Hydrolysis of furanosides

Complete hydrolysis of the polyglycoses gave the starting monomer. Hydrolysis with dilute acid gave an initial rise in optical rotation which was attributed to the hydrolysis of furanosidic bonds. The positive rotations suggest the presence of o-d as well as of 0-D-glycosidic linkages. The isolation... 

The two hexosans D-glucosan < l,4>/9< 1,6> and D-galactosan-<1,4>o<1,6> are 1,6-anhydrides of the furanose forms of the sugars and may be looked upon as internal hexofuranosides. In view of the generally more rapid acid hydrolysis of furanosides, such as methyl D-glucofuranoside or sucrose, as compared with normal pyranosides, the behavior of these hexosans toward acid hydrolysis is noteworthy. 

Many investigations have shown that furanosides are hydrolyzed faster than pyranosides, and acid-catalyzed hydrolysis of furanosides has been discussed in detail by Green. Additional discussion is given in Section III,la. Overend and coworkers have determined the entropy of activation for one furanoside, ethyl 3-D-galactofuranoside, and found it to be... 

Entropies of Activation for the Acid-catalyzed Hydrolysis of Furanosides"... 

In contrast to the pyranosides, acidic hydrolysis of furanosides is generally considered to follow the A2 mechanism.213... 

The ring forms (furanose or pyranose) of a monosaccharide can, to a limited extent, be inferred from the stability of the sugar to acid hydrolysis. The rates of hydrolysis of furanosides may be 10—10 times greater than those of the corresponding pyranosides (Aspinall, 1973). Terminal, nonreducing sugars can be characterized by methylation analysis as... 

Partial hydrolysis of dithioacetals has been found useful for the preparation of anomers, not obtained by the methods discussed above, and furanosidic thioglycosides [37,45]. 

Fischer was intrigued by the fact that emulsin caused hydrolysis of both / -glucosides and / -galactosides but had no effect on either the a- or / -xylo-sides (33). Since, at the time, Fischer expected glycosides to be furanosides, he suggested that both the enzymes required the presence of a free hydroxyl group at position 5 of a hexoside. 

Obayashi and Schlosser [43] have briefly described syntheses of erythro-sphingo-sine and threo-sphingosine from D-mannose and D-ribono-1,4-lactone, respectively. For the synthesis of ery/Aro-sphingosine (35) D-mannose was converted into benzyl 2,3 5,6-di-O-isopropylidene-manno-furanosides and hydrolysis of the 5,6-0-iso-propylidene group followed by periodate oxidation and borohydride reduction and protection gave the methoxymethyl ether (28). This was converted into the chloride... 

IV. Effect of the 1,6-Anhydro Ring on the Rate of Acid Hydrolysis of the Furanoside Structure of 1,6-Anhydrohexofuranoses... 

The rate of hydrolysis of polysaccharides is affected by several factors. Because of substituent interaction effects, furanosides are hydrolyzed much more rapidly than the pyranoside analogues. Differences in the hydrolysis rates of diastereomeric glycosides are significant. For example, the relative hydrolysis rates of methyl-a-D-gluco-, manno-, and galactopyranosides are 1.0 2.9 5.0. This can be related to the stabilities of the respective conjugate acids, which are transformed into the half-chair carbonium ions at different rates. Also, substituents bound to the C-2 position obviously prevent the formation of the half-chair conformation. 

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