DL-Pyroglutamic acid Ethyl D-(-)-pyroglutamate Ethyl L-pyroglutamate D-Pyroglutamic acid Z-PYR-OH (S)-(+)-5-(Hydroxymethyl)-2-pyrrolidinone p-toluenesulfonate (3R,4S,5R,6S)-1-AZA-4-HYDROXY-5-HYDROXYMETHYL-6-ISOPROPYL-3-METHYL-7-OXABICYCL[3.3.0]OCTAN-2-ONE (R)-1-BENZYL-5-HYDROXYMETHYL-2- PYRROLIDINONE (3R,4S,5R)-4-HYDROXY-5-[(1S)-1-HYDROXY-2-METHYLPROPYL]-3-METHYL-2-PYRROLIDINONE-5-CARBOXYLIC ACID (R)-1-BENZYL-5-CARBOXY-2-PYRROLIDINONE