Sitagliptin Impurity: 1,2,4-Triazolo[4,3-a]pyrazine, 7-[(2Z)-3-amino-1-oxo-4-(2,4,5-trifluorophenyl)-2-butenyl]-5,6,7,8-tetrahydro-3-(trifluoromethyl) (Sitagliptin enamine impurity).