| http://www.w3.org/ns/prov#value | - R2 and R4 independently, are hydrogen or optionally substituted C1-C18 alkyl, aryl or heteroaryl, and when R2 or R4 is a substituted group, the group is substituted with one or more substituents from the group consisting of hydroxy, halo, cyano, aryl, heteroaryl, C1-C18 alkoxy, carboxy, phospho, phosphino, C1-C18 alkoxycarbonyl, primary, secondary or tertiary amino, carbamido, thiol, or C1-C18 alk
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