Disaccharides cyclic acetals

The following procedures have been arbitrarily chosen as representative of classic acetals extensively used as versatile starting materials for synthesis [102], It covers aspects of the chemistry of acyclic and cyclic monosaccharides and same disaccharides. Procedures from other laboratories have been reproduced from the original publication and their authors ate acknowledged. 

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

The use of a catalytic amount of iodine in acetone was found to effect the addition of isopropylidene cyclic acetals to a wide variety of mono-199 and disaccharides.200 In the case of disaccharides, some cleavage of the glycosidic link was also observed. The proposed rationale for this acetalation is that iodine, acting as a Lewis acid, polarizes the carbon-oxygen bond (Scheme 42), increasing the electrophilicity of the carbon, leading to dehydration. In this instance, the iodine acts in the same manner as a proton in the acid-catalyzed introduction of a cyclic acetal. Hydroiodic... 

Carbohydrates Sugars and other carbohydrates most commonly exist as cyclic acetals and hemiacetals. For example, glucose is a six-carbon sugar that is most stable as a hemiacetal. Lactose is a disaccharide (composed of two sugar units) that has one acetal and one hemiacetal. We discuss the structures of carbohydrates in detail in Chapter 23. 

Glycosides are obtained by conversion of the cyclic semiacetal form of aldoses and ketoses to an acetal. We will focus on the metal complexes and esters of the following glycosides alkyl pyranosides, alkyl furanosides, non-reducing disaccharides, polysaccharides, and Anally the nucleosides as a member of the group of iV-glycosides. 

The procedure is remarkably effective for the coupling with cyclic ketones and provides a fast access to a-C-ketosides of Neu5Ac. The coupling efficiency is however moderate in the synthesis of C-disaccharides, as shown with the D-galactose-derived aldehyde 198 (O Scheme 44). Acetates 213 provide the expected compound 215 but much less efficiently than sulfide 204 (30 versus 82% yield, respectively). The moderate yield using acetates 213 is due to the competitive pinacol coupling of aldehyde 198 because the rate of the reductive metallation of the anomeric acetates is too slow. 

It is of interest that a number of studies on cyclic acetals of disaccharides have appeared.176,1773... 

Disaccharides are O-glycosides in which the hydroxyl group attached to the anomeric carbon atom of one monosaccharide is replaced with a hydroxyl group of See more on Maltose another monosaccharide, thereby forming a cyclic acetal. For example, maltose (11)... 

Acetal migration can lead to the formation of cyclic acetals that are resistant to mild hydrolysis with acid in the Smith degradation of polysaccharides. Lind-berg and his co-workers have shown that the model disaccharides (427) and (428) are degraded to the expected products (429) and (430), respectively, without the formation of significant amounts of the cyclic acetals (431) and (432). ... 

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