Textbook Question
Some (but not all) of the following keto esters can be formed by Dieckmann condensations. Determine which ones are possible, and draw the starting diesters.
(a)
(b)
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25. Condensation Chemistry
Intramolecular Aldol Condensation
Multiple Choice
Which product is the most likely intramolecular aldol addition product formed when -hexanedione is treated with base and then acid workup?
A
A six-membered cyclic -hydroxy ketone
B
A seven-membered cyclic -hydroxy ketone
C
A linear -hydroxy diketone
D
A five-membered cyclic -hydroxy ketone
Verified step by step guidance1
Identify the structure of 2,5-hexanedione: it is a six-carbon chain with ketone groups at carbons 2 and 5.
Recognize that intramolecular aldol reactions occur when an enolate ion formed from one carbonyl attacks another carbonyl within the same molecule, forming a ring.
Determine the possible ring sizes by counting the number of atoms involved in the ring formed between the enolate carbon and the electrophilic carbonyl carbon. For 2,5-hexanedione, the enolate formed at carbon 2 can attack the carbonyl at carbon 5, forming a ring.
Count the atoms in the ring formed: starting from the enolate carbon (carbon 2), through carbons 3 and 4, to the carbonyl carbon (carbon 5), and back to carbon 2, which results in a five-membered ring.
Conclude that the intramolecular aldol addition will most likely produce a five-membered cyclic β-hydroxy ketone after acid workup, as five-membered rings are generally favored due to minimal ring strain and favorable kinetics.
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