Microwave-Assisted Three-Component “Catalyst and Solvent-Free” Green Protocol: A Highly Efficient and Clean One-Pot Synthesis of Tetrahydrobenzo[b]pyrans

A green and highly efficient method has been developed for the one-pot synthesis of tetrahydrobenzo[b]pyrans via a three-component condensation of aldehydes, 1,3-cyclic diketones, and malononitrile under MW irradiation without using any catalyst and solvent. This transformation presumably occurs by a sequential Knoevenagel condensation, Michael addition, and intramolecular cyclization. Operational simplicity, solvent and catalyst-free conditions, the compatibility with various functional groups, nonchromatographic purification technique, and high yields are the notable advantages of this procedure. 1. Introduction Development of environmentally benign and clean synthetic procedures has become the goal of organic synthesis in recent times [1–5]. The multicomponent reactions (MCRs) are one of the most powerful and efficient tools in organic synthesis for the invention of biologically important scaffolds in the viewpoint of green chemistry [6–9]. One-pot multicomponent reactions (MCRs) have attracted considerable attention from the viewpoint of ideal synthesis by virtue of their efficiency, facile implementation, and generally high yield of the products [10–13]. Indeed, the concept of environmental factor (E-factor) and atom economy have gradually become included into conventional organic synthesis in both industry and academia. Solvents are the main reason for an insufficient E-factor, especially in synthesis of fine chemicals and pharmaceutical industries [14, 15]. As a result, it has become imperative both in academia and industry to design catalyst- and solvent-free MCRs, as these processes are rendered green with reduction of waste, time, manpower, and cost [16–21]. Recently MW-assisted chemistry has become a useful technique for a variety of applications in organic synthesis and transformations [22–25]. Microwave (MW)-promoted MCRs have been attracting research interest from chemists because these reactions exhibit some particular or unexpected reactivities and also because of their significant usefulness in green chemistry [5, 26]. In continuation of our research, we have reported few MW-promoted multicomponent coupling reactions in various chemical transformations for the synthesis of useful heterocyclic compounds [27–32]. Very recently, we have reported an efficient synthesis of pyrano[3,2-c]coumarin derivatives via copper(II) triflate catalyzed tandem reaction of 4-hydroxycoumarin with α, β-unsaturated carbonyl compounds under solvent-free conditions [33]. 4H-Benzo[b]pyrans are ubiquitous to a variety of biologically active molecules and have