S(=O)(=O)(O)O.[C:6]([OH:17])(=[O:16])[C:7]1[CH:15]=[CH:14][C:10]([C:11]([OH:13])=[O:12])=[CH:9][CH:8]=1.O1COCO[CH2:19]1>O>[C:11]([C:10]1[CH:14]=[C:15]2[C:7](=[CH:8][CH:9]=1)[C:6](=[O:17])[O:16][CH2:19]2)([OH:13])=[O:12]To 153 ml of fuming sulfuric acid, containing 25% of SO3, 50 g (0.3 m) of terephthalic acid are added, in small portions, at 20-22° C., then to the mixture thus obtained 23 g (0.25 m) of 1,3,5-trioxane are added portionwise thereinto. The mixture is kept about 30 minutes under stirring, without cooling, then the temperature is brought to 135-140° C. and heating is continued for 2-2.5 hours, whereby the reaction is complete, as shown by a HPLC control. The reaction mixture is cooled to 0-2° C. and 1000 ml of cold water are added thereto without exceeding 20-23° C., by keeping in mind that, during the addition of the first 15-20 ml of water, exothermia occurs, whilst afterwards the temperature may be easily controlled. The mixture is kept 1 hour under stirring at 20-25° C., the product thus obtained is filtered, still moist triturated in 300 ml of water repeatedly until the reddish colour of the mother liquors disappears. After the third trituration, the pH of the mother liquors stabilizes at values ranging from 5 to 6. The product is dried in vacuo at 45-50° C. until constant weight to give 47.5 g of 5-carboxyphthalide with a titer and a purity (HPLC)>95%.
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