Q1(f). Pulegone to a ring-contracted ester

Answer in brief

A is the vicinal dibromide across the exocyclic double bond. B is methyl pulegenate, methyl 5-methyl-2-(propan-2-ylidene)cyclopentane-1-carboxylate, as cis/trans ester epimers.

Key points

• Bromine adds across the exocyclic C=C: one Br is on the ring α-carbon and the other on the external CMe₂ carbon.
• Methoxide removes an α-H on the other side of the ketone. Intramolecular displacement of the ring-bound Br produces a cyclopropanone intermediate.
• Methoxide adds to the cyclopropanone carbonyl. Ring opening with elimination of the remaining Br regenerates an exocyclic isopropylidene group.
• The former ketone carbon becomes the ester carbonyl, outside a five-membered ring. This is the characteristic Favorskii ring contraction.