Definitive structural assignment of condensation products from anthranilamide and 3‐amino‐2‐carbamoylthiophene with ketones. Formation of tetrahydroquinazolinones and their thiophene isosteres

Products formed from dehydrative cyclization of cyclopentanone, cyclohexanone, and cycloheptanone with anthranilamide ( 2 ) under neutral or acidic conditions are established as the spiro compounds 2,2‐polymethylene‐1,2,3,4‐tetrahydroquinazolin‐4‐ones 6 by means of (a) the presence of a 13 C nmr signal at 66–79 ppm for atom C‐2 and (b) X‐ray crystallography on the tetramethylene compound 6a . Analogously, products from reactions of these cycloalkanones with 3‐amino‐2‐carbamoylthiophene are now shown by 13 C nmr spectra to have structures isosteric to 6 , i.e. 5,5‐polymethylene‐4,5,6,7‐tetrahydrothieno[2,3‐ e ]pyrimidin‐7‐ones 5 , rather than the chelate ring structures previously proposed. Additionally, conflicting literature reports on product 3 from reaction of acetone with 2 are partially clarified. Correlation of 13 C chemical shifts in systems 3, 5 , and 6 is presented.