Standard glycosidation

This class of donor is activated by soft Lewis acids, such as copper triflate at room temperature, and despite their hydrolytic instability, they appear inert to conditions of sulfoxide activation, TMSOTf or Tf20 (Scheme 4.53). Activation is achieved with stoichiometric promoter in the presence of the acceptor alcohol, and although the mechanism has not been investigated, presumably it proceeds via coordination followed by collapse to a stabilized oxacarbenium ion. The method is compatible with standard glycosidation solvents such as dichloromethane, acetonitrile and diethyl ether, and ester-directed couplings do not lead to orthoesters, perhaps as a result of the presence of the Lewis acid promoter [303,304]. 

In the above examples dealing with acceptor diols, the donors have been fully protected, in keeping with standard glycosidation practice. However the RDAS principle suggests diat this may not (always) be necessary, and we have carried out some preliminary tests (59). 

Linking anthracycline aglycones to appropriate sugar derivatives by standard glycosidation procedures has also been used to synthesize 3 -hydroxyesorubicin,28 4 o-methylrhodomycins29-31 ... 

The sorbistin analogues (3) and (4) have been synthesized by coupling azido-precursors to the separate units using modified forms of standard glycosidation procedures. ... 

Several groups have analyzed a variety of standard oligosaccharides and glycosides by using m.s.-m.s. techniques " (for a description of m.s.-m.s., see Ref. 131). None of this work gave convincing evidence that m.s.-m.s. is needed, or, indeed, is able, to solve carbohydrate problems, and for this reason, a discussion of m.s.-m.s. is not included herein. 

As relatively few standard compounds are available from commercial or other sources, identification of flavonol glycosides has to be achieved by alternative means, for example UV-, H- and C-NMR spectroscopy. Therefore hydrolysing all glycosides to aglycones followed by HPLC determination offers a practical method for the quantitative determination of flavonoids in tea (Hertog et al, 1993a Wang and Helliwell, 2001). 

Some herbs are standardized for several active constituents, while others are standardized to a single active ingredient. St. John s wort is standardized to contain 0.3% hypericin, whereas ginkgo is standardized to contain 24% flavone glycosides and 6% ter-pene lactones. However, standardizing an herb product to one or more plant component(s) that are identifiable by assay may be incorrect. Many herbalists believe that the whole plant contributes to the efficacy and that there are many unknown active compounds in each plant [6]. 

Identification of flavonoids Quantification of individual flavonoids depends heavily on the availability of standard references. Only a limited number of common flavonoids are commercially available as standards. Standard references for flavonoid glycosides are particularly difficult to find thus direct quantification of the native glycosides is nearly impossible. Analysis of the aglycones after acid or enzymatic hydrolysis is therefore common practice. When standard flavonoids are not available, or when unknown compounds are encountered in a particular fruit or vegetable, use of a DAD... 

TLC DATA OF THE ISOLATED COMPOUNDS AND OF FLAVONE, FLAVONE GLYCOSIDE AND BIFLAVONE REFERENCE STANDARDS... 

The standard conversion of alcohols into their xanthate esters through reaction with carbon disulphide and an alkylating agent is improved by the addition of a quaternary ammonium salt [69]. Excellent yields (>90%) are obtained in relatively short reaction times under mild conditions. The formation over relatively short reaction times of O-glycosyl xanthates from partly protected glycosides has been described using a stoichiometric amount of the quaternary ammonium salt [70]. 

The study of monosaccharides subjected to conditions of methanolysis is considered for two reasons. First, the decomposition of monosaccharides is indicative of the decomposition of monosaccharides liberated during methanolysis second, the ratio of methyl glycosides of a particular standard monosaccharide is the same for the same monosaccharide released during methanolysis (for a particular set of methanolysis conditions), provided that the concentration of sugars is relatively low. Up to four methyl glycosides (the a and )8 anomers of the pyranoside and furan-oside forms) of a particular monosaccharide may be formed the acyclic dimethyl acetal is an additional possibility. The ratios of methyl glycosides of 10 monosaccharides subjected to methanolysis with M hydrogen chloride for 24 h at 80° has been reported. Similar information is also available in another study."... 

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