A compound of formula I ##STR1## wherein R.sub.1 is nitro, halogeno or cyano group and R.sub.2 is a group selected from ##STR2## wherein Y and Z are independently oxygen, sulfur or NR, wherein R is hydrogen, straight or branched C.sub.1-8 alkyl, C.sub.5-7 cycloalkyl, phenylC.sub.1-8 alkyl or phenyl group and X.sub.1 and X.sub.2 are NR, where R is hydrogen, straight or branched C.sub.1-8 alkyl, C.sub.5-7 cycloalkyl, phenylC.sub.1-8 alkyl or phenyl group, or a pharmaceutically acceptable salt or ester thereof. Pharmaceutical compositions are also disclosed.