BigPatents India

The present invention relates to an improved process for preparation of Risedronic acid represented by a compound of formula I or salts thereof, by reacting 3-pyridylacetic acid with phosphorous acid and phosphorus trichloride in a solvent, which comprises reacting 3-pyridylacetic acid with phosphorous acid and phosphorus trichloride in cyclic aliphatic solvent with carbon atoms 5-8 at the temperature in the range of 75-100°C or in mixtures of cyclic aliphatic solvents at the boiling temperature of the reaction mixture, removing the solvent and isolating pure Risedronic acid, preferably by treating raw Risedronic acid without separating it from the final reaction mixture, with alkali metal or ammonium hydroxide, bicarbonate or carbonate . The invention further relates to the Risedronic acid monosodium hemipenta hydrate salt in desired crystalline form and a process for the preparation thereof.