Regiochemical Control of the Ring Opening of Aziridines by Means of Chelating Processes. Part 2. Synthesis and Ring-Opening Reactions of Aziridines Derived from 5,6-Dihydro-2H-pyran and of the Diastereoisomeric cis- and trans-Aziridines Derived from 3-(Benzyloxy) cyclohexene, 2-(Benzyloxy)-3,6-dihyd