Absolute configuration and conformation of 3 tetralone derivatives from Ammannia baccifera

The ethanol extract of Ammannia baccifera L. (Lythraceae) was subjected to vacuum liquid column chromatography over silica gel eluted with n-hexane: dichloromethane: methanol (step gradient) followed by column chromatography (silica gel eluted with dichloromethane: methanol) and multi-preparative thin-layer chromatography. (-)-(4R)-Hydroxy-1-tetralone, (-)-(4S)-acetoxy-1-tetralone, (-)-(4S)-hydroxy-1-tetralone-4-O-β-D-glucoside, β-sitosterol and β-sitosterol-β-D-glucoside have been purified and identified by the joint application of UV spectroscopy, mass spectrometry, NMR spectroscopy, specific rotation and circular dichroism spectroscopy. Further light on the precise nature of the shape of the molecule was provided by a more detailed analysis of the ¹H NMR spectrum. Since the cyclohexanone ring was flexible, the conformation of the substituted group at C-4 of 3 tetralone derivatives was an average of the pseudoaxial and pseudoequatorial orientation [1]. The absolute configuration and conformation responsible for the NMR measurement of (-)-(4R)-hydroxy-1-tetralone, (-)-(4S)-acetoxy-1-tetralone and (-)-(4S)-hydroxy-1-tetralone-4-O-β-D-glucoside were β-equatorial, α-axial and α-axial orientation respectively. This is the first report of the presence of (-)-(4R)-hydroxy-1-tetralone, (-)-(4S)-acetoxy-1-tetralone and (-)-(4S)-hydroxy-1-tetralone-4-O-β-D-glucoside in the ethanol extract of A. baccifera.

Acknowledgements: I. Techatanawat thanks the Thai Government Pharmaceutical Organisation for financial support

Reference: 1. Talapatra, S. K. et al. (1988), Phytochemistry 27: 3929–3932.