Enzymatic halogenation of pyrazoles and pyridine derivatives

Abstract Pyrazole, 1‐methylpyrazole and 3‐methylpyrazole are chlorinated by the enzyme chloroperoxidase from Caldariomyces fumago , in the presence of potassium chloride and hydrogen peroxide at p H 2.7, yielding the corresponding 4‐chloro derivatives in good yields. A 4 H ‐pyrazole is proposed as an intermediate in this reaction. 2‐Aminopyridine was converted regiospecifically by the same enzyme into 2‐amino‐3‐chloropyridine, and 8‐hydroxyquinoline gave its 5,7‐dibromo‐derivative in very good yield when bromide ion was used as the halide substrate.