prov:value
| - R1, R2, R3, and R4 independently, are hydrogen, hydroxy, halo, cyano, isocyano, C1-C18 alkoxy, C1-C18 carboxy, C1-C18 alkoxycarbonyl, primary, secondary or tertiary amino, thiol, optionally substituted phosphino, phospho, BH2 or B(OH)2, or C1-C18 alkylthio or an optionally substituted C1-C18 alkyl, aryl, heteroaryl or vinyl group; and when R1, R2, R3 or R4 is a substituted group, the group is subs
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