The amide side chain is converted to a methyl ester under the reaction conditions. Provide an explanation for why this reaction is so facile (usually it would require much hasher conditions).
Under these acidic conditions, it is relatively easy for the amide to attack one of the other carbonyl groups in 83. It is the most favorable for the nitrogen to attack the methyl ketone directly across the ring from it because of how close the two reacting sites are and because the ring strain involved in attacking the other two carbonyls is unfavorable. After the new ring is formed in the nucleophilic addition to the methyl ketone to create 84 it becomes possible for methanolysis to occur on the amide. This is because the new bond to the methyl ketone site stabilizes the nitrogen and makes it a better leaving group from the amide. After methanolysis of the amide the nitrogen can be removed from the methyl ketone site with acid catalyzed elimination making a good leaving group by giving the nitrogen a positive charge. Once in the solution the excess of acid will protonate it so that it will be unable to reverse the reaction. All of these steps happen in a slow equilibrium, which is driven forward by the excess methanol and high heat.