Synthesis of Novel 4-Acyloxy-2'-bromo-6'-chloropodophyllotoxin Derivatives Displaying Significant Insecticidal Activity against Mythimna separata.

In order to explore novel natural product-based insecticidal agent, two important intermediates (2 and 3) and 4-acyloxy-2'-bromo-6'-chloropodophyllotoxin derivatives (4a-f and 5a-f) were designed and prepared, and their structures were confirmed by 1H NMR, 13C NMR, HRMS, ESI-MS, optical rotation and melting point (mp). The stereochemical configuration of compound 4b was unambiguously confirmed by single-crystal X-ray diffraction. Moreover, we evaluated the insecticidal activity of target compounds 4a-f and 5a-f against a serious agricultural pest of Mythimna separata by using the leaf-dipping method. Among all tested compounds, compounds 4d, 5d and 5f exhibited stronger insecticidal activity with a final mortality rate exceeding 60%. Especially compound 5d exhibited the best insecticidal activity, with a final mortality rate of 74.1%. It has been proven that introducing bromine or chlorine atoms at the C-2', C-2' and C-6' positions of the E ring of podophyllotoxin can produce more potent compounds. In addition, the configuration of the C-4 position is important for insecticidal activity, and 4β-configuration is optimal. This will pave the way for further design, structural modification, and development of derivatives of podophyllotoxin as insecticidal agents.