The foregoing groups embraced by R2, R3, R4, R5 and R6, other than hydrogen and halogeno, can in turn be optionally substituted as for example by acyl with 2 to 4 carbon atoms, carbalkoxy groups with 2 to 4 carbon atoms, alkoxyalkyl groups with 1 to 4 carbon atoms in the alkoxy part and 1 to 4 carbon atoms in the alkyl part, and aralkyl groups with 1 to 4 carbon atoms in the alkyl part and 6 carbo