JEE Main · 2021 · 20 Jul 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

Alcoholic potassium hydroxide removes a proton from the methyl group of the side chain ketone, the only site whose enolate can reach a carbonyl across a workable distance. That carbanion adds to the aldehyde carbon, and the ring that closes has six members, namely the methyl carbon, the side chain carbonyl carbon, the two chain methylene groups, the quaternary carbon and the former aldehyde carbon. The quaternary carbon belongs to the original cyclohexanone ring as well, so the two rings meet at a single atom and the skeleton is spiro. Acid and heat then dehydrate the aldol, and the double bond takes the position that puts it in conjugation with the ketone of the new ring. The major product is spiro[5.5]undec-7-ene-1,9-dione, a cyclohex-2-enone joined through its $\gamma$ carbon to a cyclohexanone at the carbon next to that ring's carbonyl group.