1997) type, and the other rich in geranial (27-30 per cent) and neral (36-41 per cent). Plants collected in Tucumán, Argentina, in spring and in autumn (Catalán etal, 1977) gave piperitone (37 per cent and 24 per cent resp.), limonene (34 per cent and 47 per cent resp.) and 1,8-cineole (10 per cent and 13 per cent resp.); L. alba collected on the shores of Parana and Uruguay rivers in Entre Rios province, Argentina (Retamar, 1994) produced high levels of lippione (1,2-epoxypulegone) (2).

The absolute configuration of the natural dextrorotatory lippione (2) was established as 1S,2S by optical rotatory dispersion measurements (Shimizu etal., 1966). Lippiaphenol (3), considered to be an artefact formed by acid treatment of 2, has been detected as a natural product (Ricciardi etal., 1981) in the steam distilled essential oil of L. turbinata. Lippione (2) is an insect neurotoxin acting by inhibition of the enzyme acetylcholinesterase (Grundy and Still, 1985; Grundy and Still, 1985a). Both the epoxy and the keto groups appear to be necessary for irreversible inhibition (Grundy and Still, 1985a).

Lippia citriodora Kunth (syn. Aloysia triphylla Britton) is well known with the common name of 'lemon grass', 'lemon verbena' or 'vervain'. The plant is cultivated in many countries for preparing teas and alcoholic drinks. More than 65 components have been identified in the essential oil (Terblanché etal, 1996), the main constituents being geranial, neral, 6-methyl-5-hepten-2-one, 1,8-cineole, limonene, ^-caryophyl-lene and caryophyllene oxide. However, as it is typical in the genus, the composition of the oil can change significatively according to the region where the plants grow (Bellakhdar etal, 1993; Djerrari etal, 1993; Bellakhdar etal, 1994; Zygadlo etal, 1994; Ozek etal, 1996; Terblanché etal, 1996; Zrira and Benjillali, 1998). Some interesting minor components isolated from the oil are the photocitral isomers 4-7, the oxides 8-12 (von Roman Kaiser and Lamparsky, 1976) and the new sesquiterpenoid caryophyllane-2,6-^-oxide (13) (von Roman Kaiser and Lamparsky, 1976a). Monographs with information on the acute oral and dermal toxicity, irritation, photo-toxicity, sensitization and regulatory status of verbena oil (L. citriodora) (Ford etal, 1992) and verbena absolute (Ford et al., 1992a) have appeared and it is recommended that the essential oil not be used in fragrances (Ford et al, 1992).

L. dulcis Trev. is under extensive investigation as a source of the intensely sweet bis-abolane sesquiterpene ( + )-hernandulcin (14) (Compadre etal., 1985, 1986; Sauerwein etal., 1991, 1991a) (Figure 5.2) which is the prototype of a new class of sweetening

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