Abstract
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.
Highlights
Labiatae is a large family that occurs worldwide and has species that are adapted to almost all habitats and altitudes
In Plectranthus, the upper lip of the flower is unusually four-lobed and the large shoe-shaped lower lip is formed from a single lobe, while in Labiatae the upper lip often consisting of two lobes and the lower consisting of three [6]
Caffeic acid and its derivatives are of widespread occurrence in the Labiatae family and of particular attention as chemotaxonomic markers
Summary
Labiatae is a large family that occurs worldwide and has species that are adapted to almost all habitats and altitudes. The main phytochemical constituents of the genus Plectranthus are diterpenoids, essential oils and phenolics. The abietanoids, in turn, could be classified, according to structure variation, into royleanones (structures D1-D37), spirocoleons (structures D38-D66), vinylogous quinones ( named extended quinines, structures D67-D76), quinone methides (structures D77-D93), acylhydroquinones (structures D94-D117), (4→3) abeo-acylhydroquinones (structures D118, D119), phenolic abietanoids (structures D120-D122), 1,4phenanthraquinones (structures D123-D127), dimeric abietanoids (structures D128-D136) and secoabietanoids (structures D137-D139) Plectranthus species that have been investigated for essential oils. The main constituents of essential oils of Plectranthus are mono- and sesquiterpenes. From P. japonicus (= Rabdosia japonica), a tetrameric derivative of caffeic acid was isolated [36]
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