Sitagliptin Impurity: Acetamide, N-[(1R)-3-[5,6-dihydro-3-(trifluoromethyl)-1,2,4-triazolo[4,3-a]pyrazin-7(8H)-yl]-3-oxo-1-[(2,4,5-trifluorophenyl)methyl]propyl]- (Sitagliptin ketoamide impurity ).