The reaction of 6-methoxy-5-(trifluoromethyl)naphthalene-1-carboxylic acid (I) with SOCl2 in refluxing DMF gives the corresponding acyl chloride (II), which is condensed with methyl N-methylglycinate (III) by means of hydroxybenzotriazole and dicyclohexylcarbodiimide in DMF yielding methyl N-[[5-(trifluoromethyl)-6-methoxy-1-naphthyl]carbonyl]-N-methylglycinate (IV). The reaction of (IV) with P2S5 refluxing pyridine affords methyl N-[[5-(trifluoromethyl)-6-methoxy-1-naphthyl]thioxomethyl]-N-methylglycinate (V), which is finally hydrolyzed with NaOH in methanol water. The synthesis of [14C]-labeled tolrestat has also been described.