Methyl 4-aminobutanoate is heated (neat, with removal of methanol). The organic product is

Apyrrolidin-2-one
B4-aminobutanoic acid
Cazetidin-2-one
Dpiperidin-2-one

Answer: A

The amine nitrogen attacks the ester carbonyl intramolecularly. Counting N, C4, C3, C2, C1 gives a five-membered ring, which forms rapidly (a strain-free ring with a favourable proximity effect). Methanol is lost, giving the lactam pyrrolidin-2-one.

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