NOVELTY - Synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from ethyl cyano acetate comprises: condensation of cyanoacetic acid ethyl ester with 1-bromo-2-methyl propane in presence of base in polar solvent to give 2-cyano-4-methylpentanoic acid ethyl ester; reaction of 2-cyano-4-methylpentanoic acid ethyl ester with halo acetic acid ethyl ester (V) in presence of base in polar solvent to give diethyl 2-cyano-2-isobutylsuccinate; and reaction of diethyl 2-cyano-2-isobutylsuccinate with potassium chloride or sodium chloride in organic solvent to give 3-cyano-5-methyl-hexanoic acid ethyl ester.    USE - The processes are useful for: the synthesis of 3-cyano-5-methylhexanoic acid ethyl ester, (S)-3-cyano-5-methylhexanoic acid ethyl ester; and the enantiomeric enrichment of (S) 3-cyano-5-methylhexanoic acid ethyl ester in its racemate or partially enriched form (all claimed).    ADVANTAGE - The processes: are cost effective and eco-friendly; and provide optically pure (S)-3-cyano-5-methylhexanoic acid ethyl ester having excellent yield and high optical purity.    DETAILED DESCRIPTION - Process (A) for synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from ethyl cyano acetate comprises: condensation of cyanoacetic acid ethyl ester with 1-bromo-2-methyl propane in presence of base including alkali hydroxide, sodium hydride (preferred), sodium ethoxide, sodium methoxide or potassium carbonate in a polar solvent including dimethoxy ethane or diglyme (both preferred), 1,4-dioxane or tetrahydrofuran to give 2-cyano-4-methylpentanoic acid ethyl ester; reaction of 2-cyano-4-methylpentanoic acid ethyl ester with halo acetic acid ethyl ester of formula (X-CH2-COO-C2H5) (v), in presence of base including alkali hydroxide, sodium hydride (preferred), sodium ethoxide, sodium methoxide or potassium carbonate, preferably sodium hydride, in a polar solvent including dimethyl sulfoxide, N,N-dimethyl formamide or dimethoxy ethane (all preferred), tetrahydrofuran or 1,4-dioxane at 10-80 degrees C, preferably at 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, where at each step the intermediates are optionally isolated and purified with suitable process.    X = Cl, Br or I.    INDEPENDENT CLAIMS are also included for:    (1) a process (B) for synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from ethyl cyano acetate;    (2) a process (C) for synthesis of 3-cyano-5-methylhexanoic acid ethyl ester from dihydrofuran-2,5-dione;    (3) a stereoselective process (D) for synthesis of (S) 3-cyano-5-methylhexanoic acid ethyl ester from S-leucine;    (4) a process (E) for enantiomeric enrichment of (S) 3-cyano-5-methylhexanoic acid ethyl ester in its racemate or partially enriched form;    (5) 2-cyano-2-isobutyl-succinic acid diethyl ester; and    (6) 2-isopropylidene-succinic acid 1-benzyl ester.