Acid catalysis, glycosides

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

At the time the mechanism shown in [60] was proposed it was not known whether it was even chemically reasonable. Bimolecular general acid catalysis involving proton transfer in the transition state [equation (45)] had never been observed in the hydrolysis of glycosides or simple acetals. Bronsted and Wynne-Jones (1929) had... 

The notion of glycoside synthesis with unprotected glycosyl donors [10] has instigated the exploration of some new approaches using the trichloroacetimidate group [15], electrochemistry [16], electron transfer [17], and Lewis acid catalysis [18], The scope of applicability of these newer methods remains to be tested. 

The hydroxyl group on the anomeric carbon atom of the ring forms of sugars is reactive and can be replaced by another nucleophilic group such as -O-R from an alcohol. The product is a glycoside (Eq. 4-5). The reaction with methanol occurs readily with acid catalysis under dehydrating conditions, e.g., in 100% methanol. 

An observation consistent with general-acid catalysis by Glu-35 comes from a study of the reverse reaction. It is found that the rate of reaction of alcohols with the carbonium ion intermediate is virtually independent of their pick s. This is consistent with the general-base-catalyzed attack of the alcohol on the ion hence, by the principle of microscopic reversibility, the expulsion of the alcoho-late ion from the glycoside is general-acid-catalyzed.220... 

Catalysis by lysozyme is caused by three main factors (Figure 1) (1) general acid catalysis by glutamic acid residue 35, located in the proximity of the glycosidic bond, initiates the formation of a carbonium... 

Catalysis with protic acids has traditionally been used in Fischer glycosylations. As with the recent variations of the above procedure, Lewis acid catalysis is also valuable for dehydrative glycosylations with protected derivatives (Scheme 4.40). Trimethylsilyl triflate has been shown to promote reactions of glucuronic acid hemiacetals [420], and pyridine and TMSOTf was used for the synthesis of cw-glucosides [421]. Pyridinium / -toluenesulfonate (PPTS) was used for the synthesis of glycosides of the biologically important Kdo [422]. 

Reviews (a) K. Suzuki and T. Matsumoto, CpjMClj-AgX (M = Zr, Hf) A reagent for glycosidation, 7. Synth. Chem. Soc. Jpn. 48 1026 (1990) (b) K. Suzuki and T. Matsumoto, New methods in glycoside synthesis by using early transition metal-derived reagent, J. Synth. Chem. Soc. Jpn. 51 718 (1993) (c) K. Suzuki, Novel Lewis acid catalysis in organic synthesis. Pure Appl. Chem. 66 1557 (1994). 

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