| http://www.w3.org/ns/prov#value | - Symbol A is desirably an electron-withdrawing group, preferably a halogen atom, an aliphatic, aryl, or heterocyclic sulfonyl group, an aliphatic, aryl, or heterocyclic acyl group, an aliphatic, aryl, or heterocyclic oxycarbonyl group, carbamoyl, or sulfamoyl, and is especially preferably a halogen atom.
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