Development Of Novel And Efficient Methods For The Synthesis Of N-, O-, And S-containing Heterocyclic Compounds

In today's efficient, innovative and environment conscious chemical society, the focus of the organic research resides to develop the novel, efficient, and economical methods for the synthesis of pharmacologically important structural motifs. The chemical commodities containing sulphur, nitrogen, and oxygen atom are important skeleton structures, which frequently appear in pharmaceutical molecules, natural products and functional materials. Besides this, their importance in agriculture, electronic, and polymer industry cannot be neglected. Owing to their immense significance, the derivatives of these chemical commodities (N-, O-, and S-containing heterocycles) have been generated through cyclization reactions either via transition metal catalysis, oxidation, or microwave radiations.However, the development of novel methods under metal-free conditions or the use of inexpensive metal catalyst in cyclization reactions under mild reaction conditions is still a challenging job in organic synthesis. Following the metal-free C-C bond cleavage strategies of carbonyl group, this dissertation describes a novel method for the synthesis of benzazoles through Brensted acid catalyzed reactions of 2-amino thiophenols/anilines/phenols and ?-diketones via sequenced nucleophilic addition, dehydration, intramolecular cyclization, aromatization, and subsequent C-C bond cleavage reactions. Initially, the Br(?)nsted acid catalyzed cyclization reactions of 2-amino thiophenols/anilines with ?-diketones under oxidant-, metal-, and radiation-free conditions were investigated. Various 2-substituted benzothiazoles/benzimidazoles were obtained in 49%-95% yields. Different groups such as methyl, chloro, nitro, and methoxy linked on benzene rings were tolerated under the optimized reaction conditions. The oxidant-, metal-, and radiation-free conditions should make this novel method an efficient, economical, and environment friendly in nature for the synthesis of these privileged heterocyclic compounds.Besides this, cyclization reactions of 2-aminophenols with ?-diketones catalyzed by a combination of Br(?)nsted acid and Cul are also presented in this dissertation. Various 2-substituted benzoxazoles were obtained in 25%-89% yields through these reactions and different substituents such as methyl, chloro, bromo, nitro, and methoxy on 2-aminophenol were tolerated under the optimized reaction conditions.During the course of our research on heterocycle synthesis through cyclization reactions, an efficient method has been developed for the synthesis of isoquinolones through base KO'Bu-promoted SNAr reaction of 2-halobenzonitriles with ketones followed by Lewis acid Cu(OAc)2-catalyzed cyclization. These reactions proceeded smoothly to produce isoquinolone derivatives in 25%-88% yields. This method shows good functional group tolerance for methyl, methoxy, and different halogen atom (F, Cl, I). A variety of ketones (aliphatic, aromatic and alicyclic) were found suitable for these reactions. The wide availability of the starting materials, affordability of the Lewis acid catalyst, the mild reaction conditions, and experimental simplicity could make the present methodology worth useful in organic synthesis.