NOVELTY - Improved process for preparing sitagliptin (I), comprises (a) reacting Meldrum's adduct (VII) with 3-trifluoromethyl-5,6,7,8-tetrahydro(1,2,4)triazolo(4,3-a)pyrazine (IIIa) to afford 1-(3-trifluoromethyl-5,6-dihydro-8H-(1,2,4)triazolo(4,3-a)pyrazin-7-yl)-4-(2,4,5-trifluoro-phenyl)-butane-1,3-dione (VIII), (b) reacting (VIII) with substituted 1-phenylmethanamine (XIII) to afford enamide (XIV), (c) reducing (XIV) to substituted 3-(benzylamino)-1-(3-(trifluoromethyl)-5,6-dihydro-(1,2,4)triazolo(4,3-a)pyrazin-7(8H)-yl)-4-(2,4,5-trifluorophenyl)butan-1-one (XV), and (d) deprotecting (XV).    USE - The process is useful for preparing sitagliptin (claimed) which is useful for treating preventing type-2 diabetes. No biological data given.    ADVANTAGE - The improved process: utilizes less expensive, easily available and environment friendly reagents; is simple, eco-friendly, inexpensive, and reproducible process; prepares sitagliptin at commercial scale; and involves simple procedures for deprotection of para-methoxy benzyl group at penultimate step.    DETAILED DESCRIPTION - Improved process for preparing sitagliptin of formula (I) or its salts, comprises either: (a) reacting Meldrum's adduct of formula (VII) with 3-trifluoromethyl-5,6,7,8-tetrahydro(1,2,4)triazolo(4,3-a)pyrazine of formula (IIIa) or its salts in a solvent in the presence of a base to afford 1-(3-trifluoromethyl-5,6-dihydro-8H-(1,2,4)triazolo(4,3-a)pyrazin-7-yl)-4-(2,4,5-trifluoro-phenyl)-butane-1,3-dione of formula (VIII), (b) reacting (VIII) with substituted 1-phenylmethanamine of formula (XIII) in a solvent in the presence of an acid to afford enamide of formula (XIV), (c) reducing (XIV) to substituted 3-(benzylamino)-1-(3-(trifluoromethyl)-5,6-dihydro-(1,2,4)triazolo(4,3-a)pyrazin-7(8H)-yl)-4-(2,4,5-trifluorophenyl)butan-1-one of formula (XV) or its salt in a solvent in the presence of a reducing agent and an acid, and (d) deprotecting (XV) or its salts to (I) or its salts; or (a1) reacting (VII) with hydrochloride salt of (IIIa) in a solvent in the presence of a base to afford (VIII), (b1) reacting (VIII) with (R)-1-(4-methoxy-phenyl)-ethylamine of formula (XIIIa) in a solvent in the presence of an acid to afford enamide of formula (XIVa), (c1) reducing (XIVa) to 3-((S)-1-(4-methoxy-phenyl)-ethylamino)-1-(3-trifluoromethyl-5,6-dihydro-8H-(1,2,4)triazolo(4,3-a)pyrazin-7-yl)-4-(2,4,5-trifluoro-phenyl)-butan-1-one of formula (XVa) in a solvent in the presence of a reducing agent and an acid, (d1) optionally purifying (XVa) by forming a salt, preferably tartrate salt of formula (XVb), (e1) converting (XVa) or (XVb) to (I) by deprotection, and (f1) optionally converting free base of (I) to phosphate of (I) by treating (I) with phosphoric acid; or (a2) reacting (VII) with (IIIa) in ethyl acetate in the presence of N-methyl morpholine to afford (VIII), (b2) reacting (VIII) with (XIIIa) in toluene in the presence of acetic acid to afford (XIVa), (c2) reducing (XIVa) to (XVa) in dimethoxyethane in the presence of a reducing agent, preferably sodium borohydride and an acid, preferably methanesulfonic acid, step (d1) as above per se, (e2) converting (XVa) or (XVb) to (I) by transfer hydrogenation in anisole in the presence of phosphoric acid, and step (f1) as above per se.    M1 = H, Na or K;    R1 = substituent on phenyl, preferably OH, halo, alkyl or alkoxy;    R1 = 1-6C alkyl; and    m = 1-3.    INDEPENDENT CLAIMS are also included for:    (1) enamide (XIV); and    (2) substituted 3-(benzylamino)-1-(3-(trifluoromethyl)-5,6-dihydro-(1,2,4)triazolo(4,3-a)pyrazin-7(8H)-yl)-4-(2,4,5-trifluorophenyl)butan-1-one compound (XV).