Citrus limonoids and their semisynthetic derivatives as antifeedant agents against Spodoptera frugiperda larvae. A structure-activity relationship study

The antifeedant activity of Citrus-derived limonoids from limonin (1), nomilin (2), and obacunone (3) and theri semisynthetic derivatives was evaluated against a commercially important pest, Spodoptera frugiperda. Simple chemical conversions were carried out on the natural limonoids obtained from seeds of Citrus limon. These conversions focused on functional groups considered to be important for the biological activity, namely C-7 carbonyl and the furan ring. In particular, reduction of C-7 afforded the related alcohols, and from these their acetates, oximes, and methoximes were prepared. Hydrogenation of the furan ring was also performed on limonin and obacunone. The known antifeedant properties of the Citrus liomonoids are confirmed. Comparison with previously reproted data shows that insect species vary in their behavioral responses to these structural modifications. Highly significant antifeedant activity (P < 0.01) for two natural (1 and 3) and three semisynthetic limonoids (4, 8 and 10 ) was observed aginst S. frugiperda.