Peptide derivative, and pharmaceutically acceptable salt thereof, process for producing the same, and use thereof

A process for preparing a peptide derivative of formula (I) or a pharmaceutically acceptable salt thereof

• wherein X is —CH(OH)— or —CO—;
• R1 is hydrogen or an amino protecting group;
• R2 is hydroxyl or lower alkoxy;
• one of R3 and R4 is the side chain (R group) of lysine the amino group of which may be protected with a protecting group and the other of R3 and R4 is the side chain (R group) of arginine the guanidino group of which may be protected with a protecting group;
• and R5 and R6 may be the same or different and are hydrogen, lower alkyl or aralkyl,
• the process comprising the following step (i):
• (i) condensing a compound of formula (II)
• wherein R1 and R3 are as defined above and R2a is lower alkoxy, with a compound of formula (III)
• wherein R4, R5, and R6 are as defined above to produce a peptide derivative of formula (I-a) or a pharmacuetically acceptable salt thereof
• wherein R1, R2a, R3, R4, R5 and R6 are as defined above.