Synthesis of bridged bicyclic thiazine-2-thione and thiazole-2-thiones through DBU-promoted regioselective annulation of quinone monoacetals under mild conditions

The first regioselective [4 ​+ ​2] annulation reactions of quinone monoacetals with isothiocyanates and Na₂S have been accomplished. This convenient and novel protocol involves the synthesis of valuable bridged thiazine-2-thione and thiazole-2-thiones through DBU-promoted at room temperature. Mechanism study reveals the dearomative transformation possibly undergoes a radical cascade reaction on the structure of quinone monoacetals to an efficient synthesis of a broad range of thione-containing heterocyclic compounds with broad tolerance in moderate to excellent yields. Moreover, the reported procedure can be easily applied to a 1g scale.