Auto-oxidation of -beyer-15-en-19-al isolated from the essential oil of the heartwood of Roxb.: formation of 15,16-epoxy--beyeran-19-oic acid and other products.

Chemical investigation of the essential oil obtained from the heartwood of Roxb. yielded three beyerene type diterpenoids -beyer-15-ene (), -beyer-15-en-19-ol (erythroxylol A) () and -beyer-15-en-19-al (). -beyer-15-en-19-al () was found to be unstable at room temperature, giving rise to hitherto unknown 15,16-epoxy--beyeran-19-oic acid (). This conversion involves the auto-oxidation of a C-4 axial aldehyde group of an -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 -beyer-15-en-19-oic acid (), two new epoxy hydroperoxides, 15,16-epoxy-19---beyeran-4-hydroperoxide (), 15,16-epoxy-18---beyeran-4-hydroperoxide (), and two new hydroperoxides, -beyer-19--15-en-4-hydroperoxide (), -beyer-18--15-en-4-hydroperoxide () and -beyer-18--15-en-4-ol (). Identification of these compounds was carried out by the extensive usage of spectroscopic data including 1D and 2D NMR. The acid and the alcohol have been reported previously as natural products from and . The mechanistic basis of this auto-oxidation reaction is discussed.