Preparation method of varenicline intermediate

The invention belongs to the field of chemical synthesis of medicines, and particularly relates to a preparation method of a varenicline intermediate. Comprising the following steps: 1) in the presence of an organic solvent A, adding ammonium acetate and a reducing agent into a compound as shown in a formula 4 serving as a raw material, and reacting to obtain a compound as shown in a formula 3 or salt thereof; 2) in an organic solvent B, reacting the compound as shown in the formula 3 or a salt thereof with trifluoroacetic anhydride or trifluoroacetic ether under the action of alkali to obtain a compound as shown in a formula 2; and 3) in the presence of a solvent C and a catalyst, carrying out nitration reaction on the compound in the formula 2 and a nitrating agent to obtain the compound in the formula 1. The problems of tedious steps, low atom utilization rate, high cost and the like in the prior art are solved, the provided route is novel, amino is directly introduced, deprotection is not needed, atom eco