Cobalt(II) chloride hexahydrate (CoCl2·6H2O) has been found to be an efficient catalyst for the one-pot synthesis of biscoumarin derivatives through a combination of aromatic aldehydes and 4-hydroxycoumarin in aqueous media at 70°C. Several types of aromatic aldehyde, containing electron-withdrawing groups as well as electron-donating groups, were used in the reaction and in all cases the desired products were synthesized successfully. The present approach offers remarkable advantages such as short reaction times, excellent yields, straightforward procedure, easy purification, environment friendliness, and low catalyst loading. 1. Introduction Coumarin derivatives, especially biscoumarins, are important compounds in organic synthesis due to their wide spectrum of pharmacological properties such as antifungal, anti-HIV, anticancer, anticoagulant, antithrombotic, antimicrobial, and antioxidant [1–5]. These compounds are also utilized as urease inhibitors . A number of methods have been reported for the synthesis of these compounds in the presence of various catalysts like molecular iodine , sodium dodecyl sulfate (SDS) , tetrabutylammonium bromide (TBAB) , ([HSO4]) , tetrabutylammonium hexatungstate ([W6O19]) , sulfated titania (TiO2/) , ruthenium(III) chloride hydrate (·) , n-dodecylbenzene sulfonic acid (DBSA) , and silica chloride nanoparticles (nano SiO2Cl) . However, these methods suffer from one or more disadvantages such as low yields of products, long reaction times, use of expensive catalyst, toxic solvents, or harsh reaction conditions. Therefore, introducing a clean procedure by the use of green and environmentally friendly catalyst with high catalytic activity, moderate temperature, and short reaction time accompanied with excellent yield for the production of biscoumarin derivatives is needed. We hoped to develop a more general protocol for the efficient synthesis of biscoumarin derivatives via ·, which have recently attracted much attention as catalyst to organic synthesis due to their low toxicity and easy availability [16–18]. 2. Results and Discussion We herein present efficient and eco-friendly procedure for the synthesis of biscoumarin derivatives (3 a–m) by three-component condensation of 4-hydroxycoumarin (1) and aromatic aldehyde (2) catalyzed by · in water-ethanol solvent system 70°C (Scheme 1). Scheme 1: Synthesis of biscoumarins. For this study, a reaction between 4-hydroxycoumarin (2？mmol) and 3-nitrobenzaldehyde (1？mmol) was examined as the model reaction. Initial studies showed that better
I. Manolov, C. Maichle-Moessmer, and N. Danchev, “Synthesis, structure, toxicological and pharmacological investigations of 4-hydroxycoumarin derivatives,” European Journal of Medicinal Chemistry, vol. 41, no. 7, pp. 882–890, 2006.
N. Hamdi, M. C. Puerta, and P. Valerga, “Synthesis, structure, antimicrobial and antioxidant investigations of dicoumarol and related compounds,” European Journal of Medicinal Chemistry, vol. 43, no. 11, pp. 2541–2548, 2008.
C. C. Chiang, J. F. Mouscadet, H. J. Tsai, C. T. Liu, and L. Y. Hsu, “Synthesis and HIV-1 integrase inhibition of novel bis- or tetra-coumarin analogues,” Chemical and Pharmaceutical Bulletin, vol. 55, no. 12, pp. 1740–1743, 2007.
D. Zavr？nik, S. Muratovi？, S. ？pirtovi？, D. Softi？, and M. Medi？-？ari？, “The synthesis and antimicrobial activity of some 4-hydroxycoumarin derivatives,” Bosnian Journal of Basic Medical Sciences, vol. 8, no. 3, pp. 277–281, 2008.
K. M. Khan, S. Iqbal, M. A. Lodhi, G. M. Maharvi, M. I. Choudhary, and S. Perveen, “Biscoumarin: new class of urease inhibitors; economical synthesis and activity,” Bioorganic and Medicinal Chemistry, vol. 12, no. 8, pp. 1963–1968, 2004.
M. Kidwai, V. Bansal, P. Mothsra et al., “Molecular iodine: a versatile catalyst for the synthesis of bis(4-hydroxycoumarin) methanes in water,” Journal of Molecular Catalysis A: Chemical, vol. 268, no. 1-2, pp. 76–81, 2007.
H. Mehrabi and H. Abusaidi, “Synthesis of biscoumarin and 3,4-dihydropyrano[c]chromene derivatives catalysed by sodium dodecyl sulfate (SDS) in neat water,” Journal of the Iranian Chemical Society, vol. 7, no. 4, pp. 890–894, 2010.
J. M. Khurana and S. Kumar, “Tetrabutylammonium bromide (TBAB): a neutral and efficient catalyst for the synthesis of biscoumarin and 3,4-dihydropyrano[c]chromene derivatives in water and solvent-free conditions,” Tetrahedron Letters, vol. 50, no. 28, pp. 4125–4127, 2009.
N. Tavakoli-Hoseini, M. M. Heravi, F. F. Bamoharram, A. Davoodnia, and M. Ghassemzadeh, “An unexpected tetracyclic product isolated during the synthesis of biscoumarins catalyzed by [MIM(CH2)4SO3H][HSO4]: characterization and X-ray crystal structure of 7-(2-hydroxy-4-oxo-4H-chromen-3-yl)-6H,7H-chromeno[4,3-b]chromen-6-one,” Journal of Molecular Liquids, vol. 163, no. 3, pp. 122–127, 2011.
A. Davoodnia, “A highly efficient and fast method for the synthesis of biscoumarins using tetrabutylammonium hexatungstate [TBA]2[W6O19] as green and reusable heterogeneous catalyst,” Bulletin of the Korean Chemical Society, vol. 32, no. 12, pp. 4286–4290, 2011.
K. Tabatabaeian, H. Heidari, A. Khorshidi, M. Mamaghani, and N. O. Mahmoodi, “Synthesis of biscoumarin derivatives by the reaction of aldehydes and 4-hydroxycoumarin using ruthenium(III) chloride hydrate as a versatile homogeneous catalyst,” Journal of the Serbian Chemical Society, vol. 77, no. 4, pp. 407–413, 2012.
B. Pawar, V. Shinde, and A. Chaskar, “n-Dodecylbenzene sulfonic acid (DBSA) as a novel Br？nsted acid catalyst for the synthesis of bis(indolyl)methanes and bis(4-hydroxycoumarin-3-yl)methanes in water,” Green and Sustainable Chemistry, vol. 3, no. 2, pp. 56–60, 2013.
R. Karimian, F. Piri, A. A. Safari, and S. J. Davarpanah, “One-pot and chemoselective synthesis of bis(4-hydroxycoumarin) derivatives catalyzed by nano silica chloride,” Journal of Nanostructure in Chemistry, vol. 3, pp. 52–57, 2013.
A. T. Khan, T. Parvin, and L. H. Choudhury, “A simple and convenient one-pot synthesis of benzimidazole derivatives using cobalt(II) chloride hexahydrate as catalyst,” Synthetic Communications, vol. 39, no. 13, pp. 2339–2346, 2009.
V. Padalkar, K. Phatangare, S. Takale, R. Pisal, and A. Chaskar, “Silica supported sodium hydrogen sulfate and Indion 190 resin: an efficient and heterogeneous catalysts for facile synthesis of bis-(4-hydroxycoumarin-3-yl) methanes,” Journal of Saudi Chemical Society, 2012.
K. Niknam and A. Jamali, “Silica-bonded N-propylpiperazine sodium n-propionate as recyclable basic catalyst for synthesis of 3,4-dihydropyrano[c]chromene derivatives and biscoumarins,” Chinese Journal of Catalysis, vol. 33, no. 11, pp. 1840–1849, 2012.
P. Singh, P. Kumar, A. Katyal et al., “Phosphotungstic acid: an efficient catalyst for the aqueous phase synthesis of bis-(4-hydroxycoumarin-3-yl)methanes,” Catalysis Letters, vol. 134, no. 3-4, pp. 303–308, 2010.
K. P. Boroujeni and P. Ghasemi, “Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Br？nsted acid sites,” Catalysis Communications, vol. 37, pp. 50–54, 2013.