tert-Butyl (S)-2-pyrrolidone-5-carboxylate is a chiral building block and key intermediate in organic synthesis, particularly for pharmaceutical applications. As the tert-butyl ester of L-pyroglutamic acid, it enables stereoselective reactions essential for producing optically active compounds. It is widely used in the synthesis of nootropic drugs, such as piracetam and other racetams, where it improves reaction yields and enantioselectivity. In peptide synthesis, it acts as a protected pyroglutamic acid derivative, offering stability during synthesis and facile deprotection of the ester group under acidic conditions to yield the free carboxylic acid.