Al(H₂PO₄)₃ as an efficient and reusable catalyst for the multi-component synthesis of highly functionalized piperidines and dihydro-2-oxypyrroles

Extremely facile and efficient procedures have been developed for the synthesis of highly functionalized piperidines and dihydro-2-oxypyrroles via one-pot multi-component reactions in the presence of Al(H₂PO ₄)₃ as a heterogeneous and eco-friendly catalyst under mild conditions. The multi-component reaction of aromatic aldehydes, aromatic amines, and β-keto esters catalyzed by Al(H₂PO₄) ₃ in EtOH at room temperature provides highly functionalized piperidines in good to excellent yields. The structure as well as the relative stereochemistry of these functionalized piperidines was confirmed by single X-ray crystallographic analysis. The same catalyst was found useful for the synthesis of polyfuntionalized dihydro-2-oxypyrroles using a four-component reaction of amines, dialkyl acetylenedicarboxylates and formaldehyde in MeOH at ambient temperature. It is found that the catalyst is recyclable and can be used up to five times without significant loss of its activity.