prov:value
| - nted by the formula ##STR44## wherein at least one of R1 and R2 is an aryl group; when R1 is an aryl group, R2 is a hydrogen, alkyl, aryl, heterocyclic or phenyl group, the phenyl group optionally being substituted with an alkyl, halo, amino or thiol group; when R2 is an aryl group, R1 is a hydrogen, alkyl, aryl, heterocyclic or phenyl group, the phenyl group optionally being substituted with an a
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