Azidolysis of α,β-Epoxycarboxylic Acids. A Water-Promoted Process Efficiently Catalyzed by Indium Trichloride at pH 4.0

The catalytic efficiency of InCl3, Yb(OTf)3, and Sc(OTf)3 in the azidolysis of α,β-epoxycarboxylic acids has been studied in water and in organic solvents, for comparison using NaN3 and Me3SiN3 as the source of the azido group. In water, the catalytic effectiveness of these metal salts strongly depends on the pH of the aqueous medium and on the type of Lewis acid catalyst. In water their catalytic activity is mostly due to the corresponding aqua ion species, the concentration of which becomes significant when the pH of the aqueous medium is below the corresponding pK 1,1 hydrolysis constant. The process is more efficient in water than in organic solvents. At pH 4.0, InCl3 is a far better catalyst than Yb(OTf)3 or Sc(OTf)3 and allows the highly regio- and diasteroselective preparation of β-azido-α-hydroxycarboxylic acids, which can be isolated in pure form in very high yields.