The reaction catalysed by type II dehydroquinase is shown to be exclusively anti in contrast to the syn stereochemistry of the reaction catalysed by type I dehydroquinase.Enzymes are generally considered to catalyse reactions by optimised mechanisms. It is unexpected, and therefore of great interest, when two mechanistically distinct enzymes are found which catalyse the same reaction. 1 Dehydroquinase catalyses the dehydration of 3-dehydroquinate 1 to 3-dehydroshikimate 2.2 Recent biochemical and genetic studies have shown that there are two different dehydroquinases (type I and type 11), which appear to be a result of convergent evolution.3 Both types of dehydroquinase are found in eukaryotes and procaryotes, and in some circumstances can be coproduced in the same organism.3.4 Not only do type I and type I1 dehydroquinases exhibit different physical and bio-

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The syntheses of 2-bromo-3-dehydroshikimic acid1 (5) and 2-bromoshikimic acid2 (8), are described. Their formation from (2R)-2-bromo-3-dehydroquinic acid (2) using dehydroquinase and shikimate dehydrogenase is also reported. The corresponding enzymatic formation of 2-fluoro-3-dehydroshikimic acid1 (6) and 2-fluoroshikimic acid2 (9) from (2R)-2-fluoro-3-dehydroquinic acid (3) are reported.

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