[OH:1][C:2]1[CH2:3][CH:4]([C:17]([OH:19])=[O:18])[CH2:5][C:6](=[O:16])[C:7]=1[N:8]=[N:9][C:10]1[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=1.CO.[CH:22]1C=CC=CC=1.C1(C)C=CC(S(O)(=O)=O)=CC=1>O>[CH3:22][O:18][C:17]([CH:4]1[CH2:3][C:2](=[O:1])[C:7]([N:8]=[N:9][C:10]2[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=2)=[C:6]([OH:16])[CH2:5]1)=[O:19]60 G. of 3-hydroxy-5-keto-4-phenylazo-3-cyclohexene-carboxylic acid, 200 ml. of methanol, 1200 ml. of benzene and 5 g. of p-toluenesulfonic acid were boiled together under reflux on a water separator for 18 hours. After cooling, the solution was washed with 500 ml. of a 5% sodium bicarbonate solution, then washed with water, dried and evaporated. The residue was dissolved in ethyl acetate and purified on an aluminum oxide column (500 g.; activity stage I). After evaporation of the ethylacetate, there was obtained 3-hydroxy-5-keto-4-phenylazo-3-cyclohexene-carboxylic acid methyl ester as a solid having a melting point of 144° C.
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