F-19 NMR study on the biological Baeyer-Villiger oxidation of acetophenones

The biological Baeyer-Villiger oxidation of acetophenones was studied by F- 19 nuclear magnetic resonance (NMR). The F-19 NMR method was used to charac terise the time-dependent conversion of various fluorinated acetophenones i n either whole cells of Pseudomonas fluorescens ACB or in incubations with purified 4'- hydroxyacetophenone monooxygenase (HAPMO). Whole cells of P. f luorescens ACB converted 4'-fluoroacetophenone to 4-fluorophenol and 4'-flu oro-2'-hydroxyacetophenone to 4-fluorocatechol without the accumulation of 4'-fluorophenyl acetates. In contrast to 4-fluorophenol, 4-fluorocatechol w as further degraded as evidenced by the formation of stoichiometric amounts of fluoride anion. Purified HAPMO catalysed the strictly NADPH-dependent c onversion of fluorinated acetophenones to fluorophenyl acetates. Incubation s with HAPMO at pH 6 and 8 showed that the enzymatic Baeyer-Villiger oxidat ion occurred faster at pH 8 but that the phenyl acetates produced were bett er stabilised at pH 6. Quantum mechanical characteristics explained why 4'- fluoro-2'-hydroxyphenyl acetate was more sensitive to base-catalysed hydrol ysis than 4'-fluorophenyl acetate. All together,F-19 NMR proved to be a val id method to evaluate the biological conversion of ring-substituted acetoph enones to the corresponding phenyl acetates, which can serve as valuable sy nthons for further production of industrially relevant chemicals.