Evaluating the opportunity
A review of recent literature reveals several interesting approaches in chiral chemistry. One approach involves the synthesis of rare sugars, which have potential in several important applications, including as chiral building blocks in natural-products synthesis. Current production methods, however, use costly and complex biochemical processes to transform easily found abundant sugars into these rare ones. To address this limitation, researchers at the Massachusetts Institute of Technology (MIT) have found a way to use inorganic catalysts in place of enzymes to generate sugar epimers in a simple and robust manner, according to a Oct. 10, 2012 MIT press release.
Typically, biochemical processes used to generate rare sugars from more abundant sugars involve three main classes of enzymes, two of which, isomerases and oxidoreductases, are active on a wide range of simple substrates, but which also can result in the formation of side products. For example, xylose isomerase converts glucose into fructose and simultaneously converts fructose into mannose. These processes have generated some commercially available rare sugars, but the complex nature of that biochemical process makes it costly, according to the MIT release. The third class of enzymes, epimerases, are potentially the most useful biocatalysts for the widespread production of rare sugars because they offer high specificity for products and are capable of selectively modifying sugars at multiple carbon positions. Like all biological catalysts, however, epimerases can be fragile because they require specific reaction conditions and can only act on previously functionalized sugars.
The MIT researchers reported using inorganic catalysts in place of the enzymes to generate sugar epimers, thereby offering an alternative to biocatalysts in the selective conversion of sugars (1). The catalytic system involves using a tin-substituted microporous silicate (Sn-Beta zeolite) combined with a borate salt. The MIT researchers found that Sn-Beta zeolite in the presence of sodium tetraborate catalyzed the selective epimerization of aldoses in aqueous media. The reaction proceeds by way of a 1,2 carbon-shift mechanism, whereby C–C bonds move within the molecule's backbone, according to the MIT release.