Monoterpenes, biogenesis

During the past two decades a great number of papers have been published on the isolation, structure elucidation, synthesis and transformation, biogenesis, chemotaxonomy, and pharmacology of indole alkaloids. In this chapter we summarize the new results that appeared from 1968 to mid 1984 for the cory-nantheine-yohimbine group of monoterpene indole alkaloids with greater emphasis on their chemistry, excluding the related oxindoles and heteroyohimbines. 

The knowledge of enantiomeric composition of monoterpenes in essential oils is important from a scientific as well as from a practical point of view. It can give information related to chemotaxonomy, biogenesis and biosynthetic relations between compounds. It can also be useful in quality control genuines of essential oils. In some cases it can also answer the questions of applied extraction techniques and geographic origin of the extracted plant. 

Thus, it has been shown that the biogenesis of the thujane series of monoterpenes requires two 1,2-hydride shifts in the interconversions of the ions derived from a-terpinol and terpinen-4-... 

The book by Mann et al. (1994) on natural products provides a good introduction to the biogenesis of natural perfume ingredients and the review by Croteau (1987) gives further detail on the biosynthesis of monoterpenes. 

Rapp, A., Mandery, H., and Ullemeyer, H. (1984). New monoterpenoic alcohols in grape must and wine and their significance for the biogenesis of some cyclic monoterpene ethers, Vitis, 23,84-92. 

The biogenesis of this family of compounds constitutes a separate chapter in the metabolism of monoterpenes, and does not involve the carbocationic Ruzika s scheme but consists perhaps of an aldolic condensation-type process on an oxidated derivative of geraniol. 

The biogenesis of methylcyclopentanoid monoterpenes has been carefully studied allowing the key steps of the process to be depicted (ref.s 11-13). 

Secologanin is probably one of the best known secoiridoids, being the monoterpenic key intermediate in the biogenesis of indole and related alkaloids. 

Terpenoid Indole Alkaloids.—Monoterpenes have been the subject of a comprehensive review which includes these alkaloids. Indole alkaloids have been classified according to their biogenesis and another review discusses the application of tritium labelling in biosynthetic studies on these alkaloids as well as several others. 

C oH,tO, Mr 152.24, oil. The (R)-enantiomer [a]o -13.6° (CH3OH) is a pheromone of the bark beetle Ips confusus, a pest of Pinus ponderosa, while the (5)-enantiomer is a pheromone of Ips paraconflisus. 2,3-Dihydro-I. [ipsenol, C oH,gO, Mr 154.25, bp. 86 - 88 °C (1.99 kPa)] is also a pheromone of bark beetles of the genus Ips. Not only the ratio of I. to ipsenol but also the respective enantiomer ratios are species-specific and are used by the beetles to distinguish between species. With regard to the biogenesis not only the oxidation of the monoterpene hydrocarbon myr-cene but also de novo synthesis seem to be involved. A survey of the numerous pheromones from various bark beetles is given in Lit.. I. is also found in plants see also pheromones. 

The parent compound of the secoiridoids is secologa-nin (see secoiridoids), the most important intermediate in the biogenesis of alkaloids that are not derived from an amino acid. This includes most indole alkaloids, the ipecac, the Cinchona, and the pyrroloquinoline alkaloids as well as simple monoterpene alkaloids The best known biological property of the I. is their bitter taste. However, bitter principles are mostly not used as pure substances, instead alcoholic extracts are preferred to stimulate appetite (increased secretion of gastric juice). Furthermore, bitter substances are used to modify the taste of pharmaceutical products. Some I. exert various effects on the central nervous system as a consequence of their volatility and lipophilicity, e.g., nepetalactone, iridodial, teucrium lactones, and valepotriates. 

Irving, R. S. and R. P. Adams, Genetic and biosynthetic relationships of monoterpenes, in Terpenoids Structure, Biogenesis and Distribution (V. C. Runeckles and T. J. Mabry, eds.). Recent Advances in Phytochemistry Vol. 6,187-214, Academic Press, New York, 1973. 

Mono- and sesquiterpenes are the main constituents of the essential oils, while the other terpenes are constituents of balsams, resins, waxes, and rubber. Oleoresin is a roughly equal mixture of turpentine (85 % Cio-monoterpenes and 15 % C15- sesquiterpenes) and rosin (C2o-diterpene) that acts in many conifer species as a toxic material to invading insects and their pathogenic fungi [12]. A number of angiosperm species have inducible terpenoid defensive compounds (phytoalexins) [13]. These include both sesquiterpenoid and diterpenoid t)pes. Isoprenoid units are also found within the framework of other natural molecules. Thus, indole alkaloids, several quinones (vitamin K), alcohols (vitamin E, vitamin A formed from p-carotene), phenols, and isoprenoid alcohols (also known as terpenols or polyprenols) also contain terpenoid fragments. The biogenesis. 

The biogenesis of the thujane family of monoterpenes involves a homoallylic cyclization of the 4-terpinenyl carbonium ion (36) to a cyclopropylcarbinyl cation. Tracer experiments seem to be in accord with this scheme (56, 100). 

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