Question

Aldol condensation is a popular and classical method to prepare $\alpha, \beta$-unsaturated carbonyl compounds. This reaction can be both intermolecular and intramolecular. Predict which one of the following is not a product of intramolecular aldol condensation?

• A.
• B.
• C.
• D.

Hint:

Intramolecular aldol condensation involves the reaction within the same molecule, while intermolecular aldol condensation involves the reaction between two different molecules.

Answer 1

The Correct Option is D

The question asks: Among the four depicted α,β-unsaturated carbonyl compounds, which one cannot arise as the product of an intramolecular aldol condensation (base-catalyzed cyclization followed by dehydration)?

Concept Used:

Intramolecular aldol condensation occurs when a molecule containing two carbonyl groups forms an enolate at the α-position of one carbonyl, which then attacks the other carbonyl intramolecularly to close a ring. The favored ring sizes are 5- and 6-membered due to minimal ring strain. The subsequent dehydration gives a conjugated enone in which the double bond is typically endocyclic and more substituted (thermodynamically most stable):

\[ \text{1,5- or 1,6-dicarbonyl} \xrightarrow[\text{base}]{\text{intra-aldol}} \text{β-hydroxycarbonyl} \xrightarrow{-\mathrm{H_2O}} \text{conjugated endocyclic enone} \]

Thus, products are cyclic α,β-enones with the C=C preferably inside the ring (Zaitsev-type, more substituted and better conjugated with \( \mathrm{C=O} \)). Exocyclic methylene enones are disfavored as aldol products because the double bond is less substituted and less stabilized.

Step-by-Step Solution:

Step 1: Evaluate structure (1).

It is a fused bicyclic α,β-enone with the C=C inside the ring. Such skeletons arise from intramolecular aldol of suitably placed 1,6/1,7-dicarbonyls. Hence, (1) is feasible.

Step 2: Evaluate structure (2).

This is an indanone-type α,β-enone (benzannulated 5-membered ring) formed readily from o-acyl/ o-formyl aryl ketones via intramolecular aldol followed by dehydration. Hence, (2) is feasible.

Step 3: Evaluate structure (3).

This is cyclohex-2-en-1-one, a classic product of intramolecular aldol condensation of a 1,6-dicarbonyl (ring closure to a 6-membered β-hydroxyketone followed by dehydration). Hence, (3) is feasible.

Step 4: Evaluate structure (4).

This compound bears an exocyclic methylene (\(=\!CH_2\)) adjacent to the carbonyl, i.e., an exocyclic α,β-unsaturated ketone. Formation of such an exocyclic methylene through simple intramolecular aldol dehydration is disfavored because the more substituted, endocyclic alkene is the thermodynamic product:

\[ \text{β-hydroxyketone} \;\xrightarrow{-\mathrm{H_2O}}\; \underbrace{\text{endocyclic enone}}_{\text{more substituted, more conjugated}} \;\gg\; \underbrace{\text{exocyclic methylene enone}}_{\text{less substituted}} \]

Therefore, (4) is not a typical or expected product of intramolecular aldol condensation.

Final Computation & Result

The intramolecular aldol generally furnishes 5/6-membered cyclic α,β-enones with an endocyclic double bond. The only option that contradicts this (exocyclic methylene enone) is (4).