Iridoid monoterpene biosynthesis

OLDHAM, N.J., VEITH, M., BOLAND, W., Iridoid monoterpene biosynthesis in insects evidence for a de novo pathway occurring in the defensive glands of Phaedon armoraciae (Chrysomelidae) leaf beetle larvae, Naturwissenschaften, 1996,83,470-473. 

Since the biosynthesis of iridoid monoterpenes in insects has been shown to... 

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. 

Fig. 20.4. Biosynthesis of nonglycosidic iridoid monoterpenes derived from 10-hydroxycitronellol (Inouye and Uesato, 1986 Jensen, 1991 modified and used with permission of the copyright owner. Springer-Verlag, Vienna).

Plants in the Comanae, Gentiananae, and Loasanae, in general, contain both normal iridoid monoterpenes and secoiridoids. Plants in the Lamianae contain not only many normal iridoid monoterpenes but also those in which C-11 is decarboxylated. They lack secoiridoids. Plants of the Ericanae do not exhibit a clear pattern. Iridoid compounds with 8-P-stereochemistry were assumed to derive from pathways associated with secoiridoid biosynthesis, whereas those with 8-a-stereochemistry were associated with the route involving C-11 decarboxylation (Jensen, 1991). Some exceptions... 

Coscia CJ, Botta L, Guamaccia R (1970) On the mechanism of iridoid and secoiridoid monoterpene biosynthesis. Arch Biochem Biophys 136 498-506... 

Secologanin (1) of Fig. (1) is the most important representative of the iridoids (la), a special type of cyclopentanoid monoterpenes. (1) belongs the subclass of secoiridoids (lb), in which the bond C-7-C-8 is cleaved [1]. The number of known iridoids is more than 650 [2], The biosynthesis of iridoids like that of other monoterpenes starts from two C5 units (lc) whose precursor was considered to be mevalolactone [3], However, recently it was proved that the biosynthesis of some classes of terpenoids [4,5], and specially of secologanin [6] runned from 1-deoxy-D-xylulose-5-phosphate as an alternative or even main route. 

CioH.jN, Mr 147.22, oil, bp. 100-103 °C(1 kPa),[a]i, -7.2° (CHCI3), a monoterpene alkaloid ( iridoids) from the east Asian creeper Actinidia polygama and other Actinidiaceae, Bignoniaceae, and Valerianaceae. It is a component of the defensive secretion of black beetles, ants, grasshoppers, and flies. Only the (5) enantiomer occurs in nature. A. exhibits antimicrobial properties and a deterrent effect against birds and arthropods , while cats are strongly attracted Synthesis There are various syntheses of the racemate and the unnatural (/I) enantiomer, but only three for the (5) enantiomer. The biosynthesis presumably proceeds from iridodial. 

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. 

Monoterpenes and iridoids are important classes of secondary metabolites, and their biosynthesis as well as total synthesis approaches to access these compounds has attracted the interest of chemists for a long time. This overview will summarize the most important biosynthesis pathways and in addition will illustrate how one of the most important recent developments in organic chemistry, asymmetric organocatalysis, contributed in the development of total synthesis approaches toward these compounds. Therefore, a comprehensive introduction to asymmetric organocatalysis will be given as well. However, it should be noted that the applicability of this methodology to the synthesis of natural products is of course not only limited to monoterpenes and iridoids but holds much promise for other classes of primary and secondary metabolites as well. 

Loganin is an iridoid glucoside which occupies a central position in the biosynthesis of Corynanthe, Aspidosperma, Iboga, Ipecacuanha, Cinchona, and structurally simpler monoterpene alkaloids. For a review of the extensive researches that led to confirmation of the key role of loganin in alkaloid biosynthesis, see A. R. Battersby, Biochem. Soc. Symp., 29, 157 (1970) A. R. Battersby, Chem. Soc. Spec. Period. Rep., 1, 31 (1971) A. I. Scott, Accts. Chem. Res., 3, 151 (1970). For a useful account of the chemistry of iridoid glucosides see, J. M. Bobbitt and K. -P. Segebarth in Cyclopentanoid terpene Derivatives, Eds., W. I. Taylor and A. R. Battersby, Marcel Dekker, New York, 1969, p 1. 

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