25. Condensation Chemistry

Predict the major products of the following crossed Claisen condensations.

(b)

Propose mechanisms for the following reactions.

(b)

Show how Claisen condensations could be used to make the following compounds.

(c)

Show how you would use an aldol, Claisen, or another type of condensation to make each compound.

(a)

Esters with only one α hydrogen generally give poor yields in the Claisen condensation. Propose a mechanism for the Claisen condensation of ethyl isobutyrate, and explain why a poor yield is obtained.

The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound reacts with an aldehyde or ketone, in the presence of a secondary amine as a basic catalyst, to produce a new C=C. Show the starting materials that made each of these by a Knoevenagel condensation.

(a)

Show what esters would undergo Claisen condensation to give the following β-keto esters.

(a)

Design a synthesis for each of the following compounds using the given starting material:

b.

Indicate how each of the following compounds can be synthesized from the given starting material and any other necessary reagents:

b.

Predict the major product obtained when ethyl acetate (${\mathrm{CH}}_3\mathrm{COOEt}$) undergoes a Claisen condensation.

Predict the products of self-condensation of the following esters.

(d)

There are other condensation reactions similar to the aldol and Claisen condensations:

c. The Knoevenagel condensation is the condensation of an aldehyde or a ketone that has no a-hydrogens and a compound such as diethyl malonate that has an a-carbon flanked by two electron-withdrawing groups. Draw the product obtained from the following Knoevenagel condensation:

Propose a mechanism for the self-condensation of methyl 3-phenylpropionate promoted by sodium methoxide.

Draw the products of the following reactions:

d. diethyl 1,2-benzenedicarboxylate + sodium ethoxide: (1) slow addition of ethyl acetate; (2) HCl

There are other condensation reactions similar to the aldol and Claisen condensations:

a. The Perkin condensation is the condensation of an aromatic aldehyde and acetic anhydride. Draw the product obtained from the following Perkin condensation: