The structure and synthesis of metabolites from virgilia oroboides and chlorophora excelsa (Iroko)

Abstract

In the present study the acetone extract of the heartwood of two trees, Virgilia oroboides and Chlorophora excelsa, were investigated. The heartwood of Virgilia oroboides afforded a variety of known flavonoids, as well as a new pterocarpene and a new α-hydroxydihydrochalcone, viz.; 3-hydroxy-8, 9-methylenedioxy-6a,11a-dihydropterocarpan (αS), 2' ,4'-trihydroxy-4-methoxydihydrochalcone. A series comprising substituted hydroxygeranylstilbenes, substituted benzenoid compounds and quercitin-type flavones were isolated from the acetone extract of the heartwood of Chlorophora excelsa. The new compounds isolated from this tree are: 3,5-dihydroxy-4-geranylbenzaldehyde; 3' ,4,5'-trihydroxy-4'-geranylstilbene; 2'-methoxy-3, 4',7-tri-O-methylquercitin. A combination of solvent extraction, Craig countercurrent, column (LH 20 and silica gel) and thin layer chromatography procedures were used to isolate and purify the compounds mentioned. Structures were elucidated by high resolution (300 MHz) ¹HNMR spectroscopy (including NOE and spin-spin decoupling experiments) and mass spectrometry. The proposed structural assignments of the following compounds were confirmed by synthesis: 3, 5-dihydroxy-4-geranylbenzaldehyde; 3' ,4 ,5'-trihydroxy-4' -geranylstilbene; 2 ,3' ,4 ,5'-tetrahydroxy-4' -geranylstilbene (chlorophorin). The modified Wittig reation was used to synthesize 3' ,4 ,5' trihydroxystilbene. U.V. irradiation experiments were performed on chlorophorin in an attempt to synthesize the cis-isomer and a phenanthrene-type compound. Biosynthetic pathways showing the structural relationships of the identified compounds in Virgilia oroboides and Chlorophora excelsa were proposed. An attempt to synthesize (+)-catechin lignoid involved the coupling of (+)-catechin to sinapyl alcohol, with the latter synthesized from 2 ,6-dimethoxyphenol via vinyl quinone methide. Further investigations on lignoid currently in progress.