Patent ID: 8115004

Claim:
A process for the preparation of a crystalline form of a compound represented by Formula-5: wherein R is n-butyl and wherein the crystalline form is characterized by x-ray powder diffraction peaks at about 6.2, 8.6, 8.9, 15.0, 16.9, 18.9, 20.7, 22.6, 25.5 and 29.9 degrees two-theta ±0.2 degrees two-theta, the process comprising: a) reacting a compound represented by Formula-2, with methane sulfonyl chloride in a solvent or mixture of solvents in the presence of an organic amine base to give a compound represented by Formula-3: then reacting in-situ the compound represented by Formula-3 with a compound represented by Formula-4: in the presence of a polar aprotic solvent, optionally a C 1 -C 4 alcoholic solvent, and an alkali or alkaline earth metal alkoxide to give montelukast; b) quenching the reaction mixture with aqueous alkali solution, then extracting the reaction mixture with a water-immiscible solvent; c) lowering pH of the reaction mixture with an organic acid, then extracting montelukast with ester solvents and chloro solvents, followed by concentrating the solvent and then dissolving the obtained residue in a suitable solvent selected from keto solvents and ester solvents; d) treating the product obtained from step c) with n-butyl amine in a non-polar organic solvent, keto solvent or ester solvent or a mixture thereof under an inert atmosphere to give a crystalline form of the compound represented by Formula-5; and e) optionally purifying the crystalline form of a compound represented by Formula-5 using a hydrocarbon solvent or keto solvent or mixture thereof.