A simple method for the synthesis of new derivatives of isoindoline-1,3-diones has been developed, which consists in the interaction of N-arylbenzenecarboximidamides with phthalic anhydride in benzene at reflux. It was found that carrying out the reaction without heating leads to the formation of monoacylation products - phthalic acid amides. The structure of the isolated substances was proved using ¹H and ¹³C NMR spectroscopy. Acute toxicity and biological activity in silico was studied for all the obtained compounds using the online programs GUSAR and PASS. For 2-(phenyl(phenylimino)methyl)isoindoline-1,3-dione and 2-((phenyl(phenylimino)methyl)carbamoyl)benzoic acid, an acute toxicity and biological activity in vivo was studied in laboratory mice. It was shown that the results of in silico and in vivo methods are comparable and indicate the low toxicity of the obtained compounds. It was revealed that 2-(phenyl(phenylimino)methyl)isoindoline-1,3-dione has a high analgesic activity, 1.6 times higher than the activity of the reference drug metamizole sodium.