From a phagostimulant natural product to semisynthetic antifeedants against Spodoptera littoralis larvae: Chemical transformations of the neoclerodane diterpenoid scutegalin B

Scutegalin B (4), a natural neoclerodane diterpenoid possessing phagostimul ant activity against larvae of the lepidopteran Spodoptera littoralis, has been subjected to a series of chemical transformations obtaining several de rivatives. The activity of some of these changes to antifeedant (10, 12, 16 , and 17), although other derivatives are inactive (6, 7, and 15) or mainta in phagostimulant activity (8 and 9) of the starting material (4). The most potent antifeedant was 16, which possesses 16,15-lactone and a (19S)19,2 a lpha-hemiacetal groups instead of the 16,15-lactol and (19R)-(19-O-tigloyl) 19,2 alpha-hemiacetal of the phagostimulant precursor 4. These and other st ructure-activity relationships are discussed, establishing that the biologi cal action is strongly modulated by minimal structural variations.