The major product formed in the following conversion is: Substrate: 2-(2-oxopropyl)cyclopentan-1-one (a cyclopentanone ring bearing a $-\mathrm{CH}{}_2-\mathrm{CO}-\mathrm{CH}{}_3$ side chain at C-2). Reagents: (i) $\mathrm{NaBH}{}_4$; (ii) $\mathrm{H}{}_2\mathrm{SO}{}_4$, $\Delta$.

Solution & Explanation

\textbf{Answer:} (C) a cyclopent-1-ene ring with a conjugated $-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_3$ side chain \textbf{Solution:} Step (i): $\mathrm{NaBH}{}_4$ reduces both carbonyl groups — the ring ketone and the side-chain ketone — to give a diol (a cyclopentanol bearing a $-\mathrm{CH}{}_2-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_3$ side chain). Step (ii): $\mathrm{H}{}_2\mathrm{SO}{}_4$/$\Delta$ dehydrates both alcohols by E1, following Zaitsev / maximum-stability. Each $-\mathrm{OH}$ is lost so as to give the most stable (most substituted) alkene, and the two new double bonds are placed so that they are conjugated. The result is a cyclopent-1-ene (ring double bond at the formerly substituted carbon) with a conjugated 1-propenyl $-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_3$ side chain — the most stable conjugated diene. Hence option (C) is the major product.