Mannitol isomerization

Isomerization of sorbitol, D-mannitol, L-iditol, and dulcitol occurs in aqueous solution in the presence of hydrogen under pressure and a nickel—kieselguhr catalyst at 130—190°C (160). In the case of the first three, a quasiequiUbrium composition is obtained regardless of starting material. Equilibrium concentrations are 41.4% sorbitol, 31.5% D-mannitol, 26.5% L-iditol, and 0.6% dulcitol. In the presence of the same catalyst, the isohexides estabUsh an equihbrium at 220—240°C and 15.2 MPa (150 atm) of hydrogen pressure, having the composition 57% isoidide, 36% isosorbide, and 7% isomannide (161). 

S)-1,5-Anhydro-3,4,6-tri-0-benzyl-1 -C,2-0-(ophenylenemethylene)-D-mannitol Note. The isomeric chromene would be named as a 2-0,1 -C-substituted system. 

In conclusion, the steady-state kinetics of mannitol phosphorylation catalyzed by II can be explained within the model shown in Fig. 8 which was based upon different types of experiments. Does this mean that the mechanisms of the R. sphaeroides II " and the E. coli II are different Probably not. First of all, kinetically the two models are only different in that the 11 " model is an extreme case of the II model. The reorientation of the binding site upon phosphorylation of the enzyme is infinitely fast and complete in the former model, whereas competition between the rate of reorientation of the site and the rate of substrate binding to the site gives rise to the two pathways in the latter model. The experimental set-up may not have been adequate to detect the second pathway in case of II " . The important differences between the two models are at the level of the molecular mechanisms. In the II " model, the orientation of the binding site is directly linked to the state of phosphorylation of the enzyme, whereas in the II" model, the state of phosphorylation of the enzyme modulates the activation energy of the isomerization of the binding site between the two sides of the membrane. Steady-state kinetics by itself can never exclusively discriminate between these different models at the molecular level since a condition may be proposed where these different models show similar kinetics. The II model is based upon many different types of data discussed in this chapter and the steady-state kinetics is shown to be merely consistent with the model. Therefore, the II model is more likely to be representative for the mechanisms of E-IIs. 

Fig. 13.9 Facsimile of a molecular and kinetic scheme of the one-pot glucose into mannitol bio-chemo cascade, showing the many different species that undergo interconversion during the overall process, involving enzymatic isomerization, homogeneous mutarotation and heterogeneous hydrogenation. For simplicity, the various sugar-borate species have been omitted [23, 24],...

Michaelis-Menten enzymatic isomerization only two of the six sugar forms are substrates for the enzyme Km values for glucose, fructose, glucitol and mannitol are 0.13, 0.04, 0.4 and >1 m, respectively. 

Sorbitol is a hexahydric alcohol that is isomeric with mannitol, and which has been described as a humectant, plasticizer, sweetening agent, and tablet and capsule diluent. It is used extensively in the pharmaceutical, cosmetics, and food industries, since it has a sweet taste and approximately 50-60% the sweetness of sucrose. 

Dulcitol Hexanitrate, isomeric with Mannitol Hexanitrate and similar to it, is a... 

The addition of hydrogen at the double bond creates a new center of asymmetry, and hence two isomeric anhydroalditols should result. Zervas34 reported the isolation of styracitol (1,5-anhydro-D-mannitol) in 61 % yield, with insufficient effort expended to locate the second expected... 

Access to the isomeric (L)-protected glyceraldehydes by this approach is limited because of the general unavailability of L-mannitol. One way of circumventing this problem is to employ a 2-step degradation of 5,6-0-isopropylidene-L-ascorbic acid 8 (scheme 4) (7). 

The acetonation of 1,6-dibenzoyl-D-mannitol, using a concentrated sulfuric acid catalyst, gives only the 3,4-derivative, even when excess acetone is employed.31-181 Under similar conditions l,6-dichloro-l,6-didesoxy-D-mannitol affords the 3,4-derivative as well as two isomeric 2,3,4,5-ketals.119... 

An isomeric dimethylene-D-mannitol (m. p. 139°) was isolated by Haskins, Hann and Hudson62 following the methylenation and saponification of 1,6-dibenzoyl-D-mannitol. Haworth and Wiggins62 found later that a 1,6-dibenzoyl-monomethylene-D-mannitol, which was not the... 

MPa H2. To suppress the isomerization of D-glucose to D-mannose and D-fructose (Lobry de Bruyn-van Ekenstein transformation) (Scheme 5.2) and the Cannizzaro reaction, which were both promoted in an alkaline medium, the pH value was maintained between 5.5 and 6.5. Under the conditions that were optimized to minimize the side reactions, the formation of gluconic acid and mannitol was reduced to less than 1% each at 99.5-99.6% conversion, while with a normal nonpromoted Raney Ni 1.5-2.1% of gluconic acid and 1.3-1.9% of mannitol were formed at 99.5-99.7% conversion. 

Sorbitol Solution occurs as a clear, colorless, syrupy liquid. It is a water solution of sorbitol (C6H1406) containing a small amount of mannitol and other isomeric polyhydric alcohols. It is miscible with water, with ethanol, with glycerin, and with propylene glycol. It sometimes separates into crystalline masses. 

In this synthesis (Scheme 6), the C2-symmetri-cal triacetonide of D-mannitol (32) is converted via the epoxide 33 and its nucleophilic addition product 34 to the propargylic alcohol derivative 35. From this intermediate, the Z-configured vinyl iodide 36 is stereoselectively obtained by hydroalumination/iodination. The Pd-catalyzed Heck cyclization then affords the isomerically pure product 37, which represents a potential building block for the synthesis of la,2y5,25-trihy-droxy-vitamin D, following the classical Wittig strategy of Lythgoe. 

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