The reaction for preparing the esters is shown schematically by the condensation of 1-methyl-5-(4-methylbenzoyl)pyrrol-2-acetic acid and 7-(2-oxyethyl)theophylline as a general example, giving rise to the formation of the 1-methyl-5-(4-methylbenzoyl)pyrrol-2-acetate of 7-(2-oxyethyl)theophylline.