Preparation method of anagliptin

The invention relates to a preparation method of an anti-type II diabetes medicine anagliptin. The preparation method comprises the following reaction steps: carrying out SN2 substitution reaction on N-(2-chloracetyl)-L-proline ester prepared by taking L-proline as a raw material and an amino fragment; carrying out protective group removal reaction on the compound shown in the formula (III) under a certain condition; and carrying out condensation reaction on the compound shown in the formula (IV) and a pyrazolopyrimidine carboxylic acid fragment shown in the formula (V) under a re-condensation condition to prepare the compound shown in the formula (VI). And finally, converting the carboxyl methyl ester/benzyl ester formula (VI) into nitrile pyrrole under certain conditions to prepare the target molecule Anagliptin formula (VII). According to the synthesis process, a linear synthesis strategy is adopted, firstly, all molecular fragments are linearly connected, and cyano groups are introduced in the final stage