Stereoselective synthesis of 4α-acyloxy-2α/β-bromopodophyllotoxin derivatives as insecticidal agents

To find new natural products-based insecticidal agents, and substantially extend our previous work, we have designed and stereoselectively synthesized 4α-acyloxy-2α/β-bromopodophyllotoxin derivatives from podophyllotoxin. Interestingly, 4α-acyloxy-2α-bromopicropodophyllotoxins were easily converted to a more rigid compound 14 by an intramolecular Friedel–Crafts alkylation reaction in the presence of BF3·Et2O when the reaction time was prolonged to 4–18 h. Compounds 5g, 6h, 6i, and 14 displayed the more promising and pronounced insecticidal activity than toosendanin, a commercial insecticide derived from Melia azedarach, against the pre-third-instar larvae of Mythimna separata.