Novel one-pot procedure for the preparation of 3-substituted 2-thioxo-4(3H)-quinazolinones

3-Substituted 2-thioxo-4(3H)-quinazolinones 3 are obtained in reasonable yields by a 'one-pot' procedure involving the reaction of a mixture of an anthranilic acid and a primary amine with carbon disulfide in the presence of potassium hydroxide. This provides a much more convenient approach than methods involving prior formation and isolation of isothiocyanates and is reasonably general. It fails, however, with the weakly nucleophilic 4-nitroaniline which produces benzo-3,l-thiazine-2,4(lH)-dithione 5a instead, via direct reaction between anthranilic acid and carbon disulfide. Other benzo-3,l-thiazine-2,4(lH)-dithiones 5 are obtained in excellent yields by a 'one-pot' procedure involving the reaction of the appropriate anthanilic acid and carbon disulfide in the presence of base. This method is much more convenient than the tedious reaction of phosphorus pentasulfide with isatoic anhydride. 3-Phenyl-2,4-quinazolinedithione 6 could be obtained in a reasonable yield from the reaction of the corresponding thioxoquinazolinone 3a with Lawesson's reagent or from the reaction of benzo-3,l-thiazine-2,4(lH)-dithione 5a with aniline in the presence of base