Chemo- and Regioselective Synthesis of 3,4-Dihydropyrimidin-4-ones from 4,5-Dihydro-1,2,4-oxadiazoles and Chromium Alkoxy Alkynyl Fischer Carbene Complexes

A completely chemo- and regioselective synthesis of pyrimidin-4(3<i>H</i>)-ones by the reaction between 4,5-dihydro-1,2,4-oxadiazoles and chromium alkoxy alkynyl Fischer carbene complexes is reported. Overall, it is a formal (3+3) cycloaddition in which 4,5-dihydro-1,2,4-oxadiazoles participate for the first time in the intermolecular formation of six-membered heterocycles. Three points of diversity have been explored, and usually, synthetically useful isolated yields are reached, including one example at the gram scale. A reasonable mechanism, supported by DFT calculations, is proposed.