ウッドワード・ホフマン則
The Diels–Alder reaction between maleic anhydride and cyclopentadiene is allowed by the Woodward–Hoffmann rules because there are six electrons moving suprafacially and no electrons moving antarafacially. Thus, there is one (4"q" + 2)s component and no (4"r")a component, which means the reaction is allowed thermally.