Aldol condensation sugar formation mechanism

This possible mechanism should be evaluated in relation to the catalysts. If the catalytic action is to be ascribed to the acid character of the catalysts, the condensation under consideration may differ from the ordinary aldol condensation, which is catalyzed preferentially by basic agents. Nevertheless, many condensations of the aldol type are effected with the aid of acidic reagents. Moreover, the condensation of sugars with dicarbonyl compounds has been carried out in aqueous alcoholic media which are non-acidic hence, there also exists the possibility of a mechanism catalyzed simultaneously by acid and by base, somewhat like that suggested by Lowry46 in another connection. A transition state with an amphiprotic structure has been postulated. Its formation can be catalyzed by either acids or bases. 

The mechanisms by which D-sorbose and L-sugars are formed are not known. Conceivably 3,4- and 4,5-enediols might be formed. On the other hand, the possibility exists of the cleavage of the carbon chain through a reversed aldol condensation. If a hexose was cleaved to two glyceraldehyde molecules, they would be expected partially to isomerize immediately to dihydroxyacetone. A very rapid aldol condensation of glyceraldehyde and dihydroxy acetone occurs (p. 113) with the formation of fructose and sorbose. These ketohexoses have a trans arrangement of the hydroxyls at the new asymmetric centers 92 93). 

As indicated in the Introduction, the condensation of formaldehyde into monosaccharides by basic catalysis has been known since the last century. The synthesis starts with the formation of glycolaldehyde which is a slow reaction and is responsible for the induction period observed in the condensation of formaldehyde to sugars. Once sufficient amounts of glycolaldehyde have been formed, an autocatalytic process ensues which transforms glycolaldehyde into glyceraldehyde and dihydroxyacetone, and then into all the possible tetroses, pentoses and hexoses. The principal mechanism is a base catalyzed aldol condensation, somewhat similar to the enzyme catalyzed biochemical transformations of sugars. A common mineral, kaolinite, has been found to be an efficient catalyst for this reaction. Ribose is indeed one of the important monosaccharides formed in this reaction. 

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