Isothiazoles. Part 11: 3-Azahexatrienes from 2-Arylpropenamidines: Electrocyclization to 6-Aminonicotinic Acid Derivatives

A new synthesis of 6-aminonicotinic acid derivatives was developed starting from the readily available 3-substituted acrylamidines 1 and electron-poor alkynes 2. By nucleophilic addition of the basic amidinic NH to the electron-poor triple bond, the intermediate azatrienes 4 were obtained. Through thermally induced electrocyclization compounds 4 afforded the pyridines 3 by alcohol elimination or oxidation. Another approach was the direct cyclization of acrylamidines with the dienyne system as a backbone.