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: each of R
1
and R
2
is not hydrogen, or each of R
5
and R
6
is not hydrogen.