The diene lactone shown in part (a) has one electron-donating group (OR) and one

Chapter 15, Problem 15-29

(choose chapter or problem)

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

(a) What product would you expect to form when this diene reacts with methyl acetylenecarboxylate, a strong dienophile?

(b) The Diels–Alder product A is not very stable. Upon mild heating, it reacts to produce \(\mathrm{CO}_{2}\) gas and methyl benzoate \(\left(\mathrm{PhCOOCH}_{3}\right)\), a very stable product. Explain how this strongly exothermic decarboxylation takes place. (Hint: Under the right conditions, the Diels–Alder reaction can be reversible.)