Ketoses hydrolysis

Carbohydrates may be divided into monosaccharides, disaccharides and polysaccharides. The monosaccharides under certain conditions react as polyhydroxy-aldehydes or polyhydroxy-ketones two important representatives are glucose CjHjjO (an aldose) and fructose (laevulose) CgHuO, (a ketose). Upon hydrolysis di- and polysaccharides 3deld ultimately monosaccharides. Common disaccharides are sucrose, lactose and maltose (all of molecular formula C,2H2. 0,), whilst starch, dextrin and cellulose, (CjHjoOj), in which n > 4, are typical polysaccharides. 

It is well known that the 1-phosphates of the ketoses, L-fuculose (51) and L-rhamnulose (52) have considerable biochemical interest. Their chemical synthesis has not been described as far as is known to the writer, but the rate of acid hydrolysis of L-fuculose 1-phosphate, obtained by enzymatic synthesis, has been determined by Heath and Ghalambor (20) and that of L-rhamnulose 1-phosphate by H. Sawada (48) and by Chiu and Feingold (II). They found that the rate of... 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

Blocking the C-l OH of D-fructose and L-sorbose (Scheme 25) was effected in excellent yields through regioselective isopropylidene acetalation of the free ketoses, followed by etherification (benzylation or allylation) of the remaining primary alcohol. Acid-catalyzed hydrolysis of the isopropylidene groups and condensation with HSCN efficiently produced a sole fused bicyclic OZT. 

Reduction of a ketose yields a secondary alcohol, and reduction of an aldose yields a primary alcohol (ccilled an alditol). A possible reducing agent is hydrogenation in the presence of a catalyst, such as platinum another reducing agent is sodium borohydride (NaBH ) followed by hydrolysis. Figure 16-14 illustrates the formation of an alditol. 

Of all the selective, deprotection procedures that are available to carbohydrate chemists, the partial hydrolysis of polyacetals is probably the most familiar. Articles by de Beider4,5 and Brady6 contained examples of this type of reaction for aldose and ketose derivatives, respectively, and an article by Barker and Bourne7 gave useful information from the early literature on the graded, acid hydrolysis of acetal derivatives of polyols. A discussion of the stereochemistry of cyclic acetals of carbohydrates was included in an article by Mills 70 and in one by Ferrier and Overend,76 and a survey of the formation and migration of carbohydrate cyclic acetals was made by Clode.7c... 

Studies on the rates of hydrolysis of di-O-isopropylidene derivatives of ketoses indicated that the relative order of stability of 2,2-di-methyl-l,3-dioxolane rings is in the order cis-fused to a furanoid ring > sptro-fused to a pyranoid ring > cts-fused to a pyranoid ring > sptro-fused to a furanoid ring.8 Thus, partial hydrolysis of l,2 3,4-di-0-isopropylidene-/3-D-en/fhro-2-pentulofuranose (1) to the 3,4-acetal (2)... 

Ketones tee under Aliphatic ketones and Aromatic ketones Ketonic hydrolysis, 475, 480, 481 Ketose, 437... 

The reactions enclosed within the shaded box of Fig. 17-14 do not give the whole story about the coupling mechanism. A phospho group was transferred from ATP in step a and to complete the hydrolysis it must be removed in some future step. This is indicated in a general way in Fig. 17-14 by the reaction steps d, e, and/. Step/represents the action of specific phosphatases that remove phospho groups from the seven-carbon sedoheptulose bisphosphate and from fructose bisphosphate. In either case the resulting ketose monophosphate reacts with an aldose (via transketolase, step g) to regenerate ribulose 5-phosphate, the C02 acceptor. The overall reductive pentose phosphate cycle (Fig. 17-14B) is easy to understand as a reversal of the oxidative pentose phosphate pathway in which the oxidative decarboxylation system of Eq. 17-12 is... 

Table III gives preparations of six-, seven-, and eight-carbon ketoses, and their methylated, deoxygenated, azido, amido, and branched-chain derivatives. All these are first obtained as ketose phosphates, but this is not a severe restriction, for, apparently, enzymic hydrolysis with phosphatases is always successful. Phosphatases constitute a versatile group of enzymes, easily...

Compared with the /V-substituted glycosylamines, the l-amino-l-deoxy-2-ketoses are more stable to moist acid atmospheres, but are still heat-labile and decompose rapidly in mild alkali. They exert greater reducing power, although less than reduc-tones. They brown more easily with amino acids. Acid hydrolysis gives much compared with little HMF, but no hexose is recovered, in keeping with the reaction s being irreversible (however, see below). 

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