New labdane-type diterpenoids from Leonotis leonurus support circumscription of Lamiaceae s.l.

Abstract

The genus Leonotis (Pers.) R.Br. comprises about 10 species, and has been placed within subfamily Lamioideae of the Lamiaceae. Genus members range in habit from tall herbs to small trees. All are native to tropical and/or southern Africa, with Leonotis nepetifolia (L.) R.Br. naturalised in the tropics (Harley et al., 2004). Leonotis leonurus (L.) R.Br., commonly referred to as Wild dagga or Lion’s ear, is a robust perennial shrub which grows usually to 2 m tall and is widespread throughout eastern South Africa, growing amongst rocks in grassland (Iwarsson, 1985). The plant has found a wide variety of medicinal applications, inter alia for treating colds (Githens, 1949), bronchitis, tuberculosis (Watt and Breyer-Brandwijk, 1962), coughs, asthma (Felhaber, 1997), feverish headaches (Bryant, 1970), dysentery and chest infections (Gerstner, 1941). Based on its well-documented traditional usage profile for respiratory ailments, and its in vitro antibacterial activity (Kamatou et al., 2006), L. leonurus was identified as a potential source of novel anti-tuberculosis compounds. It was screened accordingly and profiled phytochemically. Leaf material of L. leonurus (L.) R.Br. was harvested from Clanwilliam,Western Cape Province, South Africa, and a voucher (BP00444) lodged at the National Herbarium (PRE) for verification purposes.