Mechanism of dehydration of 2-CH2R-and 2-CHR2-4-hydroxy-Δ2-thiazolines as intermediates in the Hantzsch thiazole synthesis and factors impeding the synthesis of 2-Me-, 2-Ar-, and 2-Het-substituted thiazoles and thiazolo[5,4-b]indoles

Abstract

Based on the results of studies of the deuterium exchange and dehydration of 4-hydroxy-Δ2-thiazolines and 2-R-4-acetyl-8b-hydroxy-3a,8b-dihydro-4H-thiazolo[5,4-b]indoles containing the α-methylene (methine) unit at the C(2) atom, the mechanism of dehydration of these compounds generated as intermediates in the Hantzsch synthesis of thiazoles and 2-R-4-acetyl-4H-thiazolo[5,4-b]indoles was proposed. This mechanism includes an additional step of the formation of the corresponding Δ3-thiazolines. According to the results of quantum chemical calculations, this is energetically more favorable than the dehydration in terms of the commonly accepted mechanism. In some cases, an acidic medium impedes the dehydration of 4-hydroxy-Δ2-thiazolines or their cyclic analogs. The proposed mechanism provides an explanation for the empirical data on the differences in the reactivities of both thioamides and α-haloketones, which have remained unexplained in terms of the commonly accepted mechanism. The spontaneous thiazole synthesis is virtually impossible starting from thioamides of aromatic or heteroaromatic acids and α-haloketones bearing electron-withdrawing α′ substituents or cyclic bromoindoxyl-type haloketones. In the thiazole synthesis from these starting components, it is expedient to perform dehydration under basic catalysis.