10 g of 5,7-di-tert-butyl-3-hydroxy-3H-benzofuran-2-one (prepared as described in Example 1 of U.S. Pat. No. 5,614,572) are introduced in 25 ml of 1,2-dichlorobenzene as initial charge and admixed with 0.5 g of 4-dimethylaminopyridine and 3 ml of thionyl chloride. The solution is then gradually heated to 100° C. so that the evolution of HCl and SO2 remains lively, but still controllable. Thereafter the reaction mixture is stirred at 100° C. for a further ½ h. The temperature is subsequently raised to the reflux point. After 75 min, the 1,2-dichlorobenzene is distilled off, at the end with reduced pressure. The isoxindigo is crystallized out by the addition of 30 ml of acetonitrile to the residue, filtered off, washed with acetonitrile and dried to leave 6.8 g (73% of theory) of 5,7,5′,7′-tetra-tert-butyl[3,3′]bibenzofuranylidene-2,2′-dione of the formula (XV).
