Part 1. Synthesis of Stereodefined Heterocyclic Rings. We wish to report the development of novel methodology for the synthesis of stereodefined heterocyclic rings, which could be used for the synthesis of natural products containing for example tetrahydrofuran motifs, such as members of the pamamycin family. Due to their ambivalent properties, organoaluminium reagents can easily react with acetals by transfering an alkyl group after prior coordination with the substrates. This work has led to the development of a novel cascade reaction. It involves the reaction of acetals with trialkylaluminium reagents, which is followed by a cyclisation reaction, generating consequently tetrahydrofuran or tetrahydropyran rings. In addition, investigation towards the synthesis of pyrrolidines was also carried out. Part 2. Investigation and Development of a Novel Cascade Reaction. The Bergman cycloaromatisation reaction is based on the formation of a biradical intermediate species and has been, over the years, a constant source of inspiration for scientists. Continued efforts over the last 40 years permitted, among other things, a better understanding of the mode of action of the enediyne antibiotics, a class of natural compounds with exceptional biological activities. The Parsons group recently developed a novel cyclisation reaction, which also generates a biradical species, and which could, after being trapped with a suitable alkene, lead to the formation of tricyclic molecules containing heterocyclic cores. As a result, we wish to further investigate this novel reaction and develop a tandem reaction, involving this reaction combined with a Diels-Alder reaction in order to generate pentacyclic molecules, in one synthetic operation, and from an acyclic precursor.