Metal-free based one-pot approaches to a novel class of substituted carbazoles, pyridines and fused heterocycles

Abstract

This thesis presents research into the development of metal-free based one-pot protocol for the efficient and green synthesis of a novel class of diversely substituted carbazoles, pyridines, pyranochromenones and other related fused heterocyclic scaffolds in a mild, efficient and environment-friendly manner from easily accessible starting materials. To begin with a general introduction for the synthesis and applications of carbazoles, pyridines and fused chromenones, with historical background and synthetic protocols documented for the synthesis of aforesaid class of derivatives through different synthetic modifications have been discussed in the first chapter. The second chapter demonstrates a clean, catalytic, mild, high yielding and efficient method for the stereoselective synthesis of (E)-methyl-α-(3-formyl-1H-indole-2-yl)-β-aryl/alkyl-substituted acrylates via a condensation reaction of several aromatic and aliphatic aldehydes with methyl 2-(3-formyl-1H-indole-2-yl)-acetate using L-proline as an organocatalyst. Moreover, the synthesized acrylate derivatives have been converted to carbazoles and natural product prenostodione through simple synthetic transformations. In the third chapter, a mild, easy, catalytic and green protocol for the preparation of substituted carbazoles through a one-pot, two-step sequence reaction between aryl/alkyl-substituted trans-β-acroleins and methyl 2-(3-formyl-1H-indol-2-yl)acetate in water in the presence of DABCO, followed by in situ aromatization using 6N HCl under air at heating conditions has been documented. In the next chapter a simple, mild, expedient, high yielding and eco-friendly procedure for the construction of poly-functionalized carbazole derivatives in water via a sequential one-pot two-step reaction of 2-(3-formyl-1H-indol-2-yl)acetate with variety of α-substituted-nitroolefins/nitrodienes in the presence of DABCO (20.0 mol%) as a commercially available cheap organocatalyst, followed by in situ aromatization using 2N HCl.In the fifth chapter, a one-pot Michael/hydroalkoxylation reaction of α-arylacetylenyl-β-aryl nitroolefins with enolizable cyclic 1,3- dicabonyls/cyclic β-keto ester in EtOH at room temperature using DABCO as a organocatalyst has been described. This Michael/hydroalkoxylation reaction generated 2-arylidenesubstituted 3,4-dihydropyrano[3,2-c]chromen-5(2H)- ones/pyranonaphthoquinones/3,4-dihydropyrano[4,3-b]pyran-5(2H)-ones and related pyrano-annulated heterocycles in an excellent stereoselective manner. In chapter six, a green Michael-elimination-cyclization reaction between several 5/6-membered cyclic sulfamidate imines and a variety of γ-substituted-β,γ-unsaturated α-ketocarbonyls in a neat condition irradiated by microwave using DABCO as an organic base for the synthesis of substituted pyridine scaffolds has been established. The final chapter of this thesis focuses on the conclusion and future outcomes of the developed protocols accomplished during entire course of works.