Labdane diterpenes from Marrubium thessalum

From the aerial parts of Marrubium thessalum, four labdane diterpenes, 13S-preperegrinine, 3α-hydroxymarrubiin, 9α,13R-15,16-bisepoxy-15β-methoxy-3-oxo-labdan-6β,19-olide and 15-methoxyvelutine C, have been isolated together with four known diterpenes and one methoxylated flavone, ladanein. The structures of the isolated compounds were established by means of NMR (COSY, HSQC, HMBC, NOESY, and ROESY) and MS spectral analyses. Complete NMR assignments are reported for 13R-preperegrinine. Characteristic compounds of the plant were peregrinine and ladanein.