An investigation of compounds occurring in leonotis species

Abstract

Two labdane diterpenoids 8-hydroxymarrubiin and leonitin were isolated from Leonotis dysophylla (Benth.) and Leonotis leonitis respectively. Spectral studies of 8-hydroxymarrubiin, C₂₀H₂₈ O₅ʻ showed the presence of a β -substituted furan, a Ϫ-lactone, three tertiary methyl groups and tertiary hydroxyl group (s). The NMR spectrum of 8-hydroxymarrubiin and marrubiin C₂₀H₂₈O₄ʻ were almost identical with the exception of the C₠₇- methyl group which appeared as a singlet in 8-hydroxymarrubiin and as a doublet in marrubiin. The extra oxygen atom was therefore assumed to be present as a hydroxyl group substituted in the C₈₋ position. This was further confirmed by the formation of an epoxide and a Ϫό-dilactone. Leonitin, C₂₀H₂₈O₇ʻ was shown by spectral and chemical evidence to be a diterpenoid dilactone possessing an ester function and an ether linkage. Comparison of the NMR spectra of compound X and leonitin suggested that the acetoxy function occurs in the C₂₀- position. This was further supported by the formation of a 'Ϫό -dilactone. The absence of a β -furan moiety was apparent from chemical and spectral evidence, A structure for leonitin is proposed and aspects of its stereochemistry discussed.