When the McCormack cycloaddition is conducted at room temperature with CH3PC12, and the cycloadduct hydrolyzed with the pH controlled at neutrality or on the basic side, the phospholene oxides formed have the 3, 4-position for the double bond. This procedure led to the isolation of phospholene oxides 12 and 13 from dienes 9 and 10, respectively. The ketal derivative 12 is capable