Novel Protocols for the Synthesis of Imidazopyridines, Quinolines and Fused Indazoles via Cycloaddition and C-H Activation Strategies

Heterocycles are an important class of compounds, not only due to their natural abundance, but also because of their chemical and biological significance. Natural product scaffolds have been well recognized as “privileged heterocyclic structures” in terms of their ability to be the basis for successful drugs. An assessment of all FDA approved new molecular entities (NMEs) reveals that natural products and their derivatives represent over one-third of all NMEs. Such scaffolds can be used as cores of compound libraries. In this context, the design of amenable synthetic approaches for the synthesis of new heterocyclic systems is an attractive and challenging area. Cycloadditions have emerged as the most efficient chemical processes, combining atom economy, stereospecificity, and the ability to generate molecular complexity in a single step. Hence they have become increasingly popular among synthetic chemists, and a very large fraction of complex (natural) products syntheses include cycloadditions, often as the key step. Among the cycloaddition reactions, Diels–Alder reaction, with its rich synthetic diversity, is recognized as a powerful tool for regio-, stereoselective preparation of natural and unnatural polycarbocycles and polyheterocycles. [4+1] cycloaddition is among most powerful reactions in the arsenal of the synthetic chemists and counts next to the well-known [3+2] cycloaddition reaction for the synthesis of five-membered rings owing to the ubiquity of five-membered carbo- and heterocyclic substructures in natural products. Construction of C-C and C-X via C-H bond activation/functionalization has attracted great attention in the past decades. However, selective C-H activation has been a challenging task, owing to the ubiquitous nature of C-H bond in organic molecules. In this aspect, along with transition-metal catalysed C-H functionalization, recently alternative, metal-free versions have become very important due to economic and environmental concerns. A highly atom economic, efficient and yet environmentally friendly method has been the prime synthetic target to be achieved by the chemists for synthesizing complex heterocycles, since last few decades. In this context, sequential one-pot multicomponent reactions have been widely explored. Inspired by these, we have developed novel protocols for the synthesis of imidazopyridines, 4-aminoquinolines, isocryptolepine alkaloid analogs and fused indazoles via cycloaddition and C-H activation strategies