Synthetic and structural investigations into the Diels-Alder reaction and conjugated furans

Abstract

The Diels-Alder reaction was discovered in 1928 and is still the pre-eminent method for the synthesis of six-membered carbocyclic and heterocyclic rings. This thesis comprises four chapters of results and discussion with the Diels-Alder reaction as a theme. Chapter 2 describes a reinvestigation of a long forgotten double Diels-Alder reaction. Chapters 3 and 4 explore the chemistry of conjugated furans and oligofurans. Chapter 5 explores the Diels-Alder endo/exo selectivity of the archetypal diene, cyclopentadiene. Not long after the Diels-Alder reaction was first reported, a series of papers disclosed the discovery of the double dehydro-Diels-Alder reaction. In this reaction a 1,5-dienyne reacts with two molar equivalents of an activated dienophile to give two new rings and up to six new stereogenic centres. This facinating reaction has been proposed to proceed by way of a cyclic allene intermediate. Despite the potential power of this reaction in synthesis, it seems to have been largely ignored in the literature. In Chapter 2 the scope, limitations, stereoselectivity and mechanism of this reaction are explored. The dendralenes are an emerging family of fundamental hydrocarbons. Dendralenes are of great interest in synthesis as they can engage in diene transmissive Diels-Alder reactions that produce polycyclic compounds with considerable stereochemical complexity. The incorporation of furans into a dendralene framework leads to a new sub-family of compounds called the furanodendralenes. The synthesis and Diels-Alder chemistry of these compounds is described in Chapter 3. Related to the furanodendralenes studied in Chapter 3 are compounds containing only furan moieties, the {u03B2}-oligofurans. Chapter 4 describes a general synthesis of the whole family of {u03B2}-furans up to eight furan units in length. Structural studies, primarily by way of X-ray crystallography, have revealed the ?-oligofurans to adopt intriguing secondary structures. That many Diels-Alder reactions exhibit endo-selectivity has been known for many years (Alder's "endo-rule"). This stereoselectivity has been ascribed to Secondary Orbital Interactions. These hypothetical stabilising interactions operate only in the endo transition state. In Chapter 5 the Diels-Alder reactions of the archetypal diene, cyclopentadiene, is studied with a range of common dienophiles, in order to quantify their endo/exo selectivities. These experimental results will serve as an important reference for computational investigations into the origins of endo-selectivity in the Diels-Alder reaction. Show more