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Sugar in Two Steps

Hexose sugars are naturally abundant, but it is often useful to modify their structures for chemical and biochemical studies. Standard synthetic routes tend to be long and tedious and require multiple protection steps. Northrup and MacMillan (p. 1752, published online 12 August 2004) now describe a reaction sequence for generating the sugars from achiral aldehyde precursors in just two steps, thereby offering a convenient means of preparing diverse structural variants. In the first step, α-oxyaldehydes are dimerized with L-proline as the only source of asymmetry throughout the sequence. In the second step, an aldol addition-cyclization step is controlled by variation of solvent and Lewis acid to afford any of three stereoisomeric products (glucose, mannose, or allose), all in high yield and stereochemical purity.