The present application relates to a process based on the oxidative ring opening of a compound with oxindole structure for preparation of substituted 2-(triazinylcarbonyl)sulfonanilides of the formula (1-1) proceeding from N-sulfonyl-substituted 3-triazinyloxindole of the formula (2-1), and to the 2-(triazinylcarbonyl)sulfonanilides of the formula (1-1) thus prepared, and to the use thereof as intermediates for the synthesis of fine chemicals and of active ingredients in the agricultural sector.The invention also relates to a multistage process for preparation of N-alkyl-N-[2-(1,3,5-triazin-2-ylcarbonyl)phenyl]alkanesulfonamides of the formula (4-1), proceeding from a 3-(alkylsulfanyl)-1,3-dihydro-2H-indol-2-one of the formula (7-1), wherein the multistage process also comprises, as a component step, the oxidative ring opening mentioned, and to the 2-(triazinylcarbonyl)sulfonanilides of the formula (1-1) which are obtained by the oxidative ring opening and are used as intermediates in the multistage process.