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Abstract
Chemical investigation of the essential oil obtained from the heartwood of Erythroxylum monogynum Roxb. yielded three beyerene type diterpenoids ent-beyer-15-ene (1), ent-beyer-15-en-19-ol (erythroxylol A) (2) and ent-beyer-15-en-19-al (3). Ent-beyer-15-en-19-al (3) was found to be unstable at room temperature, giving rise to hitherto unknown 15,16-epoxy-ent-beyeran-19-oic acid (4). This conversion involves the auto-oxidation of a C-4 axial aldehyde group of an ent-beyer-15-ene diterpenoid with the concurrent epoxidation of the C-15 double bond. This is the first report of the auto-oxidation of an aldehyde group to a carboxylic acid group with the concurrent epoxidation of a double bond in the same compound. Further investigation of this observation under controlled conditions resulted in the isolation and identification of ent-beyer-15-en-19-oic acid (5), two new epoxy hydroperoxides, 15,16-epoxy-19-nor-ent-beyeran-4α-hydroperoxide (6a), 15,16-epoxy-18-nor-ent-beyeran-4β-hydroperoxide (6b), and two new hydroperoxides, ent-beyer-19-nor-15-en-4α-hydroperoxide (7), ent-beyer-18-nor-15-en-4β-hydroperoxide (8) and ent-beyer-18-nor-15-en-4β-ol (9). Identification of these compounds was carried out by the extensive usage of spectroscopic data including 1D and 2D NMR. The acid 5 and the alcohol 9 have been reported previously as natural products from Elaeoselinum asclepium and Erythroxylum monogynum. The mechanistic basis of this auto-oxidation reaction is discussed.
Highlights
Erythroxylum monogynum Roxb. (Erythroxylaceae), is a small evergreen tree indigenous to Sri Lanka and India
Its 1H and 13C NMR spectra (Tables 1 and 2) together with distortionless enhancement by polarization transfer (DEPT)135 and heteronuclear single quantum coherence (HSQC) data showed the presence of three methyls attached to quaternary carbons, eight methylenes, five methines, two of which are olefinic [δH 5.68 d (J = 5.6 Hz), 5.46 d (J = 5.6 Hz); δC 134.5, 136.8] and one which is aldehydic [δH 9.75 d (J = 1.4 Hz); δC 205.9] and four quaternary carbons accounting for C 20H30O
Our results suggest that 4-hydroxy-19-nor-ent-beyer-15-ene which has been reported previously as a natural product from E. monogynum may be an artefact arising from the autooxidation reaction
Summary
Erythroxylum monogynum Roxb. (Erythroxylaceae), is a small evergreen tree indigenous to Sri Lanka and India. 1H and 13C NMR spectroscopic data (Tables 1 and 2) together with HSQC and DEPT135 of compound 7 showed the presence four quaternary carbons of which one is oxygenated (δC 84.2), four methines of which two are olefinic [δH 5.71 d (J = 5.7 Hz), 5.45 d (J = 5.7 Hz); δC135.2, 136.4], eight methylenes and three tertiary methyls (δH 0.99 s, 1.28 s, and 0.85 s; δC 24.9, 24.8, and 15.5).
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