The synthesis and structure modification of active natural lead compounds is an important approach for the discovery of novel green pesticides.Nineteen derivatives of chasmanthinine, a natural C19-diterpenoid alkaloids, were prepared, and their structures were unambiguously determined by 1H NMR, 13C NMR, and HR-ESI-MS. Moreover, the antifeedant activities of title compounds were evaluated against larvae of Spodoptera exigua (Hubner). The results illustrated that compound p with a thienyl group at the C-14 position (EC50 = 0.10 mg/cm2) showed the strongest antifeedant activities among all tested compounds. This present study is the first report on the antifeedant effects of synthetic chasmanthinine analogs aganist S. exigua (Hubner) larvae. © 2021 Japan Institute of Heterocyclic Chemistry. All rights reserved.