Abstract: A method for producing dihydroxyindoles includes: a step 1 of obtaining an aqueous first solution including dihydroxyindoles obtained by causing at least a material selected from the group consisting of 3-(3,4-dihydroxyphenyl)alanine and a derivative of 3-(3,4-dihydroxyphenyl)alanine to react with an oxidizing agent; and a step 2 of obtaining an oleaginous second solution in which the dihydroxyindoles are extracted in an extraction solvent by mixing the first solution obtained in the step 1 with the extraction solvent. Before the first solution obtained in the step 1 is mixed with the extraction solvent in the step 2, a water-insoluble by-product is removed from the first solution.

Abstract: A method for producing dihydroxyindoles includes: a step 1 of obtaining an aqueous first solution including DHIs obtained by causing DOPAs to react with hexacyanoferrate(III); a step 2 of obtaining an oleaginous second solution in which DHIs are extracted in an extraction solvent by mixing the first solution obtained in the step 1 with the extraction solvent; and a step 3 of obtaining an aqueous third solution by evaporating the extraction solvent from a mixture of the second solution obtained in the step 2 and water. The extraction of DHIs in the step 2 is performed in a tank A. After the second solution is discharged from the tank A, the second solution is supplied to a tank B, and then the step 3 is performed in the tank B.

Abstract: A method for producing dihydroxyindoles includes: a step 1 of obtaining an aqueous first solution including DHIs obtained by causing DOPAs to react with hexacyanoferrate(III); a step 2 of obtaining an oleaginous second solution in which DHIs are extracted in an extraction solvent by mixing the first solution obtained in the step 1 with the extraction solvent; and a step 3 of obtaining an aqueous third solution by evaporating the extraction solvent from a mixture of the second solution obtained in the step 2 and water. The extraction of DHIs in the step 2 is performed in a tank A. After the second solution is discharged from the tank A, the second solution is supplied to a tank B, and then the step 3 is performed in the tank B.

Abstract: A method for producing dihydroxyindoles includes: a step 1 of obtaining an aqueous first solution including dihydroxyindoles obtained by causing at least a material selected from the group consisting of 3-(3,4-dihydroxyphenyl)alanine and a derivative of 3-(3,4-dihydroxyphenyl)alanine to react with an oxidizing agent; and a step 2 of obtaining an oleaginous second solution in which the dihydroxyindoles are extracted in an extraction solvent by mixing the first solution obtained in the step 1 with the extraction solvent. Before the first solution obtained in the step 1 is mixed with the extraction solvent in the step 2, a water-insoluble by-product is removed from the first solution.