A Regioselective Synthesis of 6-Alkyl- and 6-Aryluracils by Cs2CO3- or K3PO4-Promoted Dimerization of 3-Alkyl- and 3-Aryl-2-Propynamides

A regioselective synthesis of 6-alkyl- and 6-aryluracils was developed by the dimerization of 3-alkyl- and 3-aryl-2-propynamides promoted by either Cs2CO3 or K3PO4. A range of 3-aryl-2-propynamides, with both electron-deficient and electron-rich 3-aryl substituents, were successfully reacted in high yields. Cs+ acts as a soft Lewis acid to polarize the carbon–carbon triple bond, and solid K3PO4 interacts with carbonyl oxygen, promoting intermolecular nucleophilic attack by the only weakly nucleophilic amide nitrogen. Experiments were conducted to support the proposed mechanism.