The present invention relates to a procedure for the oxidation of a thioether group to a sulfoxide group, with aqueous sodium percarbonate in the presence of a molybdenum salt as a catalyst. The procedure is of application to oxidize the thioether group of a compound (I), where R1 is a C1-C6 alkyl, a halogenated C1-C6 alkyl, or -(CH2)n-OR9, where n is an integer between 1 and 6 and R9 is H or a C1-C6 alkyl R2, R3, R4, R5, R6 and R8, independently form each other represent H, a C1-C6 alkyl, or C1-C6 alkoxy and R7 is H, a C1-C6 alkyl, a C1-C6 alkoxy or a C1-C6 fluorinated alkoxy, in order to obtain the sulfinyl derivative (II).
