NOVELTY - A (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile is claimed.    USE - (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile is used for preparation of (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyric acid for preparation of sitagliptin (all claimed).    ADVANTAGE - The (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile is economically synthesized by simple environmentally-friendly method, without using complex chiral reagents and performing chiral asymmetric catalytic hydrogenation reaction.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) synthesis of (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile, which involves reacting (S)-(2,4,5-trifluorophenyl)oxirane and cyaniding agent, to obtain (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile;    (2) preparation of (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyric acid, which involves hydrolyzing (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyronitrile in presence of base, or base and peroxide, or acid, to obtain (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyric acid;    (3) preparation of sitagliptin, which involves subjecting (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyric acid and benzyloxy hydroxylamine hydrochloride to condensation reaction under action of condensing agent (A1) by addition of organic alkali (OA1) to obtain (S)-N-(benzyloxy)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyramide, subjecting (S)-N-(benzyloxy)-3-hydroxy-4-(2,4,5-trifluorophenyl)butyramide to intramolecular cyclization to obtain (R)-1-(benzyloxy)-4-(2,4,5-trifluorobenzyl)-2-butyrolactam (VIII), carrying out hydrolysis reaction of (R)-1-(benzyloxy)-4-(2,4,5-trifluorobenzyl)-2-butyrolactam under action of alkali to obtain (R)-3-(benzyloxy)amino-4-(2,4,5-trifluorobenzyl)butyric acid, subjecting (R)-3-(benzyloxy)amino-4-(2,4,5-trifluorobenzyl)butyric acid and benzotriazole triazine compound (X) to condensation reaction in condensing agent (A2) and organic alkali (OA2) to obtain amino-protected sitagliptin derivative, and removing protecting group of amino-protected sitagliptin derivative by catalytic hydrogenation under action of catalyst to obtain sitagliptin; and    (4) synthesis of (S)-(2,4,5-trifluorophenyl)oxirane, which involves reacting 2,4,5-trifluorophenyl magnesium halide of formula (II) as Grignard reagent and (R)-epoxy chloropropane to obtain (S)-1-chloro-3-(2,4,5-trifluorophenyl)-2-propanol, and subjecting (S)-1-chloro-3-(2,4,5-trifluorophenyl)-2-propanol to cyclization process under alkaline condition, to obtain (S)-(2,4,5-trifluorophenyl)oxirane.    X=halo.