Natural glycoside

When cyclization occurs between an electron-deficient allene and a ketone, as is the case with 27, butenolides are produced [19]. Given the importance of this functional group in cardiac glycoside natural products, the simplicity of the starting materials, and the facility of the cathodic cyclization, it may be worth noting the opportunity to utilize this transformation in the construction of these natural products. 

APPLICATION OF LC-NMR-MS TO GLYCOSIDIC NATURAL PRODUCTS OF MARINE ORIGIN... 

The complementary structural information of both NMR spectroscopy and MS detection is particularly valuable for the analysis of closely related glycosidic natural products, where compounds of the same molecular mass (isobars, see Table 5.1.1), and even with identical MS/MS fragmentation patterns, frequently occur, as this example demonstrates. 

The polyketide-derived aryl C-glycoside natural products have a variety of interesting biological activities, most notably antitumor activity (7). The defining feature of members of this class is a carbon-carbon bond linking a polycyclic aromatic "aglycone" and the anomeric carbon of a carbohydrate (Figure 1). 

The conversion of vicianose [6-(L-arabinosyl)-D-glucose] to the calcium vicianobionate was effected by oxidation with bromine in the presence of calcium carbonate. Bertrand and Weisweiller" then hydrolyzed the aldonic acid and obtained n-gluconic acid and i/-arabinose, as evidence of the glycosidic nature of the L-arabinose unit in the disaccharide. 

Twenty kflograms of finely powdered, dry conns of C. autumncde were extracted first at room temperature and then at higher temperatures (max. 70°). A total of 60 1. of ethanol was used in the series of extractions. The extract was concentrated on a water bath and finally under reduced pressure at 100°. The oily concentrate was diluted to a volume of 4 1. with water and washed four times with 2-1. portions of ether. The ether extract was washed with 300-ml. portions of water, 3% hydrochlorie acid, water, 4% sodium carbonate, and finally with two 300-ml. portions of water. The aqueous solution was acidified with hydrochloric acid to pH 2-3 and extracted six times with 500-ml. portions of chloroform. Concentration of the chloroform extract afforded 48 g. of neutral and phenolic, chloroform-soluble materials. The aqueous solution was basified with ammonium hydroxide and extracted fom times with SOO-ml. portions of chloroform to yield 14.4 g. of basic, chloroform-soluble substances. The remaining aqueous solution was neutralized to pH 7 with hydrochloric acid (cooling) and concentrated to a volume of 1 1. Five extractions of this concentrate with 500-ml. portions of chloroform-ethanol (2 1) afforded 4.5 g. of material, probably of glycosidic nature. 

Das demonstrated that oral administration of labeled 3-0-methylcatechin to three volunteers was followed by plasma peak levels within 2 hours after administration [69]. Hollman et al. studied the absorption of different glycosides of quercetin and the quercetin aglycone in nine healthy ileostomy subjects to avoid losses caused by colonic bacteria [30-31]. The absorption after oral administration decreased in the following order 52% for quercetin glucosides from fried onions > 24% for quercetin aglycone > 17% for quercetin nitinoside [31]. Consequently, humans absorb considerable amounts of quercetin, but the absorption depends on the glycosidic nature of the flavonoid [31]. 

Parker KA (1994) Novel methods for the synthesis of C-aryl glycoside natural products. Pure Appi Chem 66 2135-2138... 

The actual mechanisms for the formation of the glycosidic bond(s) between the two or more monosaccharide residues cannot be accomplished by the simple Fischer type of synthesis that is used for the formation of the alcohol glycosides. Nature uses relatively elaborate, enzymatically catalyzed reactions with high-energy, activated monosaccharide units (sometimes di-, tri-, and tetrasaccharide units) to achieve the synthesis of these combined monosaccharides (see Chapter 10 for some of these biological syntheses). 

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