New Synthetic Intrigues Towards Oxidative Dearomatization and
Related Strategies

Abstract

The thesis entitled “New Synthetic Intrigues Towards Oxidative Dearomatization and Related Strategies” is divided into the following six chapters.
Chapter-I describes a new methodology of oxidative-dearomatization of planar phenols and naphthols. An economic, viable, one-pot, metal-free protocol for direct conversion of naphthols to α-ketols is reported. naphthols were found to undergo facile unprecedented oxidative dearomatization with regioselective hydroxylation with phenyl selenyl bromide in open air conditions. Quaternary stereocenters were developed along with formation of sterically demanding α- and γ-ketols with high yields. A thorough study of the stereo electronic demands of the unusual oxy-Selenium reactive intermediate involved in dearomatization of 1- and 2-naphthols is studied. The first instance of unprecedented phenoxy-selenium interaction leading to facile dearomatization of arenes is highlighted in this chapter.
Chapter-II entails the first recognition of Phenyl Trimethyl Ammonium Tribromide (PTAB) as an effective reagent for spiro-cyclizations proceeding via oxidative dearomatization. The experiment exhibits metal-free, ligand-free and one-pot accomplishment of these significant transformations. The presented methodology described the first generalized protocol for spiro-etherification reaction as well as brominative dearomatization of naphthols which can be applied towards various directions. Good to moderate diastereoselectivity was achieved with varied substitution in the hydroxy- naphthols in case of spiro-etherification. Also this chapter highlight the synthesis of bromo spiro-furanonaphtalones from 1-naphthol derivatives. In this chapter also we describe the lactonisation of 2-naphthol derivatives using PTAB as the effective reagent.
Chapter-III discloses a highly efficient methodology for generation of nitrogen containing quaternary carbon centres via aminative and azidative oxidative dearomatization of phenols and naphthols. The same protocol has also been successfully employed to achieve oxidative peroxidation as well as oxidative etherification of phenols and naphthols. The simplest metal free reaction conditions delineates an easy break through to the “Trio”- of oxidative amination, azidation and peroxidation. An array of diverse polyfunctionalised heterocycles has been synthesized in one pot.
Chapter-IV explored two new reagents Phenyl selenyl bromide (PhSeBr) and phenyl trimethylammonium tribromide (PTAB) as the effective reagents for oxidative C (sp2) – C (sp3) bond cleavage of 1-hydroxy alkyl 2-naphthols via oxidative dearomatization.Also the synthetically important bromo-azido ketone was first synthesized in this oxidative C (sp2) – C (sp3) bond cleavage.
Chapter-V describes a facile methodology which has successfully simplified the generation of sulfonamides, thiosulfonates and symmetric disulfanes. These excellent oxidative tendencies prompted us to study the tribromide mediated oxidative couplings in the S-N and S-S generation studies. The “trio” of reactions occur in an open metal free atmosphere and has also been scaled up to grams making it suitable for commercialization. The reactions also have been successfully carried out with asymmetric variants, thus contributing to the chiral pool. The user friendly “trio” enables easy generation of these versatile sulfur analogues and the reaction condition employed depict an economic outline.
Chapter-VI describes the first stereoselective synthesis of Heliannuol G employing an o-quinone methide intermediate. The key compound trans- dihydrobenzofuran was generated employing a reactive o-quinone methide intermediate via oxidative dearomatization. The present synthesis triumphs over all the drawbacks associated with the reported one and reveal an impressive overall yield of 12.37%.