A new reaction between 2‐aminobenzophenone and thiourea in dimethyl sulfoxide (DMSO) has been developed that primarily affords 4‐phenylquinazoline as a single product. This reaction is also applicable, in general, to the reactions between thiourea and conformation‐restricted β‐amino ketones, such as 1‐aminoanthracene‐9,10‐dione and 1‐amino‐9H‐fluoren‐9‐one, to prepare perimidine derivatives. Experimental data is consistent with our computational study on the thermal decomposition of thiourea to form hydrogen sulfide and carbodiimide. This reaction involves a coupling between 2‐aminobenzophenone and carbodiimide generated in situ from thiourea to form 4‐phenylquinazolin‐2(1H)‐imine intermediate, and the generation of sulfur‐containing reducing agent from hydrogen sulfide and DMSO, which reduces 4‐phenylquinazolin‐2(1H)‐imine to 4‐phenyl‐1,2‐dihydroquinazolin‐2‐amine. Elimination of ammonia from the latter yields 4‐phenylquinazoline.
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