Glucose hydride shift

The occurrence of some substitution in the deamination of 2-amino-2-deoxy-/3-D-mannopyranosides131 152 (72), and its absence in the reaction of the a-D-pyranoside150 69, must be due to the steric effect of the axial anomeric substituent which (in the a-D-pyran-oside) hinders the approach of the nucleophile (water) to either the C-2 carbonium ion or to C-2 of the diazonium ion. The glucose and glucitol tentatively detected as minor products in the deamination of 72 (R = D-glucose residue and R = D-glucitol residue) presumably arose by way of a hydride shift of H-l to C-2. 2-Deoxy-D-glucono-1,5-lactone (75) was not detected, as it would probably have. o,... 

When hydride shifts occur in the deamination of axial amines, cleavage of ether or glycosidic functions may result. The small amount of glucose tentatively identified151 in the product of deamination of 6-0-(2-amino-2-deoxy-/3-D-mannopyranosyl)-D-glucose (72 see p. 47) can be attributed to the occurrence of a hydride shift of H-l to C-2. 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

The enzyme D-xylose isomerase catalyzes the interconversion of D-xylose to E>-xylulose and D-glucose to D-fhictose by transferring a hydrogen atom between Cl and C2. Various mechanisms have been suggested including base-catalyzed transfer of a proton with a cis-ene diol as an intermediate, a hydride transfer, and a metal-assisted hydride shift [86-89]. The last of these three suggestions is the preferred mechanism at this time, but more studies of the mechanism are needed. 

Unlike fructose, glucose, one of the most common aldohexoses, is difficult to transform directly to HMF. It is the most abundant monosaccharide obtained from the depolymerization of cellulose. To achieve successful HMF production from glucose, heterogeneous catalyzed isomerization to fructose therefore appears as a key reaction. There are two mechanisms for glucose isomerization via proton transfer and intramolecular hydride shift. These are generally catalyzed by a base and Lewis acid, respectively. 

The latter isomerization via an intramolecular hydride shift is catalyzed by Lewis acids. Tin-containing P-zeoHte exhibits remarkable activity for the glucose-fructose isomerization in water. and NMR studies have elucidated this Lewis acid-catalyzed isomerization mechanism, and show a clear difference between Sn-fl (Lewis acid catalysis) and NaOH (Bronsted base catalysis). 

Gounder R, Davis ME (2013) Titanium-beta zeolites catalyze the stereospecific isomerization of D-glucose to L-sorbose via intramolecular C5-C1 hydride shift. ACS Catal 3(7) 1469-1476... 

Step 2 is the isomerization of glucose to fructose. This reaction involves the conversion of the aldohexose into the 2-ketohexose. Retro-aldol reaction of the aldohexose leads to a C4 and C2 sugar, whereas the ketohexose leads to the two trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLY). As the pathway to LA involves the trioses, selective glucose isomerization is essential, its conversion being limited by equilibrium in the operational temperature window. The isomerization of aldo- to ketoses can proceed via an acid-catalyzed hydride shift, a base-catalyzed mechanism with a proton shift (and intermediate enol), or via a concerted 1,2-hydride shift in neutral media [96, 97]. The latter isomerization mechanism occurs at mild temperatures (100°C) in the presence of Lewis acid catalysts, first... 

To conclude, the mie-pot conversion of cellulose-to-lactic acid (or lactate ester in alcoholic media) thus follows a complex cascade reaction network involving at least six reactions. These reactions have different catalytic needs, but, in general, the presence of both Lewis and Brpnsted acidity are paramount for catalytic success. Br0nsted acidity is key to the hydrolysis of cellulose (step 1) at mild temperatures (<200°C), and to some extent to the dehydration of triose (step 4), whereas Lewis acid sites play a vital role in the isomerization reaction of glucose-to-fructose (step 2), the retro-aldol (step 3), and the 1,2-hydride shift (step 6). Steps 4 and 5 are relatively less demanding they are catalyzed by both acid types. 

PROBLEM 22.40 Figures 22.25 and 22.26 give one mechanism for the Lohry de Bruijn—Alberda van Ekenstein reaction. Use the example given, in which D-glucose equilibrates with D-mannose and D-fructose, and provide another mechanism in which a hydride shift occurs. 

Replacement of the tetracoordinated Mg ion at binding site 1 by an amino acid employing site-directed mutagenesis furnished a catalytically incompetent mutant underscoring the crucial role of this catalytic entity [65]. Additional support for the hydride shift on the open-chain tautomer of the respective substrate was collected by employing 3-0-methyl-D-glucose (36) [66]. 

Initial enolisation of glucose yields the 1,2-enediolate. Expulsion of the 3-OH or 3-OR yields a l-aldehydo-3-deoxy-2,3-enol. This tautomerises to the ot-ketoaldehyde and addition of OH at Cl, followed by shift of HI as hydride, yields a pair of straight-chain, C2-epimeric, 3-deoxygluconic acids known as metasaccharinic acids. These are the predominant six-carbon products from the peeling reaction of 1 3-linked polysaccharides. 

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