NOVELTY - Preparation (M') of (R)(-)-3-(carbamoylmethyl)-5-methylhexanoic acid (II) or its salts, involves: a) reacting an ester (III) with ammonia reagent in presence of a lewis acid and a solvent; and b) optionally isolating compound (II); c) optionally converting compound (II) to its salts; and d) optionally purifying compound (II) or its salts.    USE - For preparation of (R)(-)-3-(carbamoylmethyl)-5-methylhexanoic acid useful for preparation of pregabalin (claimed) useful as CNS-active compound, and having protective effect on-seizure and analgesic, anticonvulsant, and anxiolytic activity.    ADVANTAGE - The process prepares pregabalin with low amount of undesired impurity. The process prepares pregabalin free of 4-ene impurity. The process is not only environment friendly and also provides better yield with required chiral purity. The process is further suitable to practice at on industrial scale.    DETAILED DESCRIPTION - Preparation (M') of (R)(-)-3-(carbamoylmethyl)-5-methylhexanoic acid of formula (II) or its salts, involves: a) reacting an ester of formula (III) with ammonia reagent in presence of a lewis acid and a solvent; and b) optionally isolating compound (II); c) optionally converting compound (II) to its salts; and d) optionally purifying compound (II) or its salts.    R'=1-6C alkyl, aryl or alkyl aryl.    INDEPENDENT CLAIMS are included for the following:    (1) preparation (M1) of pregabalin of formula (I), involving: a1) reacting the ester (III) with ammonia reagent in presence of lewis acid and solvent to provide compound (II) or its salts; b1) reacting compound (II) or its salts with a hypohalite in presence of base and solvent to provide pregabalin (I); c1) isolating pregabalin (I); and d1) optionally purifying pregabalin in solvent;    (2) preparation (M2) of pregabalin free of 4-ene impurity of formula (IV), involving: a2) reacting compound (II) or its salt with hypochlorite in presence of base and solvent; b2) treating the reaction mass with an acid to get pH of 2-3; c2) treating with base to result the pH of mass of 5-5.5; d2) isolating pregabalin; and e2) optionally purifying pregabalin in solvent; and    (3) pharmaceutical compositions comprising pregabalin (I) or its salts prepared by the method (M1) or (M2) together with at least one excipient, carrier and diluents.