JPH0361672B

Riboflavin of the formula I (I) is prepared by condensing a 4,5-dimethyl-N-(D)-ribityl-2-phenylazoaniline of the formula II (II) where R is H or -Cl, -NO2 or -CH3 in the o- or p-position, with barbituric acid of the formula III (III) in the presence of an acid as the condensing agent, by an improved process in which the acidic condensing agent used is an aliphatic or cycloaliphatic/aliphatic tertiary carboxylic acid of the general formula IV (IV) where R1, R2 and R3 are each a lower alkyl group, R1, R2 and R3 together containing 3 to 20, preferably 3 to 10, carbon atoms, or R1 is a lower alkyl group, in particular methyl, and R2 and R3 together form a tetramethylene or pentamethylene group. The process can be particularly advantageously carried out using trimethylacetic acid or a commercial mixture of saturated tertiary carboxylic acids, e.g. Versatic R10-acid.