Remophilanetriol: A New Eremophilane from the Roots of Rehmannia glutinosa

Abstract

Radix Rehmanniae is the fresh or dry root of Rehmannia glutinosa Libosch. (Scrophulariiaceae), and is traditionally used for antipyretic and hemostatic. The roots of Rehmannia glutinosa have been also used as a tonic and sedative after processing of the fresh root such as boiling in the rice wine or steaming. A number of constituents such as iridoids, phenethyl alcohol glycosides, glycosides, cyclopentanoid monoterpenes, and norcarotenoids, have been reported from the fresh or processed roots of R. glutinosa. In the course of chemical investigation on the dried roots of R. glutinosa, a new eremophilane-type metabolite, named remophilanetriol (1), was encountered, together with aeginetic acid (2), which has been identified from the same species. Details of the isolation and structure elucidation of new compound (1) are described in the present study. The MeOH extract of Rehmannia glutinosa was concentrated, suspended in H2O, and sequentially partitioned with n-hexane, EtOAc, and BuOH. Purification of the EtOACsoluble extract by utilizing normal-phase and reversed-phase column chromatography, followed by reversed-phase HPLC, resulted in the isolation of compounds 1 and 2. Compound 1 has the molecular formula of C15H24O4, as deduced from the C NMR and HRFABMS data, which indicated four degrees of unsaturation. Analysis of H, C and DEPT NMR data (Table 1) revealed the presence two methyls, six methylenes (two oxygenated), three methines, and four quaternary carbon including carbonyl (δ 199.8) functionality. These data accounted 21 carbon-bound protons, requiring the presence of three hydroxyl groups in the molecule. In addition, carbon resonances observed at δ 199.8, δ 161.6, and δ 124.4 were strongly suggestive of the presence of an α,β-unsaturated ketone moiety. Since the above structure information accounted for all but two unsaturation equivalents, compound 1 must be bicyclic to satisfy four degrees of unsaturation required by its molecular formula. The structure of remophilanetriol was unambiguously deduced by analysis of 2D NMR data (COSY, HMQC, and HMBC). Analysis of COSY results led to the