See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the target acetal structure: The given acetal is a spiro compound. It has a cyclopentane ring and a six-membered 1,3-dioxane ring sharing one spiro carbon. The 1,3-dioxane ring has two methyl groups (gem-dimethyl) at C5 (the carbon between the two oxygens is the spiro/acetal carbon, and the carbon at position 5 of the dioxane ring bears two methyl groups). Step 2 - Retrosynthetic analysis of an acetal: An acetal is formed by reaction of a ketone (or aldehyde) with a diol under acid catalysis with removal of water. The acetal carbon (C bonded to two oxygens) comes from the carbonyl carbon of the ketone. The two oxygen atoms and the carbon chain between them come from the diol. Step 3 - Identify the ketone component: The spiro carbon is the acetal carbon. It is part of a cyclopentane ring. Therefore, the ketone is cyclopentanone (a five-membered ring ketone). Step 4 - Identify the diol component: The 1,3-dioxane ring is six-membered: O-CH2-C(CH3)2-CH2-O with the acetal carbon closing the ring. The -O-CH2-C(CH3)2-CH2-O- fragment comes from the diol HO-CH2-C(CH3)2-CH2-OH, which is 2,2-dimethyl-1,3-propanediol (neopentyl glycol). The central carbon of this diol bears two methyl groups and two CH2OH arms. Step 5 - Match with options: (a) Uses 4,4-dimethylcyclohexanone (6-membered ring ketone) + ethylene glycol -> would give a 5-membered dioxolane spiro product with a cyclohexane ring, not the target. (b) Uses cyclopentanone + a 3-membered ring or branched diol that would give a 5-membered acetal ring, not matching the gem-dimethyl 6-membered dioxane ring. (c) Uses cyclopentanone + 2-methyl-1,3-propanediol (only one methyl on central carbon) -> would give a 6-membered dioxane ring with only one methyl at C5, not gem-dimethyl. (d) Uses cyclopentanone + 2,2-dimethyl-1,3-propanediol (two methyls on central carbon, neopentyl glycol) -> cyclopentanone reacts with HOCH2C(CH3)2CH2OH to give a spiro acetal where the dioxane ring has gem-dimethyl at C5 and the cyclopentane ring is intact. This matches the target exactly. Therefore, the correct answer is D.