Synthesis of substituted 6-imino-2-piperidinones by Rh-catalyzed [4+2] annulation of 4-alkynals with carbodiimides

Synthesis of substituted 6-imino-2-piperidinones has been achieved by cationic rhodium(I)/dppp complex-catalyzed [4+2] annulations of various 4-alkynals with carbodiimides at room temperature. The reactions of benzene-linked 4-alkynals (2-alkynylbenzaldehydes) and carbodiimides also proceeded at room temperature, affording the corresponding 6-imino-2-piperidinones.