[CH3:1][O:2][C:3](=[O:16])[CH:4]1[C:13](C)=[C:12]([CH3:15])[CH2:11][CH:10]=[C:5]1[C:6]([O:8][CH3:9])=[O:7].[Se](=O)=O.[C:20](OC(=O)C)(=O)C>C(Cl)Cl>[CH3:9][O:8][C:6](=[O:7])[C:5]1[C:4](=[CH:13][C:12]([CH3:15])=[C:11]([CH3:20])[CH:10]=1)[C:3]([O:2][CH3:1])=[O:16]232 parts of the 5,6-dimethyl-1,4-dihydro-phthalic acid dimethyl ester (I) obtained in A) are first freed from readily volatile constituents in vacuo and then diluted with 400 parts by volume of acetic anhydride. Then 133 parts of selenium dioxide are slowly added by small amounts to the reaction mixture, which is heated to 100° C, and the temperature rises to 130° C without external application of heat. The reaction mixture is subsequently refluxed for 3 hours and freed from acetic anhydride in vacuo. The residue is taken up in methylene chloride, the solution is freed from precipitated selenium by filtration, then freed from acid constituents with a saturated aqueous sodium bicarbonate solution, washed neutral with water, dried over sodium sulphate and concentrated to dryness in vacuo. Yield: 230 parts of a yellowish brown oil which a gas chromatogram shows to have a 94% content of 4,5-dimethyl-phthalic acid dimethyl ester (II). The product is distilled under a high vacuum. The fraction with a boiling range of 116°-120° C (0.04 Torr) contains 174 parts (78% of theory) of 4,5-dimethyl-phthalic acid dimethyl ester (II) with a melting point of 54°-55° C.
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