Photooxidation of nimbin and salannin, tetranortriterpenoids from the neem tree (Azadirachta indica)

Nimbin and salannin, major triterpenoids accompanying azadirachtin in extracts of neem (Azadirachta indica) seeds, were photooxidized by UV light in the presence of oxygen to more polar, unstable intermediates that rearranged on silica gel to two final products in which the furan ring had been oxidized to isomeric hydroxybutenolides. The isomeric hydroxybutenolides were also readily formed when a crude extract of triterpenoids from neem seeds was irradiated, and both isomers of salannin have been isolated from seeds. Photooxidation of nimbin and salannin proceeded much faster than that of azadirachtin. All photoproducts showed some biological activity against Spodoptera littoralis, Locusta migratoria, and Schistocerca gregaria. Isonimbinolide was as potent as azadirachtin at inhibiting feeding in all three species, and it also inhibited the growth of S. littoralis. Isosalanninolide showed potent antifeedant and growth inhibitory activity against S. littoralis.