Q.57. - Organic Chemistry

Question

Question: When A is treated with aqueous –OH, the major product is compound B, which undergoes ester hydrolysis and decarboxylation to form C. Draw a stepwise mechanism for the conversion of A to B.

Step-by-Step Solution

Answer

Answer

Step 1:

Step 2:

Step 3:

Step 1: Intramolecular Aldol reactions

Di-carbonyl compounds undergo intramolecular aldol condensation to form a five or six membered cyclic compound. Out of the two carbonyl groups, one which forms resonance stabilized enolate is protonated, and the other acts as an electrophilic center.

Step 2:Mechanism for conversion of A to B

The deprotonation does not occur at the  $\alpha$-carbonyl, it occurs in a way that the enolate is more resonance stabilized, as shown below:

Deprotonation and formation of an enolate

 

 The nucleophilic attack occurs at the keto carbonyl group since it leads to the formation of a 6-membered ring. If the attack would have occurred at the ester carbonyl group, an unstable 3-membered ring would have been formed.

Nucleophilic attack and formation of 6-membered ring

 

In the final step, dehydration occurs via the E1cb mechanism, involving deprotonation and elimination of -OH group, as shown below:

Dehydration via E1cb mechanism