The cyclization of methyl-5-(methylthio)anthranilate (I) with formamide (II) by heating at 190 C gives 6-(methylthio)-4(3H)-quinazolinone (III), which is condensed with methyl 3-chloroacrylate (IV) by means of NaH yielding methyl 3-[6-(methylthio)-4-oxo-4H-quinazolin-3-yl]-2-propenoate (V). Finally, this compound is hydrolyzed with refluxing 6N HCl. The same procedure of synthesis can also be performed from 5-(methylthio)anthranilic acid instead of methyl 5-(methylthio)anthranilate (I).