Stereoselectivity of chloroperoxidase-dependent halogenation

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Abstract
The stereoselectivity of chloroperoxidase halogenation of 4 substrates was examined. Chloroperoxidase catalyzes the bromination, but not chlorination, of racemic 2-exo-methylbicyclo[2.2.1]hept-5-ene-2-endo-carboxylic acid (to the .delta.-lactone) and racemic bicyclo[3.2.0]hept-2-en-6-one (to the 2-exo-bromo-3-endo-hydroxybromohydrin). These products are obtained in near quantitative yield and are racemic. The circumstances of the bromination strongly suggest that halogenation does not occur at the active site but rather by chloroperoxidase-catalyzed formation of Br2 and its release into solution. The inability of chloroperoxidase to halogenate these 2 alkenes at its active site most probably derives from a steric exclusion from the active site. The stereoselectivity of 2 additional substrates that undergo active site chlorination was determined. Methionine is quantitatively converted to a 50:50 ratio of the 2 methionine sulfoxide diastereomers. 2-Methyl-4-propylcyclopentane-1,3-dione is quantitatively chlorinated to 2-chloro-2-methyl-4-propylcyclopentane-1,3-dione. On the basis of optical rotation and 1H NMR, this product is present as a 40:60 ratio of the racemic diastereomers. Active site chlorination by chloroperoxidase processed without appreciable stereoselectivity.