Novel compounds of formula I, ##STR1## in which R.sub.1 is hydrogen, alkyl, phenylalkyl, carbalkoxyalkyl, acyl, carboxyalkyl, alkoxyalkyl, hydroxyalkyl, cyanoalkyl, or carbamoylalkyl,R.sub.2 is hydrogen, pivaloyloxymethyl or the residue of an easily splittable ester grouping,R.sub.3 is a pyrazolyl radical, unsubstituted or mono- or di-substituted by alkyl, phenyl, alkoxy, alkylthio, carboxy, carboxyalkyl, carbamoyl, carbamoylalkyl, alkylsulphonyl, azido, acylamino, hydrazino, acylhydrazino alkylidenehydrazino, phenylidenehydrazino, in which the phenyl nucleus is unsubstituted or substituted by NH.sub.2, lower alkoxy or lower alkyl, furylidenehydrazino, carbalkoxy or a group R.sub.5 R.sub.6 N--, in which R.sub.5 and R.sub.6 are the same or different and are hydrogen or alkyl, provided that the nitrogen atoms and the 4-position of the pyrazole nucleus are either unsubstituted or substituted by alkyl, phenyl, or carbalkoxy, andR.sub.4 is hydrogen, acetoxy, carbamoyloxy or --S--Rh, in which Rh is a heterocyclic radical.