The 5-Phenylpenta-2,4-dienaldehydes 4, and 7, show an uncatalized cis-trans-isomerization of the 4,5-double bond above 70 °C. The negative value of the activation entropy for these reactions points to the formation of the bicyclic valence isomeric 2H-Pyrans 5 and 8 respectively in the rate determining step. Intermediate 5 can be trapped as its tetracyanoethylene cycloadduct 11.
5-苯基戊-2,4-二烯醛4和7在70°C以上显示4,5双键的未催化顺式-反式异构化。这些反应的活化熵的负值表示在速率确定步骤中分别形成了双环价异构体2 H-
吡喃5和8。中间体5可被捕获作为其四
氰乙烯环加成11。