Abstract: The subject-matter of the invention consists in a method of removing specific impurities of montelukast of formula (I), which occur due to chemical instability of the target substance and also contaminate the substance in the preparation process. Further, methods of isolation of specific impurities of montelukast defined by formulae (V-A), (IV-A), (XIIIa-A), (XIIIb-A) and analytic methods used for the control of the production of montelukast in the pharmaceutical quality.

Abstract: A method of isolation of Montelukast of formula I from reaction mixtures, comprising conversion of the crude sub-stance to well-crystallizing salts with primary amines in the environment of at least one organic solvent and acetonitrile, followed by re-crystallization of these salts with simultaneous removal of chemical impurities and use of the chemically pure salts of Montelukast with primary amines for direct transformation to the pharmaceutically useful amorphous form of Montelukast sodium of formula II.

Abstract: A method of preparation of Montelukast of formula (I) by reaction of the compound of formula (III) and a compound of formula (IX), characterized in that the reaction is carried out in the presence of a base, an inert solvent and a component increasing selectivity of the process, especially of a polyether of general formula (XIII), wherein R stands for hydrogen or an alkyl and the value of n varies from 1 to 40, polyethyleneglycol of general formula (XIV), wherein n=1 to 40 or a crown-ether of formulae (XV), (XVI) or (XVII).

Abstract: The salt of oxalic acid with 5-[4-[2-(N-methyl-N-(2-pyridyl)amino)-ethoxy]-benzyl]-thiazolidin-2,4-dione of formula III including its tautomers and solvates. The method of preparation of the salt of oxalic acid with 5-[4-[2-(N-methyl-N-(2-pyridyl)amino)-ethoxy]thiazolidin-2,4-dione consists in a reaction of 5-[4-[2-(N-methyl-N-(2-pyridyl)-amino)ethoxy]benzyl]thiazolidin-2,4-dione of formula I with oxalic acid, the reaction being carried out in a solution in organic solvent or its mixture with water.

Abstract: A method for obtaining antidiabetic of formula (I), wherein the method comprises condensing of a 4-derivatized phenol or phenolate of general formula (II), wherein R is an amino group-containing organic residue, selected from the group comprising a residue of the following formula —NHR3, wherein R3 is hydrogen or a protecting group, which is removed before further treatment, and a residue of general formula (A), wherein Rb represents a carboxy group either in the free acid form or in the form of a salt or ester or another functional derivative or the nitrile group CN, and M represents a hydrogen or alkali metal atom, with a pyridine base of general formula (III, wherein Z is a leaving group other than a halogen, wherein, before or after carrying out the condensation, the following operations are carried out: (a) diazotizing the amino group present in organic residue R; (b) converting the diazotised residue R into a derivative of 2-halopropionate or 2-halopropionitrile of formula (B), wherein Rb is as defined