Chemistry of the Genus Plectranthus

This review presents the phytochemical constituents of the genus Plectranthus reported up to 1999. Only a tetrameric derivative of caffeic acid was isolated from P. japonicus, but a group of long-chain alkylphenols, of possible taxonomic significance in the genus, was also isolated. As a genus of the subfamily Nepetoideae, Plectranthus is free from iridoid glycosides and rich in essential oil (i.e. > 0.5% volatile oil on a dry weight basis). Diterpenoids are the more common secondary metabolites in Plectranthus. The majority of them are highly modified abietanoids. This seems to be similar to the pattern of diterpenoids observed for Salvia, but no clerodane diterpenoids were found in Plectranthus.


Introduction
Labiatae is a large family that occurs worldwide and has species that are adapted to almost all habitats and altitudes. The genus Plectranthus L' He'r. belongs to subfamily Nepetoideae of tribe

Essential oils
Plectranthus is one of the oil-rich genera belonging to the subfamily Nepetoideae [17]. Table 1 lists Plectranthus species that have been investigated for essential oils. The main constituents of essential oils of Plectranthus are mono-and sesquiterpenes. For example, constituents of essential oil of P.

Conclusions
Although the genus Plectranthus comprises many plants of medicinal and economic interest [80], its chemistry remains poorly known. Caffeic acid and its derivatives are of widespread occurrence in the Labiatae family and of particular attention as chemotaxonomic markers. Chlorogenic acid appears to be of almost universal occurrence within this family, whereas rosmarinic acid is restricted to the subfamily Nepetoideae [81]. Only a tetrameric derivative of caffeic acid was isolated from P. japonicus [36]. But a group of long-chain alkylphenols, of possible taxonomic significance in the genus, was isolated [28,29]. Generally, the subfamily Lamioideae is rich in iridoid glycosides, whereas they are absent from the Nepetoideae [82]. No iridoid glycosides were isolated from Plectranthus.
Generally, Plectranthus species are essential-oil-rich (i.e. > 0.5% volatile oil on a dry weight basis), in agreement with the general situation that the Nepetoideae are oil-rich, whilst the Lamioideae are oilpoor [83].
Diterpenoids are the more common secondary metabolites in Plectranthus. The majority of them are highly modified abietanoids, in addition to some phyllocladanes and ent-kaurenes. It seems to be similar to the pattern of diterpenoids of Salvia [84], but no clerodane diterpenoids were found in