Microwave-assisted Dimroth rearrangement of thiazines to dihydropyrimidinethiones: Synthetic and mechanistic aspects

The Dimroth rearrangement of a set of substituted 2-amino-6H-1,3-thiazines into 3,4-dihydropyrimidine-2-thiones was investigated. The rearrangements were carried out under microwave irradiation conditions in batch or continuous flow format, employing either toluene or 1-methyl-2-pyrrolidone as solvent. Thiazines bearing an ester group at the C5 position rearranged at a considerably higher temperature than derivatives without substituents at this position into the corresponding pyrimidinethiones. This strictly thermal rearrangement was studied in detail using differential scanning calorimetry and density functional theory computational methods. The reaction pathway involves a zwitterionic intermediate.