| http://www.w3.org/ns/prov#value | - or a salt thereof, wherein; LG is the leaving group of an active ester; X is O or S; Pg is an amine protecting group; and each Z is independently hydrogen, deuterium, fluorine, chlorine, bromine, iodine, an amino acid side chain or a straight chain or branched C1-C6 alkyl group that may optionally contain a substituted or unsubstituted aryl group wherein the carbon atoms of the alkyl and aryl grou
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