Iridoids aglycon

The iridoid natural products contain a functionally and stereochemically rich scaffold, which has inspired chemists to design a diverse range of strategies for their construction. Biichi and coworkers reported the first total synthesis of the iridoid aglycone ( )-genipin (25) in 1967 (Scheme 5). Pivotal to... 

The diversity of biological activity exhibited by several iridoids has generated much interest for their synthesis. Due to difficulties of glycosidation of iridoid aglycones, only a few synthetic approaches in this area have been reported. Some of these synthetic routes for stereoselective synthesis of natural iridoids are discussed here briefly to make it inspiring among the synthetic chemists for their future plans. 

Application of the "Ireland-Claisen" rearrangement to the D-xylal derived (25) has given access to compounds (26) which have been used (Scheme 14) in novel syntheses of iridoid aglycones... 

Glycosides can be classified on the basis of the structural types of aglycone present in the glycoside. For example, in anthraquinone, flavonoid, iridoid, lignan or steroid glycosides, the aglycones are anthraquinone, flavonoid, iridoid, lignan or steroid, respectively. 

Aucubin (iridoid Aucubajaponica (Cornaceae), Forms reactive aglycone... 

Two-phase solvent systems composed of chloroform-methanol-water at various volume ratios are frequently used for the separation of moderately hydrophobic compounds, including flavone aglycones, phenylpropanoids, iridoid glycosides, and so forth [6]. 

The appreciable synthetic potential of the de Mayo reaction has been applied to the preparation of polycyclic natural products. Some recent developments are shown in Table 4. The de Mayo reaction was used as the key step in the synthesis of hirsutene (entry 1), fusicoccin H aglycone (entry 2), taxanes (entries 3 and 4), loganin (entries 8 and 9), sarracenin (entry 10) and iridoid intermediates (entry 11). 

In conclusion, we could say that on a wide sense, the dialdehydes generated by the metabolism and hydrolysis of the aglycones of iridoids, could be in part responsible for the described activities. The non-saturated carbonyl group can react with the SH groups, from amino acids and nucleic acids, causing different effects as the cellular growth inhibition [38,77]. 

It seems that the pharmacological activities showed by different iridoids are due to their genine, like aucubin. The structural similarity between the aglycone and glutaraldehyde suggests a similar mechanism of enzyme inhibition through protein cross-linking by Schiff reactions. 

BoschnaiosMle. B. belongs to the iridoid glucosides with a C,o aglycone possessing an aldehyde function. C15H24O8, Mr 344.36, amorphous, [a]o -104°... 

Lamiide. An iridoid glucoside with a C,o aglycone occurring bound to various aromatic carboxylic acids. 

The iridoid glycosides geniposide (38) and geniposidic acid (37) and their aglycones have have been shown to inhibit growth of wheat embryos (Cameron et al., 1984). 

A number of the pharmacological activities of iridoids are due to the aglycone, because the activity appears only after treatment with p-glucosidase, or only when the pure aglycone is tested (Stermitz, 1988). 

Stermitz, F. R., Iridoid glycosides and aglycones as chiral syn-thons, bioactive compounds, and lepidopteran defenses, in Biologically Active Natural Products Potential Use in Agriculture (H. G. Cutler, ed.), ACS Symposium Series 380, 397-402, American Chemical Society, Washington, DC, 1988. 

Various iridoids have been identified as antimicrobial, laxative, choleretic and hepatoprotective—especially picroliv (Sticher 1977 Visen et al. 1993). While anti-inflammatory activity has been identified in aucubin and others, in most cases it is relatively weak. In the case of harpagoside this may depend on the degree of hydrolysis that occurs in the gut since the aglycone (harpagogenin) was found to be less active than the glycoside itself (Redo et al. 1994). 

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