Biosynthesis of iridoids

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

Loganin is the intermediate that undergoes the ring opening reaction which leads to the formation of secoiridoids. This step occurs by a mechanism that remains to be elucidated and affords secologanin (31) as the precursor of all of the secoiridoids and consequently, of the indole alkaloids that incorporate this pattern [1, 2, 3]. General biosynthesis of iridoids is given in fig. 1. 

Relevant products in the biosynthesis of iridoids such as loganin or epiloganin are frequently detected by the presence of acyl derivatives, even though they certainly are in all species that contain iridoids. From the taxonomic point of view, both the nature of the acyl moiety and the position of the acylation can be used as indicators. Though the enzymatic machinery and the metabolic enviromnents provide collections of natural products which characterize the genus, families and other taxons... 

The starting compound for the biosynthesis of iridoids is the mevalonic acid, and the first step seems to be common to all the structures, but they divert from the cyclization step onwards, where the iridane skeleton is formed. Dahlgreen [11] proposed three routes based on biosynthetic data and by an analysis of taxons of the reported iridoids (see scheme 1). 

Many reviews have dealt with the distribution, structure, properties and biosynthesis of iridoids and secoiridoids [131-137] without distinguish between the biosynthetic pathway route. In our case, some of the most studied activities, as anti-inflammatory, antitumoral-chemopreventive, and protective by iridoids biosynthesized by route IIa, have been discussed. 

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. 

Thus, contrary to earlier expectations, various routes leading to the iridane skeleton in the biosynthesis of iridoids and secoiridoids are not identical (Inouye and Uesato, 1986). 

Inouye, H. and S. Uesato, Biosynthesis of iridoids and secoiri-doids, Fortschr. Chem. Org. Naturst., 50, 169-236 (1986). 

Contents L. Jaenicke and F.-J. Marner The Irones and Their Precursors. — M. Lounasmaa and P. Somersalo The Condylocarpine Group of Indole Alkaloids. — U. Sequin The Antibiotics of the Pluramycin Group (4//-Anthra [l,2- ]pyran Antibiotics). — R. M. Wenger Cyclosporine and Analogues — Isolation and Synthesis — Mechanism of Action and Structural Requirements for Pharmacological Activity. — H. Inouye and S. Uesato Biosynthesis of Iridoids and Secoiridoids. 

Several groups carried out tracer studies for the understanding of the biosynthesis of iridoid glycosides. Now we have reached to a point from which a fairly complete... 

Recently, Li et al. also studied the biosynthesis of iridoid glucoside lamalbid (106) in Lamium harbatum using C-labeled intermediates of MEP and MVA pathways, namely, [3,4,5- C3] 1-deoxy o-xylulose 5-phosphate and [2- Ci] mevalonolactone (MVL), and observed that only the former was incorporated into lamalbid (Scheme 97.12) [132]. 

Scheme 97.11 Proposed sequences for biosynthesis of iridoids in Fraxinus, Syringa, Forsythia, and Catharanthus species...

Damtoft S, Franzyk H, Jensen SR (1994) Biosynthesis of iridoids in Forsythia spp. Phytochemistry 37 173-178... 

Uesato S, Ueda S, Kobayashi K, Inouye H (1983) Mechanism of iridane skeleton formation in the biosynthesis of iridoid glucosides in Gardenia jasminoides cell cultures. Chem Pharm... 

Biosynthesis of iridoids has been studied in much detail, essentially in connection with the biosynthesis of indole alkaloids (29, 342). In the case of loganin (19)... 

Some further features of the biosynthesis of iridoid glycosides... 

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