The chlorine addition and Diels-Alder cycloaddition of cyclopentadiene to 1, 4-benzoquinone-4-(O-methyloxime) have been studied MO theoretically. It has been shown that the reactions occur predominantly to the quinone ring double bond which is oriented anti to the nitrogen lone pair due to an n-${sigma}^*$ interaction between the nitrogen lone pair, n, and the app. vicinal bond, causing the ${pi}$ bond to be weakened and destabilized due to the less conjugation from reduced delocalization.