Standard glycosidation methods

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]. 

Being normal alkyl glycopyranosides, 3-buten-2-yl glycosides can be prepared by standard glycosylation methods, as previously mentioned for NPGs. 

Of note is a new glycosidation method applicable to the preparation of lignan glycosides for which standard methods are not suitable. The P,P-diphenyl-(A -... 

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). 

Enzymic hydrolysis (25-40°C) at the heterosidic bond of the chromogenic substrates was followed either continuously (via formation of 2 -chloro,4 -nitrophenol) at pH 5.5 (O.D. 405 nm) or discontinuously (4-methylumbel-liferone fluorescence at pH 10, emission at A > 435, excitation at A366 nm). Reaction rates were calculated from the linear increase of O.D. (em = 9000 M-1cm-1) or fluorescence (standardization with 4-methylumbelliferone) versus time. Alternatively, an HPLC method was used to follow the formation of chromophoric reaction products, phenols and glycosides (1). Concentrations were calculated from peak heights after appropriate standardization. 

A major recent growth point in substitution reactions has been the synthesis of pteridine glycosides, especially ribosides for study as probes in DNA chemistry taking advantage of the fluorescent properties of pteridines (see Section 10.18.12.4). Typically these reactions are developments of standard methods of glycosylation used with purines and pyrimidines as nucleophiles. In these and in other cases, the ambident nucleophiles within the pterin... 

Preparation of NPGs may be carried out by standard procedures for making alkyl glycosides (Scheme 3) [6]. The most direct method is the Fischer glycosidation (5 —> 1, Scheme 3), in which the aldose is treated with n-pentenyl alcohol [8] and an acid catalyst... 

Friedel-Crafts coupling of an aromatic compound with an activated glycosyl donor is a classic, standard method for C-aryl glycoside synthesis [1,2]. Early interests were primarily centered at C-nucleoside synthesis, so that data were accumulated for the ribofuranosyl series with degradable aromatics to construct the nucleic base (Scheme 2) [5]. Equation (1)... 

In relation to carbohydrate chemistry, the Koenigs-Knorr synthesis of glycosides involves the treatment of glycosyl halides with an alcohol or phenol in the presence of a heavy metal salt.267 Karrer268 discovered that reaction of silver salts of hydros acids could also be used and use of these reagents has been extended more recently. Numerous variations and improvements on the original method have now been reported, and silver oxide, silver carbonate or silver trifluoromethanesulfonate have since become the accepted standard reagents.267... 

Other methods have been used to prepare O-glycosides of desoxysugars. Bergmann and his coworkers118 converted tetrabenzoyl-2-desoxy-D-glucose into l-bromo-tribenzoyl-2-desoxy-D-glucose by the standard procedure. This latter compound, when treated with methanol in the presence of silver carbonate, afforded after debenzoylation, methyl 2-desoxy-/3-D-glucoside. 

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