prov:value
| - when one of R1 and R2 is a hydrogen atom or a C1 -C3 alkyl and the other is a group ##STR3## wherein R8 is a hydrogen atom, alkoxy, aminocarbonyl, carboxy, alkoxycarbonyl, alkylcarbonyl, alkyl optionally substituted by from 1 to 3 substituents selected among fluorine, chlorine or bromine atoms, hydroxy, cyano, alkoxy, mercapto, C3 -C6 cycloalkyl optionally substituted by 1 or 2 chlorine atoms, ami
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