Catalytic Synthesis of Pyrano- and Furoquinolines Using Nano Silica Chromic Acid at Room Temperature

Nano silica chromic acid (nano-SCA) is found to catalyze efficiently the three component-coupling reactions of aldehydes, amines, and cyclic enol ethers such as 3,4-dihydro-2H-pyran and 2,3-dihydrofuran under mild conditions to afford the corresponding pyrano- and furanoquinolines in excellent yields with high endoselectivity. Interestingly, 2,3-dihydrofuran afforded selectively endoproducts under the similar reaction conditions. Heterogeneous reaction conditions, easy procedure, short reaction time, and high yields are some important advantages of this method. 1. Introduction Aza-Diels-Alder reactions rank among the most powerful methodologies for the construction of nitrogen-containing six-membered ring compounds [1]. The pyranoquinolines and furanoquinolines are a class of nitrogen-containing heterocycles which are synthesized by Aza-Diels-Alder methodology. Whereas pyranoquinolines exhibit biological properties such as psychotropic, antiallergic, anti-inflammatory, and estrogenic activities in addition to their use as pharmaceuticals [2, 3], the furanoquinolines function as antagonists of 5-hydroxytryptamine receptors in animals and have been found to be the most potent anti-inflammatory agents in addition to being the most potent anti-inflammatory agents [4]. The hetero-Diels-Alder reaction is becoming a mainstay for heterocycle and natural product synthesis [5, 6]. Pyranoquinoline derivatives are found to possess a wide spectrum of biological activities such as psychotropic, antiallergenic, anti-inflammatory, and estrogenic activity [7]. Generally these compounds are prepared by Aza-Diels-Alder reactions of imines derived from aldehydes and amines with dihydropyran or dihydrofuran. Transition-metal complexes such as Co2(CO)8, Ni(CO)4 [8, 9], and InCl3 [8, 9] find their use for this reaction, although BF3·OEt2 has been the most commonly used catalyst. Thirteen various methods [10] are reported in the literature which include the use of GdCl3, ZrCl4, LiClO4, LiBF4, I2, and montmorillonite clay to promote this reaction. Many Lewis acids cannot be utilized for the single-step coupling of aldehydes, amines, and enol ethers because they will be decomposed or deactivated by the amines and water formed in the intermediate imine-formation step. Most imines are hygroscopic, unstable at high temperature, and difficult to purify; so, a one-pot three-component coupling protocol is highly desirable. Nano silica chromic acid (nano-SCA) is a solid acid which can be used for different reactions either as reagent or as catalyst under heterogeneous conditions.