The invention relates to a process for the preparation of a compound of formula ##STR1## useful as intermediate for the preparation of pesticides, wherein R is hydrogen, C.sub.1 -C.sub.6 alkyl, C.sub.3 -C.sub.6 cycloalkyl, C.sub.1 -C.sub.4 alkyl substituted by from 1 to 10 halogen atoms or by from 1 to 3 radicals selected from the group consisting of C.sub.1 -C.sub.3 alkoxy, C.sub.1 -C.sub.3 alkylthio and phenyl, phenyl or phenyl substituted by from 1 to 3 radicals selected from the group consisting of halogen, methyl, ethyl, methoxy, methylthio and nitro, which comprises reacting with hydrazine hydrate a compound of formula ##STR2## wherein R.sub.1 is hydrogen, C.sub.1 -C.sub.4 alkyl, C.sub.3 -C.sub.6 cycloalkyl, C.sub.1 -C.sub.4 alkyl substituted by from 1 to 9 chlorine atoms, C.sub.1 -C.sub.3 alkoxy, C.sub.1 -C.sub.3 alkylthio, C.sub.1 -C.sub.3 alkylsulfinyl, C.sub.1 -C.sub.3 alkylsulfonyl, phenyl, phenyl substituted by from 1 to 3 radicals selected from the group consisting of halogen, methyl, ethyl, methoxy, methylthio and nitro, or pyridyl, and subjecting to hydrolysis the resulting compound of formula ##STR3##