The title compounds were obtained from reaction of variously substituted 2(1H)pyrazinones with acetylenic derivatives. Experimental evidence points out to a two step mechanism : a DielsAlder cycloaddition followed by immediate decomposition of the adducts into the title products via two competitive retro DielsAlderreactions. The product distribution, which is shown to be highly dependent on the
acetylenic derivatives to give specifically substituted pyridones or pyridines. The observed selectivity can be explained in terms of HO-LU interactions for the reagents and two competitive retro Diels-Alderreactions of the primary bicycloadducts. With cyanotosylate as dienophile no pyrimidone derivative but a new 2(1H)-pyrazinone is obtained.