1. A. P. Pavar, P. Ya. Romanovskii, R. É. Vegner, G. I. Chipens, G. A. Inikhova, Yu. I. Indulen, Z. P. Auna, and V. E. Klusha, in: The Chemistry and Biology of Peptides [in Russian], Riga (1971), p. 48.
2. R. Schwyzer, B. Iselin, H. Kappeler, B. Riniker, W. Rittel, and H. Zuber, Helv. Chim. Acta,41, 1287 (1958).
3. E. Schröder, Ann. Chem.,691, 232 (1966).
4. M. A. Creswell, R. W. Hanson, and H. D. Law, J. Chem. Soc.,1967C, 2669.
5. G. I. Chipens and O. Papsuevich, in: The Chemistry and Biology of Peptides [in Russian], Riga (1971) p. 5.