Question

The major product obtained in the following reaction is :

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Intramolecular aldol condensation: the CH₂ between the chain ketone and the CH₂CO₂Et attacks the ring C=O to close a five-membered ring, then water is lost to give the conjugated enone.

Answer 1

The Correct Option is A

Substrate: 2-substituted cyclopentanone, ring C1(=O)-C2(H) with side chain C2-CH₂(a)-C(=O)(b)-CH₂(c)-CH₂(d)-CO₂Et. Base: NaOEt, heat.
The most acidic alpha hydrogens on the chain ketone are on C(c) (alpha to the ketone; C(a) is also alpha). Enolate at C(c) attacks the ring carbonyl C1 intramolecularly: the new ring contains C1, C2, C(a), C(b), C(c) = five-membered ring (favoured). Attack from C(a) would give a strained three-membered ring.
Aldol addition gives a bicyclic beta-hydroxy ketone; heating with base dehydrates it to the conjugated enone with C1=C(c), a tetrasubstituted double bond at the ring fusion carbon C1 conjugated with the C(b)=O.
The CO₂Et group remains as CH₂CO₂Et on C(c) (alpha carbon of the enone).
This is a bicyclo[3.3.0]octenone (two fused five-membered rings) with the C=C between the fusion carbon and the carbon carrying CH₂CO₂Et, adjacent to C=O: option (1).
Answer: (1) the fused cyclopentenone with C=C at the ring fusion and CH₂CO₂Et on the carbon next to C=O