A new convenient synthesis of N-carboxyanhydrides (NCAs) of alpha-amino acids was achieved by selective cyclization of urethane derivatives of alpha-amino acids. The urethanes were readily synthesized via N-carbamoylation of alpha-amino acids by bis(4-nitrophenyl)carbonate quantitatively. These urethanes having 4-nitrophenoxy moiety were tolerant to air and moisture to allow their facile purification and storage. When the obtained urethanes were heated in 2-butanone at 60 degrees C, they underwent the selective cyclization via intramolecular nucleophilic attack of the carboxyl moiety to the urethane moiety with releasing 4-nitrophenol, leading to the successful formation of the corresponding NCAs. Addition of carboxylic acids remarkably stabilized the formed NCAs during the reaction, allowing their isolation in high yields. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3839-3844, 2009