The present invention relates to a novel process for the preparation of a compound of formula (I), wherein R1, R2, R3, R4 and R5 are each independently selected from hydrogen, halogen, C1-C4-alkyl, C1-C4-haloalkyl, C1-C4-alkoxy, C1-C4-haloalkoxy, nitro, cyano, and pentafluorothio; R6 is C1-C4-alkyl, or C1-C4-haloalkyl; by oxidation of a compound of formula (II) with an oxidation agent selected from trifluoroperacetic acid and trichloroperacetic acid in the presence of a catalyst selected from hydroxides, oxides, sulfates, acetates or trifluoroacetates of lithium, magnesium, calcium, strontium, barium, titanium (IV), zinc (II) and manganese (II).