Xylogranatins F-R: Antifeedants from the chinese mangrove, Xylocarpus granatum, a new biogenetic pathway to tetranortriterpenoids

Thirteen limonoids with a new carbon skeleton, the xylogranatins F-R (1-13), have been isolated from the seeds of a Chinese mangrove, Xylocarpus granatum; two recently reported compounds, xylogranatins C and D were also isolated. Their structures were elucidated on the basis of spectroscopic data and chemical methods. The absolute configurations of these compounds were determined by using the modified Mosher MTPA ester method and by quantum chemical circular dichroism (CD) calculations. Xylogranatins F-Q are the first aromatic B-ring limonoids found in nature. They belong to two substructural classes, of which one (1-3) contains a pyridine ring while the other one (412) contains a central furan core. Xylogranatins C and R can be considered to be key biosynthetic intermediates, while xylogranatin D, the only limonoid found so far with a carbon skeleton that conatains a C-30-C-9 linkage, is apparently an artifact. The structures of these compounds suggest a new biogenetic pathway to tetranortriterpenoids. Xylogranatins F, G and R were found to exhibit marked antifeedant activity against the third instar larvae of Mythimna separata (Walker) at a concentration of 1 mg mL(-1). The most potent compound tested was xylogranatin G. Its AFC(50) (concentration for 50% antifeedant activity) values at the exposure times of 24 and 48 h were 0.31 and 0.30 mg mL(-1), respectively.