Acetal glycosides

Glycosides are named by first citing the alkyl group and then replacing the -ose ending of the sugar with -oside. Like all acetals, glycosides are stable to neutral water. They aren t in equilibrium with an open-chain form, and they don t show mutarotation. They can, however, be converted back to the free monosaccharide by hydrolysis with aqueous acid (Section 19.10). 

The direct comparison of 1 and 2 in a variety of RCM reactions also indicates a presumably close relationship between these catalysts (Table 1) [6]. Both of them give ready access to cycloalkenes of almost any ring size > 5, including medium sized and macrocyclic products. Only in the case of the 10-membered jasmine ketolactone 16 was the yield obtained with 2a lower than that with lc this result may be due to a somewhat shorter lifetime of the cationic species in solution. However, the examples summarized in Table 1 demonstrate that the allenylidene species 2 exhibit a remarkable compatibility with polar functional groups in the substrates, including ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and even free hydroxyl groups. 

Although the ruthenium allenylidene complexes 2 have not yet been as comprehensively studied as their carbene counterparts, they also seem to exhibit a closely related application profile [6]. So far, they have proven to tolerate ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and free secondary hydroxyl groups in the substrates (Table 1). 

In base, aldoses and ketoses rapidly equilibrate to mixtures of sugars (Fig. 123) (174). Most sugars react with alcohols under acidic conditions to yield cyclic acetals (glycosides). Glycoside formation, like acetal formation, is catalyzed by acid and involves cation intermediates (Fig. 124). 

The success of the mercuric acetate-glycoside syntheses led to the synthesis of oligosaccharides. He selected, for the alcoholic component, methyl... 

C(6) to give an oxonium ion. Nucleophilic attack, probably by acdtic acid, on the benzylic carbon gives a O-acetyl sugar derivative and benzyl acetate. Glycosides are also acetolysed under these reaction conditions. 

Like other acetals, glycosides are stable to basic conditions, but they hydrolyze in aqueous acid to a free sugar and an alcohol. Glycosides are stable with basic reagents and in basic solutions. 

In their behavior toward acids and bases, trityl ethers resemble the acetals, glycosides and orthoesters, being fairly stable in alkali, but easily split by acidic reagents. Thus, in cases where carbonyl compounds are used to protect pairs of hydroxyl groups by the formation of acetals, tritylation may often be used to protect suitably situated individual hydroxyl groups. 

The discovery of natural benzoxazinones was based on the finding that rye plants showed an increased resistance towards pathogenic fungi. Hence, the first aglucone and the first glucoside were reported from rye in two successive papers [1,2]. Acetal glycosides of the 2-hydroxy-2/f-l,4-... 

Chain linkage Phosphoesteric Amide (peptide) Acetal (glycosidic)... 

As weTl see later in this chapter, both disaccharides and polysaccharides are examples of glycosides in which monosaccharide units are joined together by acetal (glycosidic) linkages. 

In their reaction with alcohols under acid catalysis, the aldoses differ from the simple, or unhydroxylated, aldehydes in forming mixed monocylic acetals (glycosides) rather than acyclic dialkyl acetals. However, Fischer 138) found that, on shaking a cold solution of an aldose in concentrated hydrochloric acid with a thiol, a crystalline product separated which was the dithioacetal (mercaptal) of the acyclic structure. These substances have... 

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