1,3-Diaza-1,3-butadienes. Synthesis and Conversion into Pyrimidines by [4π + 2π] Cycloaddition with Electron Deficient Acetylenes. Synthetic Utility of 2-(Trichloromethyl)pyrimidines1

Methods have been devised to generate 1 H -1,3-diaza-1,3-butadienes bearing a leaving group at position-4 in latent, masked, and unprotected forms. A hallmark of these azadienes is that they undergo thermal [4π + 2π] cycloaddition reactions with electron deficient acetylenes to give adducts which are aromatized to pyrimidine derivatives under the reaction conditions. Thus, 1-(methoxycarbonyl)-3-acylamidines 17 on heating in solution are converted in situ into the 1,3-diaza-1,3-dienes 18 and/or 19 which react with dimethyl acetylenedicarboxylate (DMAD) to produce the pyrimidines 20 . The 1-Boc-1,3-diaza-1,3-dienes 24 are masked forms of the 1 H -dienes inasmuch as they react with DMAD under relatively mild conditions to give the dihydropyrimidine adducts 25, which are easily detectable by 1 H NMR spectroscopy, and which aromatize to pyrimidines 26 at higher temperatures. The 4-methylthio compounds 31 and 33, and the 2-(trichloromethyl) compounds 37, are isolable, relatively stable, 1 H -1,3-diaza-1,3-butadienes. These easily prepared compounds react with electron deficient acetylenes under mild conditions to provide the pyrimidines 20, 34, and 38, respectively, in fair to excellent yields. The 2-(trichloromethyl)pyrimidines 38 are very useful precursors of a wide variety of other 2-substituted pyrimidines 46 - 52 .