Antimicrobial analogues of Gramicidin S

A method of preparing a compound of formula (I),

• or a pharmaceutically acceptable salt thereof, comprising
• removing amino protection groups from a compound of formula (I′) or a pharmaceutically acceptable salt thereof
• wherein
• R1, R2, R3, and R4are each independently selected from hydrogen or —NH2;
• R1A, R2A, R3A, and R4A are each independently selected from hydrogen or —NH-GX;
• and
• G1, G2, and Gx at each occurrence are independently amino protection groups;
• wherein at least one of R1, R2, R3, and R4 is —NH2.