A Total Synthesis Of Novel Sesquiterpenoid Natural Product ( ±)-Merrilactone A And A Study Of π-Face Selectivity In Additions To Trigonal Carbon Centers In Iso-Steric Environments

Abstract

Natural product synthesis has been a most exciting and challenging branch of organic chemistry in view of its creative power and unlimited scope. Natural product synthesis witnessed an unprecedented growth and innovative developments, especially during the later half of the 20th century. This can be attributed to a number of factors, one of which has been the isolation and characterization of growing number of compounds from natural sources through availability of newer techniques of isolation and purification and advances in the incisive tools (eg. 2D NMR, X-ray, HRMS) of structure determination. Many natural products, though scarce from natural resources, possess wide ranging biological activity and need to be accessed through synthesis for clinical development and evaluation, particularly of analogs. This has been one of the main stimuli in recent years for undertaking the synthesis of natural products. Among the diverse architecture created by Nature, terpenoids are the most variegated in terms of the presence of a bewildering array of carbocyclic frameworks with unusual assemblage of rings and functionalities. This phenomenal structural diversity of terpenoids makes them challenging targets for total synthesis and for the articulation of new synthetic strategies for carbocyclic ring construction. One of the major concerns in organic chemistry, particularly of relevance in synthesis is the control of diastereoselectivity in nucleophilic and electrophilic additions to trigonal carbon atoms as this is the fundamental step in stereogenesis. Several approaches have been devised to achieve diastereoselection and to understand the interplay of underlying stereoelectronic factors. In this context, introduction of newer probe systems and search for incisive interpretations are continuously enriching the area. The present thesis addresses both the above mentioned themes of contemporary interest in organic chemistry and is presented in two main parts. Part-1: A Total Synthesis of Novel Sesquiterpenoid Natural Product (±)-Merrilactone A. Part-2: A Study of -Face Selectivity in Additions to Trigonal Carbon Centers in Iso-steric Environments. The Part-1 describes our travails towards a stereoselective construction of the complex framework present in the biologically potent and structurally novel sesquiterpene natural product Merrilactone A culminating in its total synthesis. The Part-2 narrates the results of -face selectivity in addition reactions to two novel systems, exo-5-subtituted bicyclo[2.1.1]hexan-2-ones, 5-exo-substituted 2-methylene-bicyclo[2.1.1]hexane and 1-substitued tricyclo[2.1.0.02,5]pentan-3-ones, employing various nucleophiles and electrophiles