TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The process (B) for synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from ethyl cyano acetate comprises: condensation of 2-methyl-propionaldehyde with cyanoacetic acid ethyl ester in presence of base e.g. piperidinium acetate, and further hydrogenation using noble metal catalyst including palladium on carbon or palladium hydroxide on carbon (both preferred), platinum oxide or Raney nickel in polar solvent including dimethoxy ethane and diglyme (both preferred), 1,4-dioxane or tetrahydrofuran, under hydrogen pressure of 1-5 (preferably 2) kg/cm2, further isolation of the product in solution form from the catalyst by filtration to give 2-cyano-4-methyl-pentanoic acid ethyl ester; reaction of 2-cyano-4-methyl-pentanoic acid ethyl ester with (V) in presence of base including alkali hydroxide, sodium hydride (preferred), sodium ethoxide, sodium methoxide or potassium carbonate in a polar solvent including dimethyl sulfoxide, dimethoxyethane or N,N-dimethylformamide (all preferred), tetrahydrofuran or 1,4-dioxane at 10-80 degrees C, preferably 50-60 degrees C to give diethyl 2-cyano-2-isobutylsuccinate; and reaction of diethyl 2-cyano-2-isobutylsuccinate with potassium chloride or sodium chloride in an organic solvent including dimethylsulfoxide at 140-180 degrees C, preferably 150-160 degrees C to give 3-cyano-5-methyl-hexanoic acid ethyl ester so that at each step the intermediates were optionally isolated and purified with suitable process. The process (C) for synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from dihydrofuran-2,5-dione comprises: reaction of dihydrofuran-2,5-dione with benzylalcohol in presence of para-toluenesulfonic acid to give succinic acid dibenzyl ester; reaction of succinic acid dibenzyl ester with 2-methyl-propionaldehyde in presence of a base including potassium tert-butoxide (preferred), sodium hydride, sodium ethoxide or sodium methoxide in a polar solvent including tert-butanol (preferred), tetrahydrofuran, dimethyl sulfoxide or dimethoxy ethane at 25-80 degrees C, preferably 50-60 degrees C to give 2-isopropyliden-succinic acid 1-benzyl ester; acid catalyzed esterification of 2-isopropyliden-succinic acid 1-benzyl ester with aliphatic alcohol including 1-6C hydrocarbon (optionally branched), preferably ethanol, where acid used is aryl sulfonic acid e.g. paratoluenesulfonic acid to give 2-isopropyliden-succinic acid 1-benzyl ester 4-ethyl ester; hydrogenation and hydrogenolysis of 2-isopropyliden-succinic acid 1-benzyl ester 4-ethyl ester in presence of noble metal catalyst including palladium on carbon or palladium hydroxide on carbon (both preferred) or platinum oxide in polar solvent including methanol, ethanol (preferred) or n-butanol at hydrogen pressure maintained at 3-15 (preferably 10) kg/cm2 to give 2-isopropyl-succinic acid 4-ethyl ester; reaction of 2-isopropyl-succinic acid 4-ethyl ester with thionyl chloride to give acid chloride in situ treatment of acid chloride with ammonia to give 3-carbamoyl-5-methylhexanoic acid ethyl ester; and dehydration of 3-carbamoyl-5-methylhexanoic acid ethyl ester in presence of dehydrating agent including thionylchloride, phosphorus pentoxide or phosphorus oxychloride to give 3-cyano-5-methyl-hexanoic acid ethyl ester so that at each step the intermediates were optionally isolated and purified with suitable process. The stereoselective process (D) for synthesis of (S) 3-cyano-5-methylhexanoic acid ethyl ester from S-leucine comprises: Sandmeyer reaction of (S)-2-amino-4-methyl-pentanoic acid using sodium nitrite and sulfuric acid followed by treatment of reaction mass with potassium bromide to give (S)-2-bromo-4-methyl-pentanoic acid; reaction of (S)-2-bromo-4-methyl-pentanoic acid with thionyl chloride to give acid chloride, treatment of which ammonia in situ to give (S)-2-bromo-4-methyl-pentanoic acid amide; dehydration of (S)-2-bromo-4-methyl-pentanoic acid amide in presence of dehydrating agent including phosphorus pentoxide to give (S)-2-bromo-4-methyl-pentanenitrile; preparation of enantiomerically enriched 2-((S)-1-cyano-3-methyl-butyl)-malonic acid diethyl ester by Sn2 displacement reaction of (S)-2-bromo-4-methyl-pentanenitrile with diethyl malonate in presence of base including sodium hydride (preferred), sodium ethoxide, sodium methoxide or potassium carbonate, and polar solvent including N,N-dimethylformamide or dimethoxyethane (both preferred), tetrahydrofuran or 1,4 dioxane at 10-80 degrees C, preferably 50-60 degrees C; and decarboxylation of 2-(1-cyano-3-methyl-butyl)-malonic acid diethyl ester in presence of potassium chloride or sodium chloride in presence of solvent including dimethylsulfoxide at 140-180 degrees C, preferably 150-160 degrees C to give enantiomerically pure (S)-3-cyano-5-methyl-hexanoic acid ethyl ester so that at each step the intermediates were optionally isolated and purified with suitable process. The process (E) for enantiomeric enrichment of (S) 3-cyano-5-methylhexanoic acid ethyl ester in its racemate or partially enriched form comprises: stereoselective hydrolysis of R isomer of 3-cyano-5-methylhexanoic acid ethyl ester involving enzymatic kinetic resolution technique using lipase enzyme derived from the microorganism Candida antarctica B in presence of buffer at concentration of 10-100 (preferably 40) mM, at 10-50 degrees C, preferably 15 degrees C at pH of 6-8, preferably 7.2; and isolation of unhydrolyzed (S)-3-cyano-5-methylhexanoic acid ethyl ester. In the process (E): the pH is adjusted using the base including potassium hydroxide, sodium hydroxide or ammonium hydroxide; the hydrolyzed R isomer of 3-cyano-5- methylhexanoic acid is recycled to obtain racemic 3-cyano-5-methylhexanoic acid ethyl ester which was then further subjected to kinetic enzymatic resolution to obtain S-3-cyano-5-methylhexanoic acid; and the recycling of R isomer of 3-cyano-5-methylhexanoic acid is carried out by converting it to its ethyl ester and further subjecting the ethyl ester to racemization in presence of base in an alcoholic solvent. EXAMPLE - A 50 ml reactor was charged with diethyl 2-cyano-2-isobutylsuccinate (102 g), potassium chloride (32.5 g) and dimethyl sulfoxide (500 ml). The resulting reaction mixture was heated at 150-160 degrees C and maintained at that temperature for 4 hours. The reaction mixture was cooled to 40-50 degrees C and treated with methyl tert-butyl ether (200 ml). The mixture was further cooled to 0-5 degrees C and treated with water (1 l) in small portions to maintain the temperature below 40 degrees C. After stirring for 30 minutes, the phases were separated. The aqueous phase was extracted three times with methyl tert-butyl ether (800 ml), organic phases were combined and washed twice with water (100 ml). The organic layer was decolorized by treating with activated charcoal (7 g). The resultant mixture was filtered to remove charcoal and filtrate was evaporated to give (RS)-3-cyano-5-methylhexanoic acid ethyl ester (76.1 g (98.5 % purity)) as light brown color oil.