Effective preparation of hexahydroquinolines under ambient and solvent-free conditions Mahdi Ghorbani a, ⁎, Hamid Reza Shaterian a , Soheila Noura a , Fereshte Khammar a , Kazem Behbodi b , Bahareh Reisi c , Mohsen Oftadeh d a Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan, PO Box 98135-674, Zahedan, Iran b Department of Chemistry, Shahreza Branch, Islamic Azad University, 311-86145 Shahreza, Isfahan, Iran c Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran d Chemistry Department, Payame Noor University, 19395-4697 Tehran, Iran abstract article info Article history: Received 6 October 2014 Received in revised form 5 January 2015 Accepted 8 January 2015 Available online 10 January 2015 Keywords: Ionic liquid Catalysis Dimedone Aldehyde Solvent free The mild acidic ionic liquids such as N-methyl-2-pyrrolidonium dihydrogen phosphate [NMP][H 2 PO 4 ], N-methyl- 2-pyrrolidonium hydrogen sulfate [NMP][HSO 4 ], Triethylamine hydrogen sulfate [Et 3 N-H][HSO 4 ], L-prolinium sulfate [L-Pro 2 ][SO 4 ] and L-prolinium nitrate [L-Pro][NO 3 ] were used as efficient catalysts for the one-pot four- component condensations of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free conditions. Some derivatives of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquino- line-3-carboxylate were obtained under solvent-free conditions in high yields. Simple reaction, mild condition and easy work-up procedures are advantages of the present work. Also, the ionic liquids can be recycled several times without significant loss of their activity. © 2015 Published by Elsevier B.V. 1. Introduction Multicomponent reactions (MCRs) in ionic liquids (ILs) are impor- tant in organic synthesis [1]. Ionic liquids act as solvent, co-catalyst, or catalyst [2]. IL-MCR causes the rapid synthesis of highly functionalized heterocyclic molecules with high potential applications in medicinal chemistry [3,4]. Yielding the one-pot multi-component symmetrical 1,4-dihydropyridines (1,4-DHPs) during Hantzsch reaction was first established in 1881 [5]. 1,4-DHPs form a class of heterocyclic com- pounds which present interesting pharmacological and biological prop- erties [6]. In order to research on ionic liquids and their application on IL-MCRs [7], recently several alternate and more efficient methods in- cluding the use of microwaves [8], autoclave [9], ionic liquids [10], io- dine [11], metal triflates [12], ceric ammonium nitrate (CAN) [13], L- proline [14], PTSA-SDS [15] and BINOL-phosphoric acid derivatives [16] have been developed for the synthesis of polyhydroquinoline de- rivatives. Herein, we report the new application of some ILs such as N- methyl-2-pyrrolidonium dihydrogen phosphate [NMP][H 2 PO 4 ], N- methyl-2-pyrrolidonium hydrogen sulfate [NMP][HSO 4 ], Triethylamine hydrogen sulfate [Et 3 N-H][HSO 4 ], L-prolinium sulfate [L-Pro 2 ][SO 4 ], and L-prolinium nitrate [L-Pro][NO 3 ] as catalysts (Fig. 1) for the preparation of some functionalized ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8- hexahydroquinoline-3-carboxylate derivatives during the reaction of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium ace- tate under solvent-free conditions for the first time (Scheme 1). 2. Experimental All reagents were purchased from Merck and Aldrich and used with- out further purification. The mild acidic ionic liquids such as N-methyl- 2-pyrrolidonium hydrogen phosphate [NMP][H 2 PO 4 ] [17], N-methyl-2- pyrrolidonium hydrogen phosphate [NMP][HSO 4 ] [18], Triethylamine hydrogen sulfate [Et 3 N-H][HSO 4 ] [19],[L-Pro 2 ][SO 4 ] [20] and [L-Pro] [NO 3 ] [20] as catalysts were prepared according to the reported proce- dure. The NMR spectra were recorded on a Bruker Avance DPX 300 or 400 MHz instrument. IR spectra were recorded on a JASCO FT-IR 460 plus spectrophotometer. Melting points were determined in open cap- illaries with a BUCHI 510 melting point apparatus. TLC was performed on silica-gel Poly Gram SIL G/UV 254 plates. All yields refer to isolated products after purification. 2.1. General procedure for the synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5- oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives under solvent-free conditions The mixture of the aldehydes (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol) and Journal of Molecular Liquids 204 (2015) 15–20 ⁎ Corresponding author. E-mail address: ghorbanii.mahdi@gmail.com (M. Ghorbani). http://dx.doi.org/10.1016/j.molliq.2015.01.013 0167-7322/© 2015 Published by Elsevier B.V. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq