Patent ID: 8318184

Claim:
A compound of the formula wherein R 1 is hydrogen, R 2 is hydrogen, and R 3 is hydrogen; R 1 is hydrogen, R 2 is (2S)-2-aminopropanoyl, and R 3 is hydrogen; R 1 is hydrogen, R 2 is (2S)-2-amino-4-methylsulfanyl-butanoyl, and R 3 is hydrogen; R 1 is hydrogen, R 2 is (2S)-2-amino-4-methyl-pentanoyl, and R 3 is hydrogen; R 1 is hydrogen, R 2 is 2-aminoacetyl, and R 3 is hydrogen; R 1 is benzyl, R 2 is hydrogen, and R 3 is benzyl; or R 1 is (2-fluorophenyl)methyl, R 2 is hydrogen, and R 3 is (2-fluorophenyl)methyl; or a pharmaceutically acceptable salt thereof.