Glycoside Acetates

With the exception of the Fischer reaction, the general methods employed for the formation of the glycoside bond result in the formation of glycoside acetates (or benzoates). Deacylation is accomplished by treatment with alkali, to which the glycoside bond is stable, unless the bond approximates to the ester type. The original method of deacetylation was by quantitative saponification with sodium hydroxide, potassium hydroxide, or barium hydroxide. It was noted, however, that, in anhydrous media, much less than the theoretical amount of alkali was required, and the reaction has since been made entirely catalyt.ic, with considerable gain in efficiency and convenience. The same methods have been employed for debenzoylation. 

The acetate was shaken at 0° or room temperature with aqueous sodium hydroxide, potassium hydroxide, or barium hydroxide in considerable 

The first recorded use of this type of reagent in carbohydrate chemistry was made by Fischer and Bergmann who found that only 0.2 mole of sodium ethoxide was required for complete deacetylation of a sugar or glycoside acetate in ethanol at room temperature. This method was modified by Zempl6n, mainly in connection with a study of cellobiose derivatives. The acetate was dissolved in chloroform, and a solution of sodium methoxide (0.1 to 0.2 molar proportion) in methanol was added. A gelatinous, addition compound separated at 0° and was decomposed by the addition of water. After neutralization, the aqueous layer was processed in the usual way (see Section IV, 1). 

It was subsequently shown that a solution or suspension of a glycoside acetate in absolute methanol can be completely deacetylated in a few minutes on the boiling-water bath by using catalytic quantities of sodimn methoxide (about 0.002 molar proportion), with production of methyl acetate. This method is of very wide application. 

In a typical experiment, 0.1 ml. of 0.1 N sodium methoxide was added to 0.54 g. of methyl tetra-O-acetyl-a-D-mannopyranoside in 5 ml. of cold, absolute methanol. After 2 min. on the boiling-water bath, crystals of methyl a-n-mannopyranoside 

---

FeCl3, CH2CI2, 2-3 min, 68% yield. Benzyl ethers are cleaved in 15-20 min under these conditions. Methyl glycosides, acetates and benzoates were not affected by this reagent. 

Digestion (Section 29.1) The first stage of catabolism, in which food is broken down by hydrolysis of ester, glycoside (acetal), and peptide (amide) bonds to yield fatty acids, simple sugars, and amino acids. 

Consequences of the conformational anomeric effect are largely expressed in monosaccharides and their derivatives. One recognizes the conformational endo-anomeric effect for pyranosides with a polar X group at C(l) (contrasteric electronic stabilization effect Fig. 7A) and conformational exo-anomeric effect for glycosides (acetals) in which the alkyl group of the exocyclic moiety is synclinal (Fig. 7B, C). 

It should also be noted that hydrolysis of glycosides (acetals or ketals) will occur under acid-catalysed conditions if we have an excess of water present. This is a reversal of the process for glycoside... 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

Gums are tasteless, odorless, colorless, and nontoxic. None, except the starches and starch derivatives, are broken down by human digestive enzymes. All are subject to microbiological attack. All can be depolymerized by acid- and enzyme-catalyzed hydrolysis of the glycosidic (acetal) linkages joining the monomeric (saccharide) units. 

Photolysis of D-glucose in methanol containing titanium(IV) chloride gave the D-xy/o-pentodialdose glycoside acetal 184 in 60% yield D-galactose behaved similarly.386... 

Tables II to X give the melting points and, where applicable, the optical rotations of the inositols, inososes, inosamines, and quercitols, and of all of their known O-substituted derivatives. Anhydroinositols, although not substitution products in the strict sense, are included, as are the carbonyl-functional derivatives of the inososes. Halogen- and nitro-substituted cyclitols, and the C-methyl-inositols and their derivatives, are not included most of these compounds are referred to in the text. The derivatives are arranged in the following order salts (inosamines) or functional derivatives (inososes), carboxylic esters, borates, nitrates, sulfonic esters, phosphates, glycosides, acetals (and Schiff bases), ethers (and IV-alkyl derivatives), and anhydrides.

Disaccharides contain a glycosidic acetal bona between the anomeric carbon of one sugar and an -OH group at any position on the other sugar. A glycosidic bond between C1 of the first sugar and the —OH at C4 of the second sugar is particularly common. Such a bond is called a i—>4 link. 

Degradation of dihydrostreptomycin in methanolic hydrogen chloride yielded methyl dihydrostreptobiosaminide, which formed a pentaacetyl derivative on acetylation. - - - Both anomers of this compound were isolated.The crystalline methyl pentaacetyldihydrostreptobio-saminfde was also obtained by selective hydrolysis of the non-glycosidic acetal group in methyl streptobiosaminide dimethyl acetal followed by catalytic hydrogenation and acetylation. 

Although Koenigs and Knorr isolated a small yield of methyl /3-n-gluco-pyranoside (as such) from a solution of tetra-O-acetyl-a-D-glucopyranosyl bromide in methanol that had stood at room temperature for several days, it is customary to add an acid acceptor to speed up the reaction and to prevent deacetylation of the product. Silver, in the form of the oxide or a salt, was the first acid acceptor to be employed, and is still the one in most common use. Unless the aglycon is a simple alcohol, it is usual to dissolve the reactants in a solvent, which is often an organic base to act as an additional acid acceptor. Walden inversion at Cl is almost the invariable rule when the reaction is done in the presence of silver ion. Under special circumstances, however, both anomeric glycoside acetates may be obtained. ... 

Walden inversion at Cl is also the rule when alkali is employed as the condensing agent. An organic base (nearly always quinoline) may be used in the same way, but, in the absence of silver, a mixture of the a- and jS-glycoside acetates results. These are usually easy to separate by fractional... 

With zinc chloride as catalyst, the /3-glycoside acetate sometimes pre-... 

---