Preparing method for caproic acid derivative

The invention discloses a preparing method for a caproic acid derivative and provides a preparing method for a caproic acid derivative IV. The method comprises the following steps that in a polar nonprotic organic solvent, a compound III and an iodization reagent are subjected to a nucleophilic substitution reaction under inert gas shielding, and the caproic acid derivative IV is obtained. Brivaracetam can be prepared from the caproic acid derivative IV only through two steps, the synthetic route is short, the reaction condition is mild, posttreatment is simple, the reaction yield is high, and the production cost is low. Racemization does not happen in the reaction process, further purification is conducted through crystallization instead of a chiral high-pressure liquid phase preparing column, the chiral purity of the brivaracetam I can be improved till the de value is larger than 99.80%, meanwhile, the content of other single impurities of the brivaracetam I is smaller than 0.1%, the API level is reached, a