Facile and selective synthesis of 4-methyl- and 4-phenylthiosemicarbazide (= N-methyl- and N-phenylhydrazinecarbothioamide) derivatives of benzil (=1,2-diphenylethane-1,2-dione)

A selective synthesis of 4-methylthiosemicarbazide (=N-methylhydrazinecarbothioamide; 4a) derivatives by reaction with benzil (=1,2-diphenylethane-1,2-dione; 3) is described. The reaction conditions determined the condensation product formed. The most important factor was the acid used: in the presence of conc. HCl solution, the open-chain 2:1 compound la was exclusively obtained, whereas in the presence of 2m HCl, the cyclic 1 : 1 condensation product 2a was formed. The alcohol used, the presence of H2O, and the time of heating were additional crucial factors. The new cyclic compound 2a with a MeO group was exclusively formed when working under high-dilution conditions. The reaction with the 4-phenyl derivative 4b gave new cyclic compounds as the major products under all conditions used (Scheme).