Montmorillonite K-10 clay catalyzed solvent-free synthesis of bis-indolylindane- 1,3-dione, 2-(1 0 ,3 0 -dihydro-1H-[2,3 0 ]biindolyl-2 0 -ylidene)-indan-1,3-dione and bisindolylindeno[1,2-b]quinoxaline under microwave irradiation Subhendu Naskar a , Priyankar Paira a , Rupankar Paira a , Shyamal Mondal a , Arindam Maity a , Abhijit Hazra a , Krishnendu B. Sahu a , Pritam Saha a , Sukdeb Banerjee a , Peter Luger b , Manuela Webe b , Nirup B. Mondal a, * a Department of Chemistry, Indian Institute of Chemical Biology, Council of Scientific & Industrial Research, 4 Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, West Bengal, India b Freie Universit€ at Berlin, Institut für Chemie und Biochemie/Kristallographie, Fabeckstr. 36a, D-14195 Berlin, Germany article info Article history: Received 17 March 2010 Received in revised form 20 April 2010 Accepted 21 April 2010 Available online 20 May 2010 Keywords: Montmorillonite K-10 Ninhydrin Indole Microwave Solvent-free condition abstract An environmentally benign protocol has been described for the synthesis of novel 2-(1 0 ,3 0 -dihydro-1H-[2,3 0 ] biindolyl-2 0 -ylidene)-indan-1,3-diones/bis-indolylindane-1,3-diones from ninhydrin and 3-substituted/ unsubstituted indoles. It uses montmorillonite K-10 as catalyst in a solvent-free condition under microwave irradiation. The method was also used for the synthesis of novel bisindolylindeno[1,2-b]quinoxaline derivatives. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years growing interest is being observed in the use of solid acidic catalysts in acid dependent organic syntheses. 1,2 This interest is likely to increase with demanding environmental legis- lation, public and corporate pressure, and drive toward clean technology. In this perspective, montmorillonite K-10 Clay (Mont. K-10), known to behave both as protic and Bronsted acid, 3 is a widely studied catalyst found useful in many organic reactions, viz., synthesis of g-lactones, 4 synthesis of fused heterocycles, 5 FriedeleCrafts reaction, 6 synthesis of biomarkers, 7 oxidative demethylation of methylphenols to benzoquinones, 8 (2,5) intra- molecular ene cyclization, 9 Michael addition, 10 Boc group removal from aromatic amines, 11 DielseAlder reaction 12 and so on. It has also been used in many microwave reactions both in liquid phase as well as solvent-free conditions. 13e16 Important pharmaceuticals often possess heterocyclic moieties as their building blocks. 17 The extensive use of heterocyclic com- pounds in the pharmaceutical industry is perhaps attributable to the availability of ample range of reactions that facilitate subtle structural modifications in heterocyclic compounds. 18e20 Since indole and its derivatives possess various biological activi- ties, 21 development of new methodologies for the synthesis of indole derivatives, which will yield subsets of heterocycles having potentiality to serve as templates for new biologically active mol- ecules, is of great importance. In this context, we wish to describe a convenient and simple methodology for the synthesis of 2-(1 0 ,3 0 -dihydro-1H-[2,3 0 ]biin- dolyl-2 0 -ylidene)-indan-1,3-diones/bis-indolylindane-1,3-diones (by reacting ninhydrin with 3-substituted/unsubstituted indoles) and also bisindolylindeno[1,2-b]quinoxalines (from the reaction of ninhydrin, 1,2-phenylenediamine, and indole). The reactions were carried out using montmorillonite K-10 as solid acidic catalyst in solvent-free condition under microwave irradiation. The novelty of the methodology lies in its eco-friendly operation, formation of structurally unique molecules, short reaction time, and excellent yield. 2. Results and discussion Initially, 1 mmol ninhydrin (2) and 2 mmol indole (1a) were added to the montmorillonite K-10 (0.5 g) in a mortar and mixed * Corresponding author. Tel.: þ91 33 2473 3491; fax: þ91 33 2473 5197; e-mail address: nirup@iicb.res.in (N.B. Mondal). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2010.04.084 Tetrahedron 66 (2010) 5196e5203