A theoretical B3LYP/6-31+G* study of the ozonolysis of 3-methoxy-4-methyl-N-(ethylideneamino)-2-azetidinone in dichloromethane at 195.15 K, and of the reduction of the corresponding ozonide with BH4- in EtOH/H2O 1:1 at 273.15 K to yield methyl 1-propenyl ether, renders three reaction paths proceeding under the energy level corresponding to reactants in accordance with the exceptionally mild reaction conditions experimentally found. Two of these paths lead to Z-methyl 1-propenyl ether with retention of stereochemistry whereas the other one gives E-methyl 1-propenyl ether. These theoretical results predict a high level of stereoselectivity in agreement with the experimental values obtained for the synthesis of vinyl ethers from the reaction of N-[(arylidene or alkylidene)amino]-2-azetidinones with ozone.