Addition reactions of thiazol-5(4H)-ones—II : Cycloaddition and Michael addition reactions of 4-substituted 2-phenylthiazol-5(4H)-ones

The mixture of adducts formed under mild conditions between a 4-substituted 2-phenylthiazol-5(4H)-one and an electron-deficient alkene is shown to include a stable cycloadduct and a Michael adduct formed through the 2- or the 4-position of the thiazolone. The reaction can be diverted towards the Michael adduct entirely, by adding traces of aqueous alkali to the reactants in acetone solution. A novel type of 1:2-adduct is present in the reaction mixture, and is shown to be formed through reaction of the cycloadduct with the alkene. A product formed by extrusion of carbonyl sulphide from the cycloadduct is the same as that obtained from the analogous oxazolone and the alkene, but generally the differences between the propensity of oxazolones and thiazolones to undergo various types of addition reaction with representative dipolarophiles are shown to be substantial.