Hemiterpene alkaloids

Introduction Hemiterpene Alkaloids Imidazole Alkaloids Biosynthesis... 

Another series of hemiterpene alkaloids is found in the genus Alchornea (Euphorbiaceae). In these alkaloids (1-4) the hemiterpene unit constitutes a major portion of the structure (Fig. 37.1) (Hart et al., 1970). 

IPP and DMAPP are reactive hemiterpene intermediates in the pathways leading to more complex terpenoid structures. They are also used as alkylating agents in the formation of meroterpenoids as indicated above, but examples of these structures are discussed under the section appropriate to the major substructure, e.g. alkaloids, shiki-mate, acetate. Relatively few true hemiterpenes are produced in nature, with isoprene, a volatile compound which is released by many species of plants, especially trees, being the notable example. Isoprene is formed by loss of a proton from the allylic cation (Figure 5.7). 

The restricted space available for these Reports unfortunately does not allow the discussion of a number of topics of interest to monoterpenoid chemists useful reviews of such topics include monoterpenoid alkaloids/ carbazole alkaloids/ isoprenoids and alkaloids of tobacco/" naturally occurring plant coumarins/ the biosynthesis of aromatic hemiterpenes/ and recent developments in the field of naturally occurring aroma components/ The poor quality of the Chemical Abstracts makes it difficult to assess the significance of a number of reviews of potential industrial interest/ A volume in the Methodicum Chimicum series includes a very brief discussion of some monoterpenoids/"... 

Biosynthesis In plants the quinoline system can be formed by several different biogenetic routes (biochemical convergence). In the Cinchona alkaloids tryptophan acts as the precursor while simple quinolines, furoquinolines, and acridines originate from an-thranilic acid. The second biosynthetic parmer is often a hemiterpene (for furoquinolines) or an iridoid monoterpene (secologanin, see secoiridoids) for Cinchona alkaloids. 

Many alkaloids contain an indole nucleus as a part of their structure. Monoterpene-derived indole alkaloids see Chapter 34) are the most important of these compounds, but several other significant types of indole alkaloids exist. Ergot alkaloids formed from tryptamine and a mevalonate-derived hemiterpene unit are of medical importance. Others, such as harmane alkaloids, physostigmine and a series of structurally similar alkaloids, Calycanthus alkaloids (Calycanthaceae) and related compounds (principally from the Rubiaceae), and relatively minor types of alkaloids from the Ascelpiada-... 

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