Zn(L-proline)2: An Efficient and Recyclable Catalytic System for the Asymmetric Multicomponent Synthesis of 2-amino-4H-chromenes in Water Under Controlled Microwave Heating

Abstract

Background: Bemzo and naphthopyranes are biologically relevant heterocycles that demonstrate a wide variety of biological activity. 2-amino-4H-chromenes is among the most compounds that exist in drugs. It is well documented that for chiral drugs one enantiomer possesses biological activity while it’s enantiomer is either inactive or less active. Catalytic asymmetric synthesis of such scaffold is very rare utilizing expensive catalyst or lengthy reaction procedure. The aim of this article is to develop a simple, green methodology for the asymmetric synthesis of naphthopyran derivatives via the reaction of malononitrile, aromatic aldehydes and β-naphthol in water under rmicrowave irradiation in the presence of catalytic amount of Zn(L-proline)2. Methods: We will develop further utility of Zn(L-proline)2 as simple, recyclable, water soluble and easy to be handled chiral catalyst for the asymmetric synthesis of naphthopyrans in a multicomponent reaction of malononitrile, aromatic aldehydes and β-naphthol in water under microwave heating. The recyclability of the catalyst will be investigated and the enantioselectivity of the products will be determined. Results: A variety of asymmetric 2-amino-4H-chromenes were obtained in excellent yields and moderate enantioselectivity. The catalyst could be reused up to four times without pronounced decrease in reactivity. Conclusion: We have developed a simple and environmentally friendly technique for the asymmetric synthesis of 2- amino-4H-chromenes utilizing for the first time Zn(L-proline)2 As asymmetric Lewis acid catalyst through a tandem Michael addition cyclization of aromatic aldehydes, malononitrile and β-naphthol. The process proved to be green and of high atom economy. Keywords: 2-amino-4H-chromenes, asymmetric synthesis, enantioselectivity, environmentally friendly technique microwave heating, Zn(L-proline)2.