Studies on the Synthesis and Reactivity of Diradical Generating Molecules

Abstract

This thesis is a compilation of mechanistic studies and synthetic potential of diradical generating cycloaromatization reactions, in particular, the Garratt-Braverman cyclization (GBC) and the Bergman cyclization (BC). The first chapter contains a brief review on sequential developments on the mechanistic aspects of BC and GBC. The second chapter deals with the mechanistic investigation on GBC. By using various experimental techniques like 2H NMR, LA-LDI and from the fate of deuterium labeled substrates we have been able to propose a diradical pathway for bis-propargyl sulfones and an anionic intramolecular Diels Alder (IMDAR) pathway for bis-propargyl ethers (Scheme 1). The third chapter comprises the validation of enediyne moiety to act as a photoaffinity label in protein capture. The probable mechanism of capture via a photo-Bergman cyclization of enediynes has also been described (Scheme 2). The fourth chapter contains differential reactivity of bis-propargyl ethers appended with aliphatic substituents. These systems may undergo either GBC to aryl (dihydro) naphthalenes or follow a 1,5-H shift pathway to yield 3,4 disubstituted furans. Strategies have been developed to shift the preference from GBC to 1,5-H shift process to yield 3,4 disubstituted furans that are otherwise difficult to obtain and also constitute an important skeleton in medicinal chemistry (Scheme 3). Unlike the ethers which follow a GBC pathway under base treatment, the corresponding propargyl alkenyl sulfones follow a base-mediated 6π-electrocyclizaton reaction to substituted thiopyran dioxides. The mechanistic investigations (briefly represented in Scheme 4) of this 6-electrocyclization process have been discussed in the fifth chapter. In the sixth chapter, the synthetic potential of GBC was further exploited by carrying out a one pot GBC and Scholl oxidation reaction to polyaromatic compounds having low lying ELUMO level without sacrificing the band gap and the aerial stability of the compounds (Scheme 5).