive catalyst/reagent, toxic organic solvents, low product yields, long reaction times, and co-occurrence of several side products. Thus, a simple, rapid and efficient procedure is still * Corresponding author. Tel.: þ82 51 629 6411; fax: þ82 51 629 6408; e-mail address: ytjeong@pknu.ac.kr (Y.T. Jeong). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.06.045 Tetrahedron 68 (2012) 6820e6828