Glycosides, in nature

Lechtenberg, M. and Nahrstedt, A. (1999) Cyanogenic glycosides, in Naturally Occurring Glycosides (ed. R. Ikan) John Wiley Sons, Ltd, Chichester, UK, pp. 147-191. 

The raison d etre for the proliferation of flavone and flavonol glycosides in nature continues to intrigue plant scientists. The ability of UV-B radiation to damage DNA, RNA, and proteins as well as to impair processes like photosynthesis is well known. Most flavonoids are very efficient antioxidants and chemoprotectants, and are therefore used in a plethora of food supplements and nutraceuticals [30]. Some flavonoids, however, have a Janus face - they could be excellent antioxidants but also have negative features. 

The raison d etre for the proliferation of flavone and flavonol glycosides in nature continues to intrigue plant scientists. The ability of UV-B radiation to damage DNA, RNA and proteins as... 

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. 

Having brought home from Berlin Fischer s traditions and an overriding interest in natural products, particularly carbohydrate and glycoside chemistry, Zemplen established and ran his laboratory very much in the Fischer style, adding, however, his personal touches. The laboratories had tall ceilings and large windows (not unlike the Fischer... 

Several other naturally occurring antioxidants have been identified in oils. Sesamol [533-31-3] (6) occurs as sesamoline [526-07-8], a glycoside, in sesame seed oil. FemUc acid [1135-24-6] (7) is found esterified to cycloartenol [469-38-5] in rice bran oil and to 3-sitosterol in com oil. Although it does not occur in oils, rosemary extract has also been found to contain powerful phenoHc antioxidants (12). 

Sahcyl alcohol [90-01-7] (saligenin, o-hydroxybenzyl alcohol) crystallizes from water in the form of needles or white rhombic crystals. It occurs in nature as the bitter glycoside, saUcin [138-52-3] which is isolated from the bark of Salix helix S. pentandra S. praecos some other species of willow trees, and the bark of a number of species of poplar trees such as Folpulus balsamifera P. candicans and P. nigra. 

Triterpenes. The triterpenes (30 carbon atoms) are widely found in nature, especially plants, both in the free state and as esters or glycosides. A smaller but important group, including lanosterol [79-63-0] (114), occurs in animals. The triterpene hydrocarbon, squalene [111-02-4] (115), occurs in the hver oils of certain fish, especially those of sharks. 

By far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures. 

Replacement of heterocyclic rings in nucleosides by ring systems which do not occur in nature represents another approach to compounds which may have activity against viral and neoplastic diseases. One of the early successes in this category involves replacement of a pyrimidine ring by a triazine. The synthesis starts with a now classical glycosidation of a heterocycle as its silylated derivative (146) with a protected halosugar (145), in this case a derivative of arabinose... 

Particularly in naturally occurring glycosides, the compound ROH from which the carbohydrate residue has been removed is often termed the aglycone, and the carbohydrate residue itself is sometimes referred to as the glycone . 

Davoll, J., Lythgoe, B., Todd, A.R. (1946) Experiments on the Synthesis of Purine Nucleosides. Part XII. The Configuration of the Glycosidic Centre in Natural aud Syuthetic Pyrimidine and Purine Nucleosides. Journal of the Chemical Society, 833-838. 

---