Title: Additional Bioactive Triterpenoids from Melia volkensii (Meliaceae)
Abstract: Meliavolen (1) and melianinone (2), new triterpenes with an apotimcallane skeleton, together with two known compounds, 3-episapelin A (5) and nimbolin B (6), have been isolated from the root bark of Melia volkensii (Meliaceae) by using brine shrimp lethality for bioactivity-directed fractionation.The structures have been elucidated by spectral data.The relative and absolute stereochemistries of 2 were determined by correlating 2 with melianin A (3).The stereochemistry of 3 was determined by X-ray crystallographic analysis and Mosher methodology.The stereochemistry of 5 was also determined by Mosher methodology.The four isolated compounds, meliavolen (I), melianinone (2), 3episapelin (5), and nimbolin B (6), were all moderately active in the brine shrimp lethality test and were significantly cytotoxic among human solid tumor cell lines with noted selectivities toward the colon cell line (HT-29).In a previous paper,' we reported two new bioactive triterpenes, meliavolin and meliavolkin, isolated from the root bark of Melia volkensii Gurke (Meliaceae) obtained from Kenya.Further fractionations of the root bark, directed by a test for brine shrimp lethality (BST),2, have yielded two new apotimcallane triterpenes, meliavolen (1) and melianinone (2), together with two known compounds, 3-episapelin A (5) and nimbolin B (6).The structures of meliavolen (1) and melianinone (2) were determined by spectral data, and the stereochemistry of 2 was determined by correlating 2 with melianin A (3). through oxidation.By using advanced Mosher's methodology, the absolute stereochemical structures of melianin A (3) and 3-episapelin (5) were determined.The relative stereochemistry of melianin A (3) was also determined by X-ray crystallographic analysis.Meliavolen (1) was isolated as a white powder.The molecular weight of 1 was indicated by a peak at mlz 683 (M+Na+) in the m-nitrobenzyl alcohol (m-NBA) FABms.High resolution FABms gave mlz 683.3918 (calcd 683.3923) for the M+Na+, corresponding to the formula, C41H5607.The ' H nmr and '3C nmr spectra of 1 showed signals due to five tertiary methyls, two methyls attached to a double bond, ., ,