See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Let us analyze each step in the synthesis: Step (A): The starting material is methyl 4-oxocyclohexane-1-carboxylate. The product has the ketone carbonyl converted to a 1,3-dioxolane (cyclic acetal) while the ester group CO2CH3 is retained. This is a ketone protection reaction. The standard reagent for forming a 1,3-dioxolane acetal is ethylene glycol (HOCH2CH2OH) with an acid catalyst. Option (a) proposes HOCH2CH2SH (2-mercaptoethanol), which would form a mixed acetal/hemithioacetal, not a 1,3-dioxolane (two oxygen atoms shown in the ring). Therefore option (a) is incorrect. Step (B): The ester group CO2CH3 is converted to CH2-OH (primary alcohol). This is a reduction of an ester to a primary alcohol. NaBH4 normally does NOT reduce esters under standard conditions (NaBH4 reduces ketones and aldehydes selectively). To reduce an ester to an alcohol, LiAlH4 (LAH) would be required. However, since the dioxolane acetal is present and must be preserved, LiAlH4 would be used carefully. NaBH4 cannot reduce esters, so option (b) is incorrect. Step (C): The primary alcohol CH2-OH is oxidized to an aldehyde CHO. KMnO4 is a strong oxidizing agent that would over-oxidize a primary alcohol to a carboxylic acid, not stop at the aldehyde stage. A milder oxidant such as PCC (pyridinium chlorochromate) or Swern oxidation would be needed to selectively oxidize a primary alcohol to an aldehyde. Therefore option (c) is incorrect. Step (D): The 1,3-dioxolane acetal is hydrolyzed back to the ketone while the aldehyde CHO group is retained. Acetal hydrolysis requires acidic aqueous conditions, i.e., H3O+ (dilute acid/water). This is the correct reagent for selective acetal deprotection. H3O+ cleaves the acetal to regenerate the ketone without affecting the aldehyde under mild conditions. Therefore option (d) is correct. Options (a), (b), and (c) describe incorrect reagents for their respective steps, while option (d) correctly identifies H3O+ as the reagent for acetal deprotection in step D. Therefore, the correct answer is D.