Glycosidation, acid catalyzed

Methyl glycosides of 2 deoxy sugars have been prepared by the acid catalyzed addition of methanol to unsaturated sugars known as glycals... 

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

The removal of the carbohydrate auxiliary group and the hydrolysis of the amino nitriles is achieved by acidolytic cleavage of the hemiaminal /V-glycosidic bond and the concomitant acid-catalyzed solvolysis of the nitrile using either hydrogen chloride in formic acid or hydrogen bromide in acetic acid56 57. 

The first total synthesis of the marine dolabellane diterpene (+)-4,5-deoxy-neodolabelline (70) was accomplished by D. R. Williams et al. [58]. The trans-disubstituted dihydropyran moiety in key intermediate 69 was efficiently prepared from mixed acetal 66 by RCM with second-generation catalyst C and subsequent Lewis acid-catalyzed allylation of ethyl glycosides 67 with allylsi-lane 68 (Scheme 12) [59]. 

Because of its importance to carbohydrate technology, biochemistry, and physical organic chemistry, the hydrolytic cleavage of glycosides has been extensively studied with respect to both acid and enzymic catalysis. Reviews on the acid-catalyzed hydrolysis have been presented by BeMiller, Capon, ... 

Scheme I.—Acid-catalyzed Hydrolysis of) -Glycoside (Hydroxyl substituents omitted).

PTFAs are generally less reactive than the corresponding trichloroacetimidate donors presumably because of the lower N-basicity or the presence of an N-sub-stituent. Although most trichloroacetimidate activators could also be used to promote PTFA glycosidations, such as TMSOTf [384], BF3Et20 [384,547], TBSOTf [548], Yb(OTf)3 [549,550] and acid-washed molecular sieves [551], the activation of PTFA usually requires more forceful conditions. Several representative Lewis-acid-catalyzed PTFA glycosidation reactions are listed m Scheme 3.58. It is worth... 

Furthermore some O- and. S -glycosylated pyropheophorbide-a derivatives have been prepared.75 The O-glycosylated pyropheophorbide derivative 97 was obtained by carbohydrate per-acetate glycosylation acid catalyzed method of derivative 96 (Fig. 10). The reagent proportions and the reaction times have been shown to rule the reaction yield. The best results were found when the reagents ratio was 2 1 2 (glycoside pyropheophorbide Lewis acid). Under these conditions the reaction yield was 36% with 83% of anomeric purity. The glycosylated derivatives 97 are diastereomeric mixtures. 

The base lability of succinoyl diester hnker severely limits the selection of protecting groups available for an oligosaccharide synthesis, so a more versatile tether was required. Diether bonds of benzylphenol or dibenzyl of 1,4-di(hydroxymethyl)-benzene satisfy this requirement because they are stable to both bases and to acids. A sufficient acid stability is important since the formation of a glycosidic bond is an acid-catalyzed reaction, not surprisingly, as it is an acetal functionality. For instance, DOX,34 the dibenzyl hnker a,a -DiOxyXylyl diether, -0CH2C6H4CH20-, is not limited by restriction of the succinoyl hnker (1) when bound via a hydroxyl or as an... 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

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