Rearrangements keto-sugars

Oxidation to the intermediate selenoxides 147 and 151 and -elimination provided the allyl enol ethers 152 and 154, which under mild conditions (elimination in refluxing benzene) could be isolated and characterized. When the elimination was performed in refluxing xylene, it was immediately followed by sig-matropic rearrangement to the C-allyl keto sugars 153 and 155. As is shown in Scheme 29, the main products in each case were those with the side chain in axial orientation. The thermodynamically more stable equatorial isomers, however, were readily formed upon treatment with silica gel, so that both configurations can be obtained selectively with this methodology. 

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) ... 

More complex examples exist where the 2-substituent on the furan is a polyhydroxyalkyl or glycosyl moiety <89CAR(l9l)209>. In these cases, the rearrangement products provide useful intermediates for the preparation of heterocycle-substituted sugar derivatives. Sometimes Baeyer-Vil-liger-type products are observed, such as 3-keto enoic esters instead of 3-keto enol esters <81TL1443>. 

The first step of the reaction is the condensation of amino acids to carbon atom 1 of aldoses (or C-2 of ketoses) and the rearrangement to the keto (aldo)-sugar (Amadori or Heyns-rearrangement). 

In this section, a highly efficient synthetic system for the preparation of 2-C-hydroxymethylaldopentoses is discussed. The reactions of four different keto-hexoses (o-psicose, D-fructose, L-sorbose and D-tagatose) were studied imder various conditions in order to elucidate the relationship between the two key processes, the coordination of the sugar to the nickel complex, and the rearrangement of the carbon skeleton of the substrate ketose. The system was then compared with that of the epimerization of the aldose/nickel(II)/ethylenediamine complex. 

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