Carbohydrates acetyl derivatives

Figure 13-10. Glucosamine (2-amino-D-glucopyra-nose) a form). Galactosamine is 2-amino-D-galactopy-ranose. Both glucosamine and galactosamine occur as N-acetyl derivatives in more complex carbohydrates, eg, glycoproteins.

P. A. McNicholas, M. Batley, and J. W. Redmond, Synthesis of methyl pyranosides and furanosides of 3-deoxy-D-marcrco-oct-2-ulosonic acid (KDO) by acid-catalysed solvolysis of the acetylated derivatives, Carbohydr. Res., 146 (1986) 219-231. 

Alcohol Compounds.—That the carbohydrates are, in fact, alcohol compounds is shown, both by their relation to poly-hydroxy alcohols and by their reactions. Carbohydrates possess alcohol characters in that they undergo distinctly alcoholic reactions. Like all alcohols they react with acetyl chloride or acetic anhydride. In.practice the latter reagent is used. They form acetyl derivatives, or esters, just as ethyl alcohol does. 

I. By the formation of acetyl derivatives and by the reduction to normal hydrocarbons containing the same number of carbon atoms, passing through the poly-hydroxy alcohols of the same number of carbons, carbohydrates are poly-hydroxy alcohols in which each carbon but one has one and only one hydroxyl group linked to it. 

An ever-increasing number of amino acid derivatives is found to occur naturally in proteins. These include s-N-methyllysines, e-N-acetyl-lysine, a-N-acetyl derivatives of various residues, N-methylhistidines, N-phosphohistidines, N-methylarginines, phosphoserine, phospho-threonine, hydroxyproline, hydroxyJysine, trimethylhydroxylysine, tyrosine sulfate, iodotyrosines, carbohydrate moieties and coenzymes... 

Another very interesting behavior is observed when comparing loss spectra of both studied herein carbohydrates in the range below Tg (see Fig. 3). There are two, well separated, secondary relaxation processes in lactose. In octa-O-acetyl-lactose only one is detected. A maximum of the secondary relaxation of octa-O-acetyl-lactose is almost the same as that of the y- relaxation of lactose at the chosen temperature. This can suggest that the considered secondary modes in lactose and its acetyl derivative may be of the same origin. Thus, in the further part of this paper we will label the secondary relaxation as a y- processes (just as in the case of lactose). 

Thus it is clearly visible that they are about 40-50 K higher than the value of Tg evaluated on the basis of dielectric measurements for acetyl derivative of lactose. It is surely related to the fact that molecules of the reduced disaccharide form strong hydrogen bonds and mainly these interactions are responsible for so significant difference in the glass transition temperatures of the both investigated herein carbohydrates. Influence of hydrogen bonds on structural... 

In a recent paper, the protected a-D-glucofiiranose (1) was selected as a carbohydrate model compound by Oscarson et al. [5] for acetylation, under the action of microwave irradiation, with acetyl chloride (2 equiv.) and pyridine, N,N-(diisopropyl)aminoethylpolystyrene (PS-DIEA), or N-(methylpolystyrene)-4-(methylamino)pyridine (PDS-DMAP) as conventional bases. The acetyl derivative 2 was produced in good to excellent yields in a very short time (Scheme 12.1). 

I. K. Baird, M. I. Holroyde, and D. C. Ellwood, Analysis of the products of Smith degradation of polysaccharides by g.l.c. of the acetylated, derived aldononitries and alditols, Carbohydr. 

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