Isolation and Characterization of Limonoids from Kigelia africana

Two new limonoids, 1-O-deacetyl-2 alpha-methoxykhayanolide (1) and ldgelianolide (2), together with deacetylkhayanolide E (3), 1-O-deacetyl-2 alpha-hydroxykhayanolide E (4) and khayanolide B (5) were isolated from the ethyl acetate-soluble fraction of the methanolic extract of Kigelia africana. The structures of these limonoids (1-5) were elucidated by the combination of 1D (H-1 and C-13 NMR) and 2D (HMQC, HMBC and COSY) NMR spectroscopy and mass spectrometry (EIMS, HREIMS), and in comparison with literature data of related compounds. The structure of compound 1 was further confirmed by X-ray crystallography, and the absolute stereochemistry of compounds 1 and 2 was determined by electronic circular dichroism (ECD) spectroscopy. Limonoids 1-5 showed weak inhibitory activities against the enzymes acetylcholinesterase (AChE), butyrycholinesterase (BChE) and lipoxygenase (LOX) in a concentration-dependent manner with IC50 values in the ranges 137.5-225.2 mu M for AChE, 185.4-241.5 mu M for BChE and 281.2-189.6 mu M for LOX.