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Organic Chemistry

23.6 Intramolecular Aldol Reactions

Organic Chemistry23.6 Intramolecular Aldol Reactions

23.6 • Intramolecular Aldol Reactions

The aldol reactions we’ve seen thus far have all been intermolecular, meaning that they have taken place between two different molecules. When certain dicarbonyl compounds are treated with base, however, an intramolecular aldol reaction can occur, leading to the formation of a cyclic product. For example, base treatment of a 1,4-diketone such as 2,5-hexanedione yields a cyclopentenone product, and base treatment of a 1,5-diketone such as 2,6-heptanedione yields a cyclohexenone.

The first reaction shows a 1,4-diketone (2,5-hexanedione) with sodium hydroxide in ethanol, forming 3-methyl-2-cyclopentenone and water. The second shows 2, 6-heptanedione (1,5-diketone) with the same reagents forming 3-methyl-2-cyclohexenone and water.

The mechanism of intramolecular aldol reactions is similar to that of intermolecular reactions. The only difference is that both the nucleophilic carbonyl anion donor and the electrophilic carbonyl acceptor are now in the same molecule. One complication, however, is that intramolecular aldol reactions might lead to a mixture of products, depending on which enolate ion is formed. For example, 2,5-hexanedione might yield either the five-membered-ring product 3-methyl-2-cyclopentenone or the three-membered-ring product (2-methylcyclopropenyl) ethanone (Figure 23.4). In practice, though, only the cyclopentenone is formed.

The reaction shows formation of 3-methyl-2-cyclopentenone through path a, from 2,5-hexanedione, using sodium hydroxide and water. Path b converts the same starting material using same reagents to (2-methylcyclopropenyl)ethanone (not formed).
Figure 23.4 Intramolecular aldol reaction of 2,5-hexanedione yields 3-methyl-2-cyclopentenone rather than the alternative cyclopropene.

The selectivity observed in the intramolecular aldol reaction of 2,5-hexanedione is due to the fact that all steps in the mechanism are reversible, so an equilibrium is reached. Thus, the relatively strain-free cyclopentenone product is considerably more stable than the highly strained cyclopropene alternative. For similar reasons, intramolecular aldol reactions of 1,5-diketones lead only to cyclohexenone products rather than to the more strained acylcyclobutenes.

Problem 23-9
Treatment of a 1,3-diketone such as 2,4-pentanedione with base does not give an aldol condensation product. Explain.
Problem 23-10

What product would you expect to obtain from base treatment of 1,6-cyclodecanedione?

An illustration shows a chemical reaction with the product yet to be identified (?). The structure of the reactant 1 6-Cyclodecanedione shows a cyclohexane ring fused with another cyclohexane with the bond missing at the fused position. The top and bottom positions of fusion is double bonded to an oxygen atom.
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