Preparation method of linagliptin and intermediate thereof

The invention belongs to the field of pharmaceutical chemicals, and in particular relates to a preparation method of linagliptin and an intermediate thereof. The preparation method comprises the following steps: carrying out Mitsunobu reaction on a compound I and 2-butyne-1-alcohol in an organic solvent in the presence of trialkyl phosphine and an azo-reagent to obtain a linagliptin intermediate compound II; carrying out substitution reaction on the compound II and (R)-3-aminopiperdine by taking isopropanol as a solvent and tri-n-butylamine as an acid-binding agent to obtain a compound III; and carrying out alkylation reaction on the compound III and 4-methyl-2-chloro-methyl-quinazolin to obtain linagliptin. According to the preparation method provided by the invention, the Mitsunobu reaction is innovatively adopted to prepare the compound II, so that the preparation method has the advantages of mild reaction condition, reasonable operation, high selectivity, high product quality and the like. Moreover, by controlling the reaction condition, the compound III and R-3-aminopiperdine or an inorganic acid salt or an organic acid salt thereof are directly subjected to nucleophilic substitution to generate linagliptin, and the method does not have protection and de-protection processes of amino groups, so that the reaction steps are reduced, the reaction process is simpler, the cost is reduced, and the purity of an obtained product is relatively improved.