[CH3:1][O:2][C:3](=[O:17])[CH2:4][CH2:5][C:6]1[CH:11]=[CH:10][C:9]([OH:12])=[CH:8][C:7]=1C(C)(C)C.O.[C:19]([OH:23])(=O)[CH:20]=[O:21].O>ClCCCl.C1(C)C=CC(S(O)(=O)=O)=CC=1>[CH3:1][O:2][C:3](=[O:17])[CH2:4][CH2:5][C:6]1[CH:7]=[C:8]([C:6]([CH3:11])([CH3:7])[CH3:5])[C:9]2[O:12][C:20](=[O:21])[CH:19]([OH:23])[C:10]=2[CH:11]=123.6 g of 3-(2-tert-butyl-4-hydroxy-phenyl)-propionic acid methyl ester (U.S. Pat. No. 43070-A2), 10.1 g of glyoxylic acid monohydrate and 0.08 g of p-toluenesulfonic acid are boiled in 80 ml of 1,2-dichloroethane for 7 hours using a water separator. The reaction solution is then cooled, washed three times with 50 ml of water each time and freed of solvent using a rotary evaporator. 29.2 g of 3-(7-tert-butyl-3-hydroxy-2-oxo-2,3-dihydro-benzofuran-5-yl)-propionic acid methyl ester are obtained in the form of a yellowish oil. That yellowish oil is placed in 50 ml of toluene with 15 ml of thionyl chloride and 3 drops of DMF and the reaction mixture is then slowly heated to 100° C. at such a rate that the evolution of HCl and SO2 remains vigorous yet controllable. The reaction mixture is then stirred at that temperature for a further hour. Approximately 50 ml of liquid are then distilled off in order to remove the excess thionyl chloride. The residue is diluted with 160 ml of toluene and, at room temperature, 14 ml of triethylamine are added dropwise thereto. The thick, red reaction mixture is then heated under reflux for 30 min. After cooling to room temperature, the triethylamine hydrochloride which has precipitated is filtered off and the filtrate is concentrated to an oily consistency using a rotary evaporator. The isoxindigo is crystallised out by adding 100 ml of acetonitrile, is filtered off, washed with acetonitrile and dried. 7.4 g (27% of the theoretical yield) of 3-[7,7′-di-tert-butyl-2,2′-dioxo-[3,3′]bibenzofuranyliden-5-yl]-5,5′-propionic acid methyl ester (LVIa) are obtained.
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