Iridoids, Monoterpenoid Alkaloids

Cyclopentanes, Iridoids, Monoterpenoid Alkaloids. ( —)-Bromodihydro-umbellulone (5) ( —)-isoiridomyrmecin (6) ( + )-iridomyrmecin (7) at — 150 loganin penta-acetate Rionomethyl ether bromide the rubidium 

Masaki, M. Hirabayashi, K. Fuji, K. Osaki, and H. Inouye, Tetrahedron Letters, 1967, 2367. 

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Geraniol 10 hydroxylase catalyzes the the cytochrome P450 dependent hydroxylation of geraniol at the C-10 position to commit this substrate to the formation of iridoid monoterpenoids (Fig. 8.2). In Catharanthus roseus, this intermediate is converted to secologanin for producing the tryptamine containing monoterpenoid indole alkaloids characteristic of this plant. 

Different species of Lonicera have been chemically investigated and various iridoids, bis-iridoids, sulfur containing monoterpenoids, alkaloids,... 

A 4-nor-7,8-dehydro-10-hydroxy-skytanthine (10) has been obtained from A. radiata [22]. Compotmd 10 and actinidine-type compound 5 are the alkaloid compounds that have been isolated from this plant species for the first time, and it is a reasonable hypothesis that the monoterpenoid structures of 10 and 5 should be derived from the previously isolated iridoids in this plant, which are known to be the biogenetic precursors of this type of alkaloids [50]. A novel skytanthine-type alkaloid named delavayine A (11) that comprises a benzoate ester has been isolated from 7. delavayi [25]. Similarly, another novel skytanthine-type alkaloid, incarviUateine E (12), possessing three incarvilline moieties has been isolated and characterized from the aerial parts of 7. sinensis [26]. Incarvine E (13) and incarvine F (14) [27] isolated from 7. sinensis and isoincarvilline (15) [28] and mairines A-C (16-18) [29] from 7. mairei are other new skytanthine-type monoterpenoid alkaloids. Compounds 16 and 18 contain a rare (5-skytanthine Af-oxide in the structure [29]. 

Monoterpenoid alkaloids are stmcturaUy related to iridoid compounds, the oxygen heterocycle being replaced by a nitrogen-containing ring [1]. A possible biosynthesis of actinidine and jS-skytanthine starting from geraniol is proposed in Scheme 28.1. 

Information is needed regarding the biosynthesis and catabolism of alkaloids in each taxon if scientists are going to gather some meaningful information on the chemotaxonomy of the distribution of alkaloids. An example is provided by the closely related iridoid compounds, which include methylcyclopentane monoterpenoid alkaloids and indole alkaloids (Hegnauer, 1967 Waller, 1969 Gross, 1969 Bisset, 1975). They present a rather complicated picture of alkaloid (and terpenoid) distribution among many families. ... 

Strictosidine synthetase catalyzes the stereospecific condensation of trypt-amine and the iridoid glucoside secologanin to form strictosidine. The product is the precursor of the monoterpenoid-derived indole and quinoline alkaloids. 

Iridoids are derivatives of monoterpenes and occur usually, but not invariably, as glycosides.100,101 Structurally, they are cyclopentano [r] pyran monoterpenoids and they provide a biogenetical and chemotaxonomical link between terpenes and alkaloids. The cleavage of the cyclopentane ring of iridoids produces secoiridoids.10... 

In the case of iridoids, they are ring opened to secoiridoids and are incorporated as monoterpenoid part of important groups of alkaloids indole, ipecacuanha, cinchona, pyrroloquinoline alkaloids and related compounds (ref 8 ). 

Gentianin (p-coumaiic acid ester of delphinidin 3-O-glucoside). C30H27O14, Mr 611.54, blue-violet powder. G. occurs in the flowers of blue gentian (Gentiana verna). The major representative of the Gentiana alkaloids, a monoterpenoid iridoid, also has the name gentianine. 

Isoprenoids (isopentenoids). The name for a group of natural products made up of isoprene units (e.g., ses-qui-, di-, and triterpenes, iridoids, carotinoids, steroids, natural rubber, etc.). Many non-isoprenoid compounds, however, do possess isoprenoid side chains, e.g., tocopherols, ubiquinones, chlorophyll, or contain isoprenoid structures incorporated into their skeletons, e.g., monoterpenoid indole alkaloids, penitrems, Cinchona alkaloids. 

Iridoids are a large and stmcturally diverse class of secondary metabolites of monoterpenoid origin [10, 11]. Their sttuc-tural parent system is the iridane skeleton, which is derived from geraniol (6) by a cyclization pathway that is mechanistically different to the cyclization reactions that are usually found in classical terpene chemistry [1, 12], Further enzymatic transformations then give the fundamental iridoid skeleton or the thereof derived secoiridoid motive (with an alternative connectivity as shown in Scheme 6.10), which often serves as a building block for the synthesis of more complex monoterpenoid indole alkaloids [1]. Noteworthy, hereby the isoprene mle is often not fulfilled anymore as the terpenoid parts of such compounds sometimes contain nine carbon atoms only due to a decarboxylation step somewhere in the sequence. Very often iridoids and secoiridoids are present as glycosides in nature. 

ORGANOCATALYSIS IN THE TOTAL SYNTHESIS OF IRIDOIDS AND MONOTERPENOID INDOLE ALKALOIDS... 

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