Monoterpene secologanin

Biosynthesis A. is formed from tryptamine and the monoterpene secologanin (see secoiridoids) via stric-tosidine, catalysed by ca. 10 known enzymes (see also figure under monoterpenoid indole alkaloids). 

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. 

Ipecac alkaloids are derived from the amino acid tyrosine and the monoterpene secologanin and are therefore termed terpenoid-isoquinoline alkaloids. They occur in the eudicot families Alangiaceae and Rubiaceae. Two species, Psychotria ipecacuanha (Rubiaceae) and Alangium lamarckii (Alangiaceae), have been investigated in detail with respect to their metabolites and biosynthesis of their alkaloids (Fujii and Ohba, 1998). Roots and rhizomes of P. ipecacuanha are the source of cephaeline and emetine (Fig. 9), two compounds with emetic, expectorant, and amebicidal properties. 

The formation of the first committed intermediate, strictosidine, requires to coordinate two sources of precmsor biosynthesis the MEP pathway which provides the monoterpene secologanine, and the tryptophane decarboxylase reaction which yields tryptamine (Fig. 7). 

Within the natural products field, the investigation of complete biosynthetic pathways at the enzyme level has been especially successful for plant alkaloids of the monoterpenoid indole alkaloid family generated from the monoterpene gluco-side secologanin and decarboxylation product of tryptophan, tryptamine [3-5]. The most comprehensive enzymatic data are now available for the alkaloids ajmalicine (raubasine) from Catharanthus roseus, and for ajmaline from Indian Rauvolfia serpentina [6] the latter alkaloid with a six-membered ring system bearing nine chiral carbon atoms. Entymatic data exsist also for vindoline, the vincaleucoblastin (VLB) precursor for which some studies on enzymatic coupling of vindoline and catharanthine exist in order to get the so-called dimeric Catharanthus indole-alkaloids VLB or vincristine [7-9] with pronounced anti-cancer activity [10, 11]. 

Strictosidine is produced, stereospecifically, from tryptamine and secologanin by strictosidine synthase, isolated from several species producing monoterpene indole alkaloids. The enzyme was cloned and can be expressed in large quantity (Fig. 37). 

Gentiananine (93) represents the first of a new series of monoterpene alkaloids in which the pyridine ring is fully substituted. Biogenetically, the alkaloid may arise from a secologanin precursor through complete opening,... 

The terpenoid portion of TIAs is derived from secologanin, whose monoterpene precursor geraniol, is produced by the recently discovered Rommer or triose phosphate / pyruvate pathway, responsible for the synthesis of isoprenes like geraniol in the plastids [58-60]. 

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

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. 

Secoiridoid glycosides are monoterpenes that are derived from secologanin, their biosynthetic precursor. They represent the bitter principles of many plant families, especially in the Comales, Dipsacales and Gentianales orders. They may occur as esters with different acids, and especially with biphenylcarbonic acids, which enhance strongly their bitter taste. Plant containing these compounds are used in many herbal preparations for their stomachic properties. 

The parent compound of the S. is secologanin, a key compound in the biosyntheses of most indole alkaloids, the Cinchona, ipecac, and pyrroloquinoline alkaloids as well as simple monoterpene alkaloids. Over 1000 indole alkaloids are formed from secologanin in vivo - comprising almost a quarter of this large group of natural products. 

Emetine and cephaeline possess two phenylethylamine units derived from phenylalanine, and a central structure consisting of nine carbons of a monoterpene unit, which is also found in the ajmaline (Seaion 2.10), strychnine (Section 2.14), and quinine (Section 2.17) alkaloids.This partial structure is derived from geraniol through secologanin by the loss of one carbon, as was established in the case of cephaeline [1]. 

Formation of Deoxyloganic Acid from Geraniol Formation of 8-e/ i-Deoxyloganin and Related Iridoids Iridoids Derived from 10-Hydroxycitronellol Conversion of Loganin to Secologanin Secologanin as a Precursor for Indole Alkaloids Distribution of Iridoid Monoterpenes Biological Activity... 

Loganin (6) is a key intermediate in the biosynthetic pathway leading to other iridoid monoterpenes as well as complex indole alkaloids. Many of these additional compounds are derived by conversion of this bicyclic iridoid monoter-pene into secologanin (18). The mechanism by which ring cleavage occurs to yield secologanin is not well understood, but apparently the cleavage happens after oxidation of the... 

A third major group, including emetine and a series of structurally related alkaloids, involves condensation of the same amine precursor (3,4-dihydroxyphenylethylamine or dopamine) with a carbonyl group from secologanin—an iri-doid monoterpene precursor. Most of these compounds have been isolated from two families, the Rubiaceae and the Alan-giaceae. 

Monoterpenes Constituents of essential oils, e.g., geraniol, menthol, and thymol, and of iridoids, e.g., loganin and secologanin... 

A number of important alkaloids are derived from monoterpenes. Monoterpenoid alkaloids are comparably simple compounds formed by addition of nitrogen atom to an existing monoterpene, mainly of the secologanin type. Several monoterpenoid alkaloids have been isolated and characterized and classified based upon their structural characteristics. The more common and pharmaceutically important types such as actinidine, skytanthine, and gentianine are discussed herein. 

When the aldehyde partner of the Pictet-Spengler reaction with tryptamine is the terpene secologanin, strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32, 33]. Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde, while imin-ium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from car-bocyclization). These alkaloids are referred to as from the Corynanthe type, with the monoterpene carbon skeleton unmodified. Although it misses one carbon and has a very... 

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