Praziquantel is a broad spectrum insecticide, and is especially suitable for the treatment of acute and chronic schistosomiasis along with complications, which is a choice drug for treating schistosomiasis at present. The structural formula of praziquantel is formula V as below:

Francisco Yuste et al. (Journal of Heterocyclic Chemistry, 1986, Vol. 23, p 189-190.) discloses a method for preparing praziquantel with the following reaction scheme, wherein the key intermediate compound of formula IV was subjected to many reaction steps to obtain the praziquantel compound of formula V.

As to the key intermediate 4-benzyl-1-phenethyl-piperazine-2,6-dione (the compound of formula IV) in the above reaction, Francisco Yuste et al. (Journal of Heterocyclic Chemistry, 1986, Vol. 23, p 189-190.) and Malcolm D. Brewer et al. (Journal of Medicinal Chemistry, 1989, Vol. 32, No. 9, p 2058-2062.) disclose a method for preparing this compound.

As disclosed in the method, the compound of formula I was directly mixed with β-phenethylamine and heated to 200° C., to obtain the compound of formula IV at a yield of 65%. This method has a low yield and produces many impurities, thus the product is hard to purify. Meanwhile, the high-temperature reaction not only is energy-consuming, but also has a high requirement on the device, which is hard to achieve industrial production.
In view of the importance of the compound of formula IV in synthesizing praziquantel compound of formula V, it is greatly meaningful to develop a preparation method having high yield, high purity and being easily applicable in industrial production.