Patent ID: 8614193

Claim:
A Compound of structure IX or X: or a pharmaceutically acceptable salt thereof, wherein: X is —C(═O)N(R 10 )— or —CH(OPr 3 ) —; R 7 , R 9 and R 10 are the same or different and represent hydrogen or an amino acid side chain moiety from either a natural or an unnatural amino acid; Pr 1 and Pr 2 are the same or different and represent hydrogen or a thiol protecting group; Pr 3 is hydrogen or an alcohol protecting group; R 1 , R 2 , R 5 and R 6 are the same or different and represent hydrogen or an amino acid side chain moiety from either a natural or an unnatural amino acid, or R 1 and R 2 and/or R 5 and R 6 , taken together with the carbon atom to which they are attached, form a spirocyclic moiety, with the proviso that: both R 1 and R 2 or both R 5 and R 6 are not hydrogen.