| abstract |
A compound of formula (IX) or (X): including pharmaceutically acceptable salts thereof; wherein: X is -C(=O)N(R 10 )- or -CH(OPr 3 ) -; R 7 and R 9 are the same or different and represent hydrogen or a natural or unnatural amino acid side chain moiety; each R 10 is the same or different and represents hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl or C 2 -C 6 alkynyl; 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 and R 5 are the same or different and represent Y or a natural or unnatural amino acid side-chain moiety, provided that R 1 and R 5 are not both a natural or unnatural amino acid side chain moiety; and Y is as defined in the application. |