Pericyclic reactions in organic synthesis

Abstract

Part I of this thesis describes a formal, metal-free, [2 + 2 + 2] cycloaddition strategy based on a cascade of two pericyclic processes. An intramolecular propargylic ene reaction of a 1,6-diyne is used to generate a vinylallene, which then reacts in an inter- or intramolecular Diels-Alder reaction with an alkenyl or alkynyl dienophile. Reactions involving unsymmetrical alkenyl and alkynyl dienophiles proceed with good to excellent regioselectivity, and endo products are formed exclusively. The mechanism of several earlier fully intramolecular related transformations have been shown to involve an analogous process rather than the diradicalmediated pathways proposed previously. Part II of this thesis describes a [4 + 4] annulation strategy involving the intramolecular [4 + 2] cycloaddition of conjugated enynes with activated cyclobutene derivatives to access intermediates containing bicyclo[4.2.0]-2,4-octadiene moieties that then undergo electrocyclic ring opening reactions to provide 1,3,5-cyclooctatrienes. Five novel cyclooctatrienes have been prepared using this method. Part III of this thesis describes the use of supercritical carbon dioxide as an environmentally friendly alternative to conventional solvents for the synthesis of a variety of carboxylic amides. The addition of amines to ketenes generated in situ via the retro-ene reaction of alkynyl ethers provides amides in good yield, in many cases with ethylene as the only byproduct of the reaction. With the exception of primary, unbranched amines, potential side reactions involving addition of the amines to carbon dioxide are not competitive with the desired C-N bond-forming reaction. The amide synthesis is applicable to the preparation of p-hydroxy and P-amino amide derivatives, as well as amides bearing isolated carbon-carbon double bonds.