This thesis is concerned with inter- and intramolecular cyclisation reactions of cyclopropenylsilanes mediated by Lewis acids towards the formation of novel heterocyclic compounds and intermolecular methylenecyclopropane reactions with N-acyliminium species in order to form novel bicyclic compounds. Chapter 1 contains background information on synthetic chemistry involving methylenecyclopropanes. Information on allyl- and vinylsilane chemistry involving Lewis acids is also described. Chapter 2 discusses synthesis and cyclisation studies of a range of cyclopropenylsilanes with different Lewis acids. Some novel and unexpected cyclisations have been discovered but on the whole these cyclopropenylsilanes were found to be too unreactive for cyclisation to occur in the anticipated manner. Chapter 3 presents the synthesis and cyclisation studies of methylenecyclopropanes with N-acyliminium ions. Various Lewis acids and reaction conditions were studied for optimisation of the cyclisation reactions.