16. Conjugated Systems

Show the product of the Diels–Alder dimerization of cyclobutadiene. (This reaction is similar to the dimerization of cyclopentadiene, discussed in Section 15-11.)

Give the structures of the products represented by letters in this synthesis.

Part 3:

Predict the products of the following reactions.

(h) cyclopentadiene + methyl acrylate, CH₂=CH–COOCH₃

Furan and maleimide undergo a Diels–Alder reaction at 25 °C to give the endo isomer of the product. When the reaction takes place at 90 °C, however, the major product is the exo isomer. Further study shows that the endo isomer of the product isomerizes to the exo isomer at 90 °C.

b. Which isomer of the product would you usually expect from this reaction? Explain why this isomer is usually favored.

Cyclopentadiene can react with itself in a Diels–Alder reaction. Draw the endo and exo products.

Furan and maleimide undergo a Diels–Alder reaction at 25 °C to give the endo isomer of the product. When the reaction takes place at 90 °C, however, the major product is the exo isomer. Further study shows that the endo isomer of the product isomerizes to the exo isomer at 90 °C.

a. Draw and label the endo and exo isomers of the Diels–Alder adduct of furan and maleimide.

1,4-Benzoquinone is a good Diels–Alder dienophile. Predict the products of its reaction with

a. buta-1,3-diene

b. cyclopenta-1,3-diene

While attempting to recrystallize maleic anhydride, a student dissolved it in freshly distilled cyclopentadiene rather than in freshly distilled cyclopentane. Was her recrystallization successful?

What are the products of the following reactions?

b.

The diene lactone shown in part (a) has one electron-donating group (-OR) and one electron-withdrawing group (C=O). This diene lactone is sufficiently electron-rich to serve as the diene in a Diels–Alder reaction.

b. The Diels–Alder product A is not very stable. Upon mild heating, it reacts to produce CO₂ gas and methyl benzoate (PhCOOCH₃), a very stable product. Explain how this strongly exothermic decarboxylation takes place. (Hint: Under the right conditions, the Diels–Alder reaction can be reversible.)

Predict the major product for each proposed Diels–Alder reaction. Include stereochemistry where appropriate.

(a)

Furan and maleimide undergo a Diels–Alder reaction at 25 °C to give the endo isomer of the product. When the reaction takes place at 90 °C, however, the major product is the exo isomer. Further study shows that the endo isomer of the product isomerizes to the exo isomer at 90 °C.

c. Examine your answer to (b) and determine whether this answer applies to a reaction that is kinetically controlled or one that is thermodynamically controlled, or both.

Predict the products of the following proposed Diels–Alder reactions.

(d)

Predict the products of the following proposed Diels–Alder reactions.

(e)