Methodologies for the alpha-monohalogenation of
acid sensitive ketones, especially cyclic, acid-sensitive, ketalized ketones. As one approach, the
ketone is reacted with a
halogen donor compound, e.g., N-chlorosuccinimide, in
anhydrous, highly polar organic reagents such as
dimethylformamide (DMF). As another monohalogenation approach, it has been observed that organic salts generated from amines and carboxylic acids catalyze the monohalogenation of ketalized
ketone in reagents comprising
alcohol solvent (
methanol,
ethanol, isopropanol, etc.). The monohalogenation is fast even at −5° C. The salt can be rapidly formed in situ from ingredients including amines and / or carboxylic acids without undue degradation of the
acid sensitive ketal.
Aryl ketones are monooxygenated using iodosylbenzene. This methodology is applied to monohalogenation of an
acid sensitive monoketal
ketone. The ability to prepare monohalogenated, acid sensitive ketones facilitates syntheses using halogenated, acid sensitive ketones. As just one example, facile synthesis of halogenated, acid sensitive ketones provides a new approach to synthesize the S-ketal-acid S-MBA (S-methylbenzylamine) salt useful as an intermediate in the manufacture of a
glucokinase activator. As an overview of this scheme, a monohalogenated, cyclic, ketalized ketone is prepared using monohalogenation methodologies of the present invention. The halogenated compound is then subjected to a Favorskii rearrangement under conditions to provide the racemic acid counterpart of the desired chiral salt. The desired chiral salt is readily recovered in enantiomerically pure form from the
racemic mixture.