1-tert-butyl 3-ethyl (3S,4S)-4-aminopyrrolidine-1,3-dicarboxylate; (3S,4S)-Ethyl 1-Boc-4-aminopyrrolidine-3-carboxylate; cis-4-Amino-1-N-Boc-pyrrolidine-3-carboxylic acid ethyl ester;