2,3,4,6-Tetramethyl- /-gluCOSE

Another fundamental idea that has been invoked to explain enzymatic catalysis is that such reactions utilize bifunctional or multifunctional catalysis that is, several functional groups in the active site are properly aligned with the substrate so that concerted catalysis may occur. Mutarotation of tetramethyl glucose is frequently cited as an example of bifunctional catalysis. Lowry and... 

A third possible type of catalysis requires that a base and an acid act synchronously to effect the breaking and formation of bonds in a single step. Thus, tetramethyl-glucose mutarotates very slowly in benzene containing either pyridine (a base) or phenol (an acid). However, when both pyridine and phenol are present, mutarota-tion is rapid. This suggested to Swain and Brown132 a concerted mechanism (Eq. 9-92) in which both an acid and a base participate. 

The remaining four hydroxyl groups can be methylated in basic solution by dimethyl sulfate or by methyl iodide and silver oxide in N,N-dimethyl-methanamide, HCON(CH3)2, solution. Hydrolysis of either of these penta-methyl glucose derivatives with aqueous acid affects only the acetal linkage and leads to a tetramethylated glucose, 20, as shown in Figure 20-4. 

Polysaccharides. Part XXVIII. The End-group Method as Applied to Starch. An Improved Method for the Estimation of Tetramethyl Glucose in Admixture with Trimethyl Glucose, E. L. Hirst and G. T. Young, J. Chem. Soc., 1247 (1938). 

If all the hydroxyl groups are converted into —O.CH3 by methylation, the two end glucose units will each have four methoxy groups compared with three in all the other components in the chain. When the methylated cellulose is hydrolysed, B above breaks down with the loss of one methyl group. There will therefore only be one tetramethyl glucose unit in the product of hydrolysis, and this can be separated by distillation under reduced pressure. There has, however, been some dispute about the accuracy of this method. More recently Wolfram J. Amer. Chetn. Soc. 1937, 1938, 1939) formed mercaptals with the aldehyde structural isomer... 

Early workers were quite concerned that the fractionation factors of sugar hydroxyl groups would change during reaction, so much work was done on tetramethyl glucose. 

Figure 1.18 Transition states for the acetate-catalysed, acetic acid-catalysed and water reactions in the mutarotation of tetramethyl glucose. The additional waters for the acetate and acetic acid reactions are drawn to indicate solvation, rather than a change in bonding that would alter fractionation factors. Isotope effects are taken from ref 34 and fractionation factors calculated from their data using 1.0, rather than 1.23, for the fractionation factor of the anomeric hydroxyl. The latter was based on an implausible, equilibrium isotope effect of 4.1 on the acid dissociation constant of tetramethyl glucose. ...

Figure 1.20 Possible transition states in the catalysis of tetramethyl glucose muta-rotation by 2-pyridone, and calculated transition state for formic acid catalysis of 2-hydroxytetrahydropyran ring opening. In the calculated transition state, the proton is largely transferred to the endocyclic oxygen, the endocyclic C-O bond has started to break, but the hydroxyl proton is not transferred to the catalyst, i.e. the reaction is concerted but not synchronous.

The methods of methylation were first launched by the preparation of crystalline tetramethyl-glucose (7). Galactose (8), sucrose, and maltose (9) were subjected to methylation, and in 1906 the method was extended to determine die stmcture of the natural glucoside, salicin (10). Denham and Woodhouse (11) in 1910 were able to introduce methyl groups into cellulose. A modification of die mediylation method enabled it to be apphed to the preparation of glucosides and mediylated glucosides (12) and the simplification of work in such cases facilitated the eonstitutional study of sucrose, lactose, maltose, and cellobiose (13, 14, 15). 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

The presence of the pyranose ring in the two D-glucose units of melezitose was inferred by Zempl6n and Braun from good evidence, and made certain by Miss Leitch29 through the crystallization of the requisite amount of 2,3,4,6-tetramethyl-D-glucose from the scission products of the fully methylated trisaccharide. The revised interpretation of Miss Leitch s experimental data concerns only the structure of the... 

D-Glucose residue 1 would therefore appear as 2,3,4,6-tetramethyl-D-glucose on hydrolysis of methylated dextran D-glucose 2 as 2,3-dimethyl-D-glucose and the other three D-glucose residues as 2,3,4-tri-... 

On methylation of the disaccharide with dimethyl sulfate and sodium hydroxide a hexamethyl ether of the carbohydrate was obtained. When this fully methylated derivative (VII) was hydrolyzed with acid, 2,3,4,6-tetramethyl-D-glucose (VIII) and dimethyl-L-arabinose (IX) were produced. Since position 3 in the L-arabinose component of VI was shown to be occupied in glycosidic linkage with D-glucose, the dimethyl-L-arabinose could be either the 2,5- or 2,4-dimethyl derivative (IX),... 

Tetramethyl-D-glucose is frequently isolated after hydrolysis of the methyl ethers of naturally-occurring glucose polymers (e.g. maltose,... 

The Physical Properties of 3,3,6,6-Tetramethyl-D-glucose and Some of Its Derivatives... 

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