JEE Main · 2023 · 12 Apr 2023 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

The substrate is octane-$2,7$-dione, $\mathrm{CH}{}_3\mathrm{CO}(\mathrm{CH}{}_2){}_4\mathrm{COCH}{}_3$. Ethoxide removes an $\alpha$-hydrogen and the resulting enolate adds intramolecularly to the second carbonyl; closing a five-membered ring is strongly favoured. Dehydration of the $\beta$-hydroxy ketone then gives the conjugated enone. The product A is $1$-acetyl-$2$-methylcyclopent-$1$-ene, a cyclopentene ring bearing a methyl group and an acetyl group on the two doubly bonded carbons.