See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The oxyallyl cation is a 1,3-dipolar species (or zwitterion) with a positive charge on carbon and a negative charge on oxygen. In this molecule, the oxyallyl cation is tethered to a cyclohexane ring with an adjacent OMe group. The rearrangement involves intramolecular electrocyclic ring closure or 1,3-dipolar cyclization. Step 1: Identify the reactive species. The starting material is an oxyallyl cation where there is a carbocation at C1 of the cyclohexane ring and an oxyanion (enolate-like oxygen) on the exocyclic =CH2 group. This is effectively a trimethylenemethane-type zwitterion or an oxyallyl dipole: [C+]-C=CH2 with O- on the terminal carbon, i.e., the system is +C-C(=CH2)-O-. Step 2: The oxyallyl cation (1,3-zwitterion with C+ and O-) undergoes intramolecular ring closure. The negatively charged oxygen (O-) attacks the positively charged carbon (C+) on the ring. Since these two charged centers are 1,3-related (separated by one carbon bearing the exocyclic methylene), their combination forms a three-membered ring. The C+ is at C1 of cyclohexane and O- is on the carbon alpha to C1 (the exocyclic carbon). Ring closure between C+ and O- gives a three-membered ring containing oxygen... but that would give an epoxide-type structure. Step 3: Re-examining the connectivity. The oxyallyl cation has the structure: ring-C+(positive) connected to C(=CH2) with O-(negative) on that same carbon. The 1,3-relationship is: C+(ring carbon) - C(exocyclic) - O-. Intramolecular combination of C+ and O- closes to form a three-membered ring: a cyclopropanone (the C+ attacks the O- or the O- attacks C+, forming a C-O bond in a three-membered ring). However, if the bond forms between C+ and O-, and the exocyclic carbon is between them with a =CH2, the product would be a cyclopropanone (cyclopropane ring with a ketone C=O). Step 4: Actually, the correct interpretation of the oxyallyl cation rearrangement is well-known in organic chemistry: an oxyallyl cation (2-oxidoallyl or trimethylenemethane oxide) undergoes disrotatory electrocyclic ring closure to give a cyclopropanone. The C1(+)-C(=CH2)-O(-) system closes to form a cyclopropanone ring where the C1 of cyclohexane and the O bond together, giving a spiro-cyclopropanone fused at C1 of cyclohexane, with the OMe group remaining at C2. Step 5: This matches option (c): a cyclohexane ring with a cyclopropanone (three-membered carbocyclic ring bearing C=O) attached at C1 and OMe at C2. Why other options fail: - (a) shows a methylenecyclopropane product (no oxygen incorporation into ring), which would require a different rearrangement pathway. - (b) shows an epoxide, which would require O- attacking an adjacent C-H or forming a two-membered ring closure that doesn't fit the geometry. - (d) shows an aromatic ring, implying aromatization occurred, which is not a simple rearrangement of the oxyallyl cation under these conditions. Therefore, the correct answer is C.