Sugar synthesis zinc iodide

The rare sugar D-ribulose has been prepared from the chiral aldehyde (18) as shown in Scheme 2. An important feature of this synthesis is the first step in which 2-lithiofuran can be added with very high specificity (>95%) to (19) in the presence of zinc iodide, whereas in its absence little specificity is observed. It is therefore presumed that addition actually takes place to a zinc chelate of (18). 

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