Five-membered heterocycles act as dienes in Diels–Alder reactions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص738-740
2025-07-14
424
Five-membered heterocycles act as dienes in Diels–Alder reactions
All of the reactions of pyrrole, furan, and thiophene we have discussed so far have been variations on reactions of benzene. But heterocycles also do reactions totally unlike those of benzene and we are now going to explore two of them.
The first is a reaction you will meet in detail. It is known as the Diels–Alder reaction, and although it has a number of subtleties we will not discuss here, it has a simple cyclic mechanism in which six electrons (three curly arrows) move around to form a new six membered ring. Here is an example with the Boc derivative of pyrrole. The electron-deficient Boc group makes pyrrole less nucleophilic and promotes the Diels–Alder reaction with an alkynyl sulfone. Benzene, and even many other heterocycles, will not do this sort of reaction.
The product is a useful intermediate in the synthesis of the analgesic epibatidine. Selective reduction of the non-conjugated double bond is followed by addition of a pyridine nucleophile (a lithium derivative can be prepared from a bromopyridine) to the vinyl sulfone.
Furan is particularly good at Diels–Alder reactions but it gives the thermodynamic product, the exo adduct, because with this aromatic diene the reaction is reversible.
Aromaticity prevents thiophene taking part in Diels–Alder reactions, but oxidation to the sulfone destroys the aromaticity because both lone pairs become involved in bonds to oxy gen. The sulfone is unstable and reacts with itself but will also do Diels–Alder reactions. With an alkyne, loss of SO2 gives a substituted benzene derivative.
Similar reactions occur with α-pyrones. These are also rather unstable and barely aromatic and they react with alkynes by Diels–Alder reactions followed by reverse Diels–Alder reactions to give benzene derivatives with the loss of CO2.
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