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 flavone. The structures of the isolated compounds were established by means of NMR [1H-1H-COSY, 1H-13C-HSQC, HMBC, NOESY, and ROESY] and MS spectral analyses. Complete NMR assignments are reported for 13R-preperegrinine.