Rapid Peptide Synthesis in Liquid Phase. Preparation of Angiotensin II and Delta-sleep-inducing Peptide by the “Hold-in-Solution” Method

A new technique for rapid peptide synthesis has been demonstrated by the preparation of a protected angiotensin II, Z–Asp(OBzl)–Arg(NO2)–Val–Tyr(Bzl)–Ile–His(Bzl)–Pro–Phe–OBzl (1), and a protected delta-sleep-inducing peptide, Z–Trp–Ala–Gly–Gly–Asp(OBzl)–Ala–Ser(Bzl)–Gly–Glu(OBzl)–OBzl (2). Elongation of the peptide chain was carried out in 1,2-dichloroethane by repeating the following series of operations: acylation of a benzyl ester of amino acid or peptide with Boc-amino acid by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride and 1-hydroxybenzotriazole; washing of the organic layer; acidolysis of the α-amino protector; neutralization; and washing. Throughout the synthesis, the growing pep tides were held in the organic layer without being isolated. The protected peptides, 1 and 2, were obtained in overall yields of 78% and 53% based on their C-terminal amino acid esters, and were then hydrogenated to give angiotensin II and delta-sleep-inducing peptide, respectively, thus demonstrating the usefulness of this method for rapid peptide synthesis.