with water and concentrating the aqueous solution by evaporation the free tetracarboxylic acid is obtained from its mono anhydride consumption 14 0 ml of n 10 sodium hydroxide solution calculated 14 1 ml 700 ml of 60 nitric acid are heated to 75 u00b0 80 u00b0 c 360 g of bicyclo 2 2 2 octene 7 2 3 4 5 tetracarboxylic acid 2 3 anhydride are added thereto within two hours while stirring vigorous evolution of nitrous gases takes place the reaction mixture is heated subsequently at 75 u00b0 80 u00b0 c for about four hours on standing in the cold a crystalline reaction product precipitates from the clear reaction solution it is filtered off by suction washed with glacial acetic acid until a colorless filtrate runs off and dried in a vacuum at 80 u00b0 c on concentrating the mother liquor by evaporation to half its volume 20 g of a crystalline compound are additionally obtained it is also possible to combine steps b and c of the above described reaction procedure in order to simplify the synthesis of the desired compound for this purpose 2 3 dihydrophthalic acid and maleic acid anhydride are reacted in a diluting solvent which is inert against.