O- and C-Glycosides

Almost half of the isoflavonoids identified from the Leguminosae, in which prenylated isoflavones constitute >50% of the total (pterocarpanoids, rotenoids, isoflavanones, isoflavans, coumestans, 3-arylcoumarins, and coumaronochromones), exhibit a high degree of structural diversity and complexity of prenylation.8,86"148 More than half of the isoflavones are monoprenylated, whereas 40% are diprenylated, and very few are triprenyl derivatives, with prenylation at positions 6>8>3 5 >6,>2 and O-methylation at 4 >7=6=2 =3 >5 =5>8. A number of novel isoflavonoid structures with rare skeletons, as well as complex isoflavone O- and C-glycosides have recently been reported.8,148... 

Phytopolyphenols are a major class of popular phytochemicals and secondary plant metabolites that commonly exist as multiple O- and C-glycosidic derivatives but also may be present as aglycone. They are an important part of human dief and are also recognized as the active principle in a number of medicinal plants. ... 

Another interesting example of the preparation of unsaturated O- and C-glycosides 36 was presented recently [44]. This was based on the Sn2 attack of the nucleophile on unsaturated epoxide 35. Substitution at C-1 proceeded with simultaneous migration of the double bond and opening of the three-member ring providing 2,3-unsaturated derivative 36 (O Scheme 20). Conversion of 4-deoxy-2,3-unsaturated sugars into 3,4-unsaturated ones reported by Banaszek and Zamojski already in 1972 [3] was one of the key-steps in total s)mthesis of monosaccharides by a hetero Diels-Alder approach. 

Flavonoids and biflavonoids based on apigenin and kaempferol moieties were isolated together with O and C glycoside from methanolic extract of this plant... 

Cascara bark should contain not less than 8.0% of hydroxyanthracenes. Not less than 60% of this mixture must be in the form of cascarosides which are complex mixed O and C glycosides based on the anthrone nucleus. Loss of the O -glucose gives rise to secondary glycosides notably Aloin A-rB, which also are found in Aloes. 

The diazirino-mannose derivative 80 has been synthesized from the corresponding mannonolactone oxime (i, MsCl ii, NH3. MeOH iii, I2, EtjO). The use of this and other diazirino-sugars in die synthesis of O- and C-glycosides is covered in Chapter 3. oc-D-Glucopyranosyl-phenyldiazomethane 81 has been synthesized as a mechanism-based a-glucosidase inhibitor. ... 

The quinones are a rather heterogeneous collection of compounds with structures based on an unsaturated system of cyclic diketones. The biologically important plastoquinones, ubiquinones, and vitamin K are included in this group however, as pigments the most widespread and important quinones are the 1,4-naphthaquinones (Fig. 18) and the 9,10-anthraquinones (Fig. 19, Table 10). Methyl, methoxyl, and hydroxyl groups are the most common substituents, and O- and C-glycosides (see Fig. 20) are frequently present in the anthraquinone group. Several structural modifications exist due to reduction, dimerization (Fig. 21), and addition of side chains. 

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