J. E. Kowalchick, B. Leiting, K. D. Pryor, et al. Design, synthe- sis, and biological evalua-tion of triazolopiperazine-based β- amino amides as potent, orally active dipeptidyl peptidase Ⅳ (DPP-4) inhibitors. Bioorganic and Medicinal Chemistry Letters, 2007, 17(21): 5934-5939.