The Nitrogen is now attached to the 2 position of the chain; its lone pair again attacks the carbon of the other carbonyl, joining the chain, and again protonating the oxygen and deprotonating the nitrogen.
Again joining the chain, protonating the oxygen and deprotonating the nitrogen. The Nitrogen is now attached to the 2 position of the chain; its lone pair again attacks the carbon of the other carbonyl.
The Nitrogen is now attached to the 2 position of the chain; its lone pair again attacks the carbon of the other carbonyl. Again joining the chain, protonating the oxygen and deprotonating the nitrogen.
The Nitrogen is now attached to the 2 position of the chain; its lone pair again attacks the carbon of the other carbonyl. Again joining the chain, protonating the oxygen and deprotonating the nitrogen.