Regiochemical control of the ring opening of aziridines by means of chelating processes .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-dihydro-, and 2-(benzyloxy)-6-methyl-3,6-dihydro-2H-pyran