Carbohydrates keto sugars

PCC in the presence of 4 A MS to afford the corresponding keto sugars (175, 176), which after Tebbe methylenation led to the corresponding exoglycals, valuable intermediates for the synthesis of C-disaccharides. Dondoni el al.159 made use of PCC in the presence of powdered 4 A MS to introduce suitable formyl groups in the carbohydrate, and allowing the synthesis of >6)-linked oligosaccharides,... 

OZTs from keto sugars. An original approach for the synthesis of OZTs from carbohydrate-based a-hydroxyketones was recently published by Silva et al.4Sa who investigated the reactivity of carbohydrate-based a-hydroxyketone in the presence of thiocyanic acid. 

Several reviews describing methods for oxidation of carbohydrates leading to the corresponding aldehydo and keto sugars have appeared.18-22 Also, a short discussion published in 1970 was devoted to... 

R. E. Arrick, D. C. Baker, and D. Horton, Chromium trioxide-dipyridine complex as an oxidant for partially protected sugar preparation of aldehydo and certain keto sugar derivatives, Carbohydr. Res., 26 (1973) 441 147. 

V. M. Parikh and J. K. N. Jones, Oxidation of sugars with ruthenium dioxide-sodium periodate a simple method for the preparation of substituted keto sugars, Can. J. Chem., 43 (1965) 3452-3453 compare R. H. Hall and K. Bischofberger, The preparation of aldonolactones and lactols by oxidation of aldoses with ruthenium(VIII) oxide, Carbohydr. Res., 65 (1978) 139-143. 

As shown above, many of the methods that are available to form carbon-carbon bonds rely on the reaction of a carbanion with a suitable electrophile. Keto-sugars are readily available by simple oxidation of hydroxyl groups. Thus, the reaction of carbohydrate-derived ketones or aldehydes with carbanions has been extensively explored. However, keto groups are suitable substrates in olefinations, such as Wittig reactions, leading to versatile intermediates for the construction of complex structures. This section will detail some application of keto-sugars in total syntheses along these two main lines. 

There are a number of cofactor independent carbohydrate epimerases that act on activated substrates, such as keto-sugars and keto-sugar nucleotides, although there is a paucity of details about their mechanisms. D-ribulose-5-phosphate 3-epimerase catalyzes the stereoinversion of substrate about the C-3 carbon to form D-xylulose 5-phosphate (as in Fig. 7.15) [102, 103]. Solvent hydron is completely incorporated into the product at the C-3 carbon, during epimerization in the d-xylulose 5-phosphate to o-ribulose 5-phosphate direction [102], This was taken as evidence for a two-base mechanism. 

Examination of the active hydrocolloid followed the usual systematic pattern. Proximate analyses were first noted, and then the carbohydrate nature of the components was determined. Galactose was identified as the major product of hydrolysis by oxidation to mucic acid (5, -4, 12, 17) and by optical rotation (17). Glucose (16, 28), fructose (17) or keto sugars (28, UO), pentoses (28, 35), and nonreducing sugars (9) were also found. However, the presence of glucose is attributed by... 

Muramatsu W (2014) Catalytic and regioselective oxidatitm of carbohydrates to synthesize keto-sugars under mild ctmditions. Org Lett 16 4846... 

Figures 22.7 and 22.8 show only aldopentoses and aldohexoses, but there are also many keto sugars among the pentoses, the five-carbon carbohydrates, and the hexoses, the six-carbon carbohydrates. Figure 22.9 shows D-fructose, the most common ketohexose.

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

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