Monoterpenes iridoids

Among the first synthetic achievements, based on the use of tricyclooctanones, are the following preparations of mono- and sesqui-terpenes. Four members of the monoterpene iridoid family have been obtained by a single approach (Scheme 17). The disadvantage of the stereo/io/iselective reduction of (21b), affording (82) and (83) in a 1 1 ratio, is compensated for by the benefit of simultaneous access to four natural products ( )-boschnialactone and the epimer (84), this being a suitable precursor to attain ( )-al-... 

Many species of Castilleja (Scrophulariaceae) contain iri-doid monoterpenes (Arslanian et al, 1985) as well as pyrrol-izidine, quinolizidine, and monoterpene iridoid alkaloids (Stermitz et al., 1986a). Both pyrrolizidine and quinolizidine alkaloids appear to be taken from the hosts of these hemipar-asitic plants see Chapter 30). The larvae of Platyptilia pica (Pterophoridae) excrete but do not sequester iridoids. The adult moths contained rhexifoline, an iridoid monoterpene alkaloid (Stermitz et al, 1986a). 

The stems and leaves of Castilleja rhexifolia (Scrophula-riaceae) contain senecionine (5) and its A -oxide (obtained from Senecio triangularis) as their main alkaloids. The major alkaloid of the blossoms and seeds is rhexifoline, an iridoid-monoterpene-derived alkaloid, which is found in many Castilleja species. These plants serve as a host for the plume moth, Platyptilia pica. The larvae primarily eat the green seeds. Both the larvae and the adults of this insect contain rhexifoline (Roby and Stermitz, 1984). Plants of C. hispida contain quinolizidine alkaloids, and occasional plants of C. sulphurea contain quinolizidine alkaloids, whereas those of C. occidentalis do not contain alkaloids other than rhexifoline. The iridoid monoterpene content of all these alkaloids is similar. The larvae were found to excrete the monoterpene iridoids (Stermitz et al., 1986). 

In addition to Camptotheca acuminata (Nyssaceae), camptothecine (6) has been found in Nothapodytes (Mappia) foetida and Merilliodendron megacarpum (Icacinaceae), Ophionhiza mungos (Rubiaceae), and Ervatamia heyneana (Apocynaceae) (Cordell, 1981). This distribution is of considerable interest to plant systematists and phylogenists, as it reflects the same dichotomy seen in the distribution of other types of monoterpene iridoids. 

Isopentenyl pyrophosphate Hemiterpenes (isoprene). Monoterpenes (iridoid components of volatile oils). Sesquiterpenes (bitter principles, components of volatile oils). Diterpenes (components of resins, gibberellins, phytol). THterpenes (squalene, sterols, etc.). Tetraterpenes (carotenoids, xanthophylls). Polyterpenes (caoutchouc, gutta percha). 

This is an example of a bicyclic ring system with the fusion of benzene and pyridine rings. Biosynthetically they are related to indole alkaloids since both groups are derived from the same two precursors, tryptophan and loganin—a monoterpene iridoid (Samuelsson 1992). 

This is a very large group of alkaloids whose basic structure contains a pyrrole ring fused to a benzene ring. Biosynthesis of pure indoles involves the amino acid tryptophan as precursor, while that of a major subgroup of the indoles, including the Catharanthus and Rauwolfia alkaloids, involves a second precursor— the monoterpene iridoid loganin. 

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