Labdane diterpenes and phenylethanoid glycosides from the Greek endemic species Marrubium thessalum (Lamiaceae)

Continuing our phytochemical investigation on the endemic species of the Greek flora, Marrubium thessalum, we report on the isolation and identification of fifteen secondary metabolites from the aerial parts. The isolation has been carried out by chromatographic methods of separation, i.e. TLC, CC and HPLC and their structures were established mainly by means of 1D and 2D NMR (¹H-NMR, ¹³C-NMR/DEPT, ¹H-¹H COSY, NOESY, HSQC, HMBC). As expected from previous investigations of other Marrubium species, the biosynthetic mechanisms of the plant are focused mainly towards diterpenes and phenylethanoid glycosides [1–4]. From the dichloromethane extract three new labdane diterpenes were isolated, 3α-hydroxymarrubiin, 9α,13R: 15,16-bisepoxy-15β-methoxy-3-oxo-labdan-6β,19-olide and 15-methoxyvelutine C. Also, from the same extract four known labdane diterpenes were isolated; peregrinine, preperegrine and 9α,13R: 15,16-bisepoxy-15α-hydroxy-3-oxo-labdan-6β,19-olide and 9α,13R: 15,16-bisepoxy-15β-hydroxy-3-oxo-labdan-6β,19-olide as an inseparable mixture, together with one flavone, ladanein. From the methanol extract of the plant six known phenylethanoid glycosides, namely acteoside, leucosceptoside A, echinacoside, forsythoside B, alyssonoside and 2-(3-hydroxy-4-methoxyphenyl)ethyl-O-[α-L-rhamnopyranosyl-(1→3)]-O-[β-D-glucopyranosyl-(1→6)]-4-O-E-feruloyl-β-D-glucopyranoside were isolated along with one flavonoid, apigenin.

Acknowledgements: Argyropoulou C gratefully acknowledges financial support from the Alexander S. Onassis foundation. The authors wish to thank Dr. Th. Constantinidis for the identification of the plant.

References: 1. Karioti, A. et al. (2003) Phytochemistry 64:655–660.

2. Karioti, A. et al. (2005a) Z. Naturforsch. 60b:328–332.

3. Karioti, A. et al. (2005b) Phytochemistry 66:1060–1066.

4. Karioti, A. et al. (2007) Phytochemistry 68:1587–1594.