Method for preparing trelagliptin

The invention relates to a method for preparing trelagliptin. The method comprises the following steps: 1) in a reaction solvent, adding 2-(6-chlorine-3-methyl-2,4-dione-3,4-dihydropyrimidine-dihydro-1-methyl)-4-F-benzonitrile into R-3-piperidinamine dihydrochloride, dropwise adding 1,8-diazabicyclo[5,4,0]-undec-7-ene in ice bath, at temperature of 15-25 DEG C, insulating the materials for reacting for 2 hours, 2) using hydrochloric acid to adjust a reaction solution to acidity, 3) adding an organic solvent and adjusting a pH value of a water phase to alkalescence, extracting the material to obtain a trelagliptin crude product, and 4) using a solvent for recrystallization to obtain trelagliptin. The obtained trelagliptin can effectively control chromatographic purity being no less than 99.0% and yield being no less than 75%, process is simple, and cost is low.