l-Proline catalyzed synthesis of densely functionalized pyrido[2,3-d]pyrimidines via three-component one-pot domino Knoevenagel aza-Diels–Alder reaction

Highly functionalized hitherto unreported pyrido[2,3-d]pyrimidines have been concisely synthesized in good yields via l-proline catalyzed one-pot three-component domino coupling of 6-amino-1,3-dimethyluracil, aldehydes, and dialkyl acetylenedicarboxylates under mild conditions for the first time. The MCR process involves Knoevenagel condensation followed by [4+2] cycloaddition reaction. No co-catalyst or activator is required for this MCR. The molecular structures of two representative pyrido[2,3-d]pyrimidines 4a and 4h were confirmed by single crystal X-ray diffraction.