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Carbohydrate chemistry

Through the looking glass

Angew. Chem. Int. Ed.http://doi.org/f2n5mn (2013)

Naturally occurring sugars are a readily available, cheap source of chirality and have frequently been used as the starting point for stereoselective syntheses. They are often, however, only easily available in one mirror-image form, with their respective enantiomers being scarce and expensive. Moreover, the unnatural isomers may be endowed with interesting and potentially useful biological properties. Now, Sarah Jenkinson and co-workers from the University of Oxford, alongside colleagues from Kagawa University, have reported a rapid and easily scalable conversion of natural D-glucose to either L-glucose or L-glucoronic acid.

D-Glucose has a six-carbon chain and four stereocentres, but Jenkinson and co-workers planned a synthetic conversion to the enantiomer by forming a new carbon–carbon bond at one end of the chain, and breaking one at the other. The first step in the sequence is a Felkin–Anh Kiliani ascension — a reaction known since the 1930s — in which cyanide is diastereoselectively added to the aldehyde before hydrolysis of the nitrile to produce a seven-carbon sodium carboxylate. Reduction of the carboxylate would lead to a meso-intermediate, but Jenkinson and co-workers avoid this with a short and highly effective protecting group sequence. In one step, the six hydroxy groups (three diols) are protected as a triacetonide group and the carboxylate is converted to an ester. Only one of three possible triacetonide isomers is formed, and then one acetonide is unmasked in preparation for the key carbon–carbon cleavage reaction. Reduction of the ester, oxidative cleavage of the diol and global deprotection leads to L-glucose. A small change to the sequence, with oxidative cleavage of the diol first, before an unmasking of the hydroxy groups and cleavage of the ester leads to L-glucoronic acid.

The L-enantiomer of glucose is known to be almost as sweet as its natural counterpart, but has no calorific value. Some nasty side-effects, however, will prevent use in food, but this synthesis will provide a useful starting point for that of other L-sugars and investigations of their biological properties.

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Davey, S. Through the looking glass. Nature Chem 6, 87 (2014). https://doi.org/10.1038/nchem.1865

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