16. Conjugated Systems

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

Part 3:

E (C8H12) + H (C3H4O2) + Heat —> I (C11H16O2)

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.

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(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.)

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.

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

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.

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

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

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

Recall (Section 16-10) that two positions of anthracene

sometimes react more like polyenes than l

ike aromatic compounds.

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(b) The Diels–Alder reaction of anthracene with

maleic anhydride is a common organic lab experiment.

Predict the product of this Diels–Alder reaction.

Show the product of the Diels–Alder dimerization of 

cyclobutadiene. (This reaction is similar to the dimerization 

of cyclopentadiene, discussed in Section 15-11.)

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.

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(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.