Action of Acetylenic Reagents

Action of Acetylenic Reagents. Isothiazoline-5-thiones (39) react rapidly with acetylenic reagents e.g. dibenzoylacetylene) to form mono-adducts of type (40). A-Phenylmaleimide gives an adduct, but diethyl azodicarboxylate gave only 

The electron-deficient heterocycle 4-nitroisothiazole (42) reacts with the electron-rich compound l-dimethylamino-2-phenylacetylene (43 R = R = Me) to give a mixture of the [2 + 2]-cycloadduct (44) and the nitrone (45), their ratio depending on the polarity of the solvent. The regio-selective character of the cycloaddition, as shown by the formation of only one of the two possible isomers of (44), is consistent with the polarization of the two 7r-systems.  

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