I decided to make isoxazole from a nitrile oxide cycloaddition. The precursor was chlorinated acetaldehyde oxime shown above. Into chloroform goes the acetaldehyde oxime, (E/Z mixture from Aldrich) and 1 equiv. N-Chlorosuccinimide at room temp. Right away the thing turns bright turquoise blue. The bluest thing I've ever seen out of an organic reaction. We're talking like CuSO4 or TiCl4 blue. Go try it yourself and impress your local biologist.
So what makes the reaction mixture blue? As usual, I haven't had time to look into the chemical literature since that would take time away from the pile 'o analogues still waiting to be made. It must be a radical or charge-transfer intermediate or something of the sort.