Secoiridoids, monoterpene alkaloids

G. In Vitro Conversion of Iridoids and Secoiridoids to Monoterpene Alkaloids... 

The principal groups of natural products belonging to this family of monoterpene derivatives are glycosidic and non-glycosidic iridoids ( see fig 1, refs 5-7 ), together with related compounds secoiridoids and bisiridoids, and monoterpene alkaloids ( see fig 2). 

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. 

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... 

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

The procedure has been utilised to synthesise the hydroxyloganin, the key ring intermediate in the metabolism of iridoids to secoiridoids and subsequently to indole and related alkaloids having this monoterpenic part. 

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. 

The order Gentianales (Apocynaceae, Gentianaceae, Lo-ganiaceae, Rubiaceae, and Theligonaceae) contains normal iridoid monoterpenes and secoiridoids, as well as iridoid-derived alkaloids in families other than the Asclepiadaceae. Multiple types of alkaloids are found in the Apocynaceae (indole and steroidal types), the Loganiaceae, and Rubiaceae (emetine, quinine, and indole types). 

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