Short and efficient synthesis of 3-substituted 4-oxazolin-2-thiones and their reactivity

A new synthesis of the substituted 4-oxazolin-2-thiones (1) and (14) is described by a regioselective tandem condensation reaction between α-ketols (4a-4b) and isothiocyanates (8). The use of dioxane as the solvent promotes the formation of the 4-methylene-1,3-oxazolidin-2-thiones (7), while the mixture of hemiaminals (9/10) is obtained in the presence of DMF; this mixture undergoes dehydration to give compounds (1). The latter are also prepared by an alternative solvent-free process by MW irradiation. Treatment of heterocycles (1) with alkyl iodides leads to the generation of the oxazolium iodides (21). The reactivity and regiochemistry of this process is explained in terms of FMO calculations.