NOVELTY - Preparing pregabalin (I) comprises (a) condensing 2-methyl-propionaldehyde (IV) with cyano acetic acid alkyl ester compound (A); (b) reacting pentanoic acid alkyl ester compound (B) with halo acetic acid ethyl ester compound (VI); (c) reacting 4-ethyl-1-alkyl-2-cyano-2-isobutylsuccinate compound (C) with alkalimetal chloride; (d) hydrolyzing (RS)-3-cyano-5-methyl-hexanoic acid ethyl ester (XI); (e) treating (RS)-3-cyano-5-methyl-hexanoic acid (XII) with cinchonidine (XIII) and (f) treating (S) 3-cyano-5-methylhexanoic acid salt of cinchonidine (XIV) with ethyl acetate and hydrochloric acid.    USE - (I) is useful for treating pain, fibromyalgia, physiological conditions associated with psychomotor stimulants, inflammation, gastrointestinal damage, insomnia, alcoholism and various psychiatric disorders, including mania and bipolar disorder.    ADVANTAGE - The process is an improved and economical process and provides (I) in high yield and optical purity (enantiomeric excess of greater than 99%). (I) does not use harmful, hazardous and poisonous chemicals such as potassium cyanide or its equivalent, hence ecofriendly.    DETAILED DESCRIPTION - Synthesizing pregabalin (I) from cyano acetic acid alkyl ester compound of formula (R-O-C(=O)-CN) (A) comprises either (a) condensation of 2-methyl-propionaldehyde (IV) with (A) in the presence of organic or inorganic base such as piperidinium acetate or cesium acetate, and further hydrogenation using noble metal catalyst such as platinum oxide, palladium on carbon, palladium hydroxide on carbon and also with Raney nickel in polar solvent such as methanol, ethanol, water, 1,4-dioxane, tetrahydrofuran, dimethoxy ethane or diglyme under hydrogen pressure of 1-5 kg/cm2 with subsequent isolation of 2-cyano-4-methyl-pentanoic acid alkyl ester compound of formula (CH3-CH(CH3)-CH2-CH(CN)-C(=O)-O-R) (B) in solution form from the catalyst by filtration, or (a1); (b) reaction of (B) with halo acetic acid ethyl ester compound of formula (X-CH2-COOC2H5) (VI) in presence of base such as sodium carbonate (preferred), potassium carbonate or cesium carbonate without solvent or in an organic solvent comprising N,N-dimethyl formamide, dimethyl sulfoxide (both preferred), tetrahydrofuran, 1,4-dioxane or dimethoxy ethane at 10-90 degrees C to give 4-ethyl-1-alkyl-2-cyano-2-isobutylsuccinate compound of formula (CH3-CH(CH3)-CH2-C((CN)(CH2-C(=O)-O-CH2-CH3))-C(=O)-O-R) (C); (c) either reaction of (C) with alkalimetal chloride such as cesium chloride, potassium chloride or sodium chloride in an organic solvent such as dimethyl sulfoxide at 130-180 degrees C or reaction of (C) with cesium carbonate along with thiol at 130-150 degrees C to get (RS)-3-cyano-5-methyl-hexanoic acid ethyl ester (XI) or (c1) hydrolysis of (C) in the presence of base such as lithium hydroxide (preferred), potassium hydroxide or sodium hydroxide at 20-80 degrees C, preferably 65-70 degrees C to get 2-cyano-2-isobutylsuccinic acid (XVI); either (d) hydrolysis of (XI) in the presence of base such as lithium hydroxide (preferred), potassium hydroxide or sodium hydroxide at 20-80 degrees C to get (RS)-3-cyano-5-methyl-hexanoic acid (XII), or decarboxylating (XVI) in presence of mineral acid such as sulfuric acid in organic solvent such as ethyl acetate to obtain (XII) at 70-80 degrees C; (e) treatment of (XII) with cinchonidine (XIII) in presence of organic solvent such as methanol, ethanol, 1,4-dioxane, ethyl acetate, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethoxy ethane or diglyme at 20-80 degrees C to precipitate out (S) 3-cyano-5-methylhexanoic acid salt of cinchonidine (XIV), followed by separation of (XIV) through known separation techniques such as filtration, centrifugation, sedimentation, followed by optional purification of (S)-3-cyano-5-methylhexanoic acid salt of (XIV) in ethyl acetate or through re-salt formation; (f) treatment of (XIV) with biphasic mixture of ethyl acetate and dilute hydrochloric acid (1:1) at room temperature to obtain (S)-3-cyano-5-methylhexanoic acid (II) from ethyl acetate layer optionally accompanied with recovery of (XIII) from aqueous phase through basification with sodium hydroxide or potassium hydroxide; and (g) hydrogenation of optically pure (II) in presence of Raney nickel such that at each step the intermediates were optionally isolated and purified with suitable processes, or (a3) treatment of (XII) with (XIII) in presence of organic solvent such as methanol, ethanol, 1,4-dioxane, ethyl acetate, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethoxy ethane or diglyme at 20-80 degrees C to precipitate out (XIV), followed by separation (XIV) through known separation techniques such as filtration, centrifugation or sedimentation, followed by optional purification of (XIV) in ethyl acetate or through re-salt formation.    R = CH3 or C2H5; and    X = Cl, Br or I.