16. Conjugated Systems

Use FMOT to predict the product of following cycloaddition reaction.

Use FMOT to predict the product of following cycloaddition reaction.

There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)

Show that the [6 + 2] cyclization of hexa-1,3,5-triene with maleic anhydride is thermally forbidden but photochemically allowed.

Show the Diels–Alder product that would actually result from heating hexa-1,3,5-triene with maleic anhydride.

Explain why maleic anhydride reacts rapidly with 1,3-butadiene but does not react at all with ethene under thermal conditions.

Show how the reactant can be converted to the product in two steps.

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two $\pi$ electrons in one reaction and four $\pi$ electrons in the other?

a.

b.