Mechanism of the inverse-electron demand Diels–Alder reaction of 2-aminopyrroles with 1,3,5-triazines: detection of an intermediate and effect of added base and acid

In base, a 2-aminopyrrole reacted with a 1,3,5-triazine to give a zwitterion (Meisenheimer complex). Acid promoted its conversion to the pyrrolo[2,3-d]pyrimidine. A cascade mechanism with reversible steps is proposed to explain why both a base and an acid are needed for the cycloaddition to occur.