2,5-Cyclohexadienones as a Useful Launching Point for the Synthesis of the Briarane Diterpenoids and A Hypervalent Iodine-Mediated Synthesis of Oxazolines

Abstract

The briarane diterpenoids are a large class of natural products derived from gorgonians and other corals from throughout the world. Despite the extremely large number of briaranes that have been isolated, along with the potent and diverse range of biological activities that have been observed, the total synthesis of the briaranes remains underexplored. A facile synthetic route to the briarane diterpenoids will aid in the further exploration of these molecules. Herein, we will describe a number of synthetic approaches that were evaluated to access a key fragment of the briarane diterpenoids. A key feature of all routes involves the use of 2,5-cyclohexadienone substrates as a diverse platform for the launching of the synthesis. Chapter 1 will provide background information on 2,5-cyclohexadienones. Methods for their synthesis, a survey of their diverse reactivity, and selected examples of their use in natural product synthesis will all be described. Emphasis will be given to reactivity patterns which aided us in our research. Chapter 2 will provide a brief survey of the briarane diterpenoids as well as some of the major biologically active families. Previous synthetic efforts used to access these molecules will also be described. Chapter 3 will describe our efforts to synthesize a key fragment of the briarane diterpenoids (referred to as the briarane stereotetrad) utilizing intermediates containing a bicyclic lactone. Chapter 4 will describe our successful efforts to access the briarane stereotetrad using monocyclic intermediates. The important influence of torsional strain in key steps, as well as a successful route to access the briarane stereotetrad will be described. Chapter 5 will report the results of a separate research project in which an iodine(III) promoted cyclization of N-allylamides to form oxazolines was studied. The development of optimum reaction conditions and the evaluation of the substrate scope will be described. Key results that suggest novel mechanistic details for this electrophilic oxidative cyclization will also be described.