How is Alogliptin Impurity 21 (CAS: 147754-12-9) typically synthesized?

Alogliptin Impurity 21 is synthesized through a multi-step process involving the use of various solvents and reagents. Commonly, it involves the condensation reaction of amino acids and aldehydes, followed by purification steps such as recrystallization. The synthesis typically requires the use of catalysts like palladium or rhodium to enhance the reaction selectivity. The overall yield can range from 80-90%, with high purity achieved through chromatographic methods.