The subject application pertains to a compound of structure IX or X: or a pharmaceutically acceptable salt thereof, wherein: X is -C(=O)N(R10)- or -CH(OPr3) -； R7, R9 and R10 are the same or different and represent hydrogen or an amino acid side chain moiety from either a natural or an unnatural amino acid； Pr1 and Pr2 are the same or different and represent hydrogen or a thiol protecting group； Pr3 is hydrogen or an alcohol protecting group； R1, R2, R5 and R6 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 R1 and R2 and/or R5 and R6, taken together with the carbon atom to which they are attached, form a spirocyclic moiety, with the proviso that: each of R1 and R2 is not hydrogen, or each of R5 and R6 is not hydrogen.
