Aldopyranosides

Nucleophilic displacements at carbon atoms adjacent to the ano-meric center of aldopyranosides do not normally proceed satisfactorily, but, surprisingly, treatment of methyl 3,4-0-isopropylidene-2-0-(p-nitrophenylsulfonyl)-a-D-glucoseptanoside with tetrabutylammon-ium fluoride in acetonitrile afforded the corresponding mannoside, from which 2-deoxy-2-fluoro-/3-D-mannopyranose was obtained by subsequent deprotection.153 Whether this is a general property of sep-... 

Beispiele fur Regel 2 Aldosen, Ketosen, Alkyl-, Benzyl- und Iso-propyliden-aldopyranoside... 

The effect of alkoxyl or phosphate groups on the acidic degradation of sugars has not yet been systematically studied. The aldopyranoses substituted at 0-1, that is, the aldopyranosides, are only degraded... 

Cope Degradation of Methyl 3,4-Dideoxy-3-(dimethylamino)-DL-aldopyranoside... 

Levene and coworkers, 1 using the ease of hydrolysis (10 minutes in 0.1 N hydrochloric acid at 100°) of the aldofur anosides as an analytical tool, studied the rate of glycoside formation (see Fig. 1) and confirmed the preliminary formation of aldofuranosides and subsequent conversion into aldopyranosides. 

Jansen and Lindberg162 have determined the effect on various aldosides of 10% sodium hydroxide at 170° the rate constants are given in Table XI. The aldofuranosides are much less stable in alkali than are the corresponding aldopyranosides, and the (rans-l, 2-anomers are less resistant than the ct s anomers. The lower stability of the Irons-glycosides has been attributed to the formation of a 1,2-anhydride, with elimination of the methoxyl group. 

The ratio of the rate coefficient for the hydrolysis of the methyl aldopyranoside (fc) to that of methyl a-D-mannopyranoside, under the reaction conditions indicated. 

Conversion of aldopyranosides into 5-thioaldopyranosides via acyclic monothioacetals was proposed by Hashimoto et The method involves the... 

Fig. 13.—The Mean Valence Geometry Parameters for Aldopyranosides (from Ref. 97).

Hydrolysis of Aldopyranosides and Aldofuranosides with 0.01 N Hydrochloric Acid at 95-100°... 

These data extend to the pyranoside series the relationship between steric strain and reactivity, and indicate that the conformational instability and non-bonded interaction of the axial substituents in the favored chair conformation result in a higher rate of hydrolysis. Another aspect of these theoretical considerations is that the methyl /3-d and fi-L aldopyranosides are only hydrolyzed faster than their corresponding a anomer when the normal chair conformation is favored, and thus, when the glycosidic group of the (3 anomer assumes an exposed, equatorial position (see page... 

Ratio of the rate constant (k) for the hydrolysis of the methyl aldopyranoside to that of methyl a-D-glucopyranoside (k = 1.98 X 10 min ). The numbers represent the position on which the substituent is axial. In the terminology suggested by Reeves, the normal chair conformation is denoted by Cl for d sugars and IC for L sugars. Indirectly obtained value. 

In their chemical properties, both these types of compound resemble to a certain degree the methyl /3-D-aldopyranosides or the methyl a- or /3-D-aldofuranosides. They do not have reducing properties, and are relatively stable in alkaline media, but are hydrolyzed by acids to the free hexoses. 

---