Chirality, monosaccharides ketoses

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

A potential limitation on the use of FDP aldolases for the synthesis of monosaccharides is that the products are always ketoses with fixed stereochemistry at the newly generated chiral centers on C-3 and C-4. There are, however, methods for establishing this stereochemistiy at other centers and for obtaining aldoses instead of ketoses. This technique makes use of a monoprotected dialdehyde as the acceptor... 

Monosaccharides are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). They are classified as trioses, tetroses, pentoses, hexoses, etc. according to the number of carbon atoms. With the exception of dihydroxy acetone, monosaccharides contain one or more chiral C-atoms. Most monosaccharides carry an oxygen-containing group at each carbon atom. However, there are also deoxy sugars which have one or several oxygen-free carbon atoms. 

Sugar enantiomers have been separated as their diastereoisomeric a-methylbenzylaminoalditols (produced by reductive amination with a chiral amine) by capillary g.l.c. of either the peracetate or per-TMS derivatives. Twenty of twenty-four monosaccharides, including ketoses and N-acetylamino-sugars, were resolved as the above TMS-derivatives. 

Monosaccharides can be reduced to the corresponding alcohols by NaBH4, electrolytically or via catalytic hydrogenation. Two new alcohols are obtained from ketoses, e. g., fructose, since a new chiral center is formed ... 

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