Enzyme-Catalyzed Preparation Of Arylhydroxylamines And Chiral Substituted Fluorenols

Arylhydroxylamines are an important class of compounds frequently used as key intermediates in the construction of numerous fine chemicals, many natural products and other useful biologically active compounds. They also exhibit a wide range of pharmacological and physiological activities. Several methods have been developed for the preparation of arylhydroxylamines, including catalytic transfer hydrogenation and metal-mediated reduction of the corresponding nitro compounds. However, it is known that during reduction of aromatic nitro compounds, intermediate derivatives are rapidly reduced to the corresponding amines and often form side products such as hydrazines, azoarenes and azoxyarenes.Enzyme-catalyzed preparation of arylhydroxylamines was explored for the first time. Using bakers' yeast as a biocatalyst, the chemoselective reduction of aromatic nitro compounds bearing electron-withdrawing groups gave the corresponding hydroxylamines with good to excellent conversion (hydroxylamine/amine = 70/0 ~ >98/2) under mild conditions. In all cases, apart from amines, only a few side products (< 2 %) were detected by the ~1H NMR spectrum of the crude products.In the past decades, plant cells have been widely used as the most promising biocatalysts for various organic reactions such as hydroxylation, glycosylation, hydrolysis, oxidation of alcohol, and reduction of ketone and olefin. However, the synthetic potential of plant cells as a reducing agent of the nitro group has never been discovered, despite the fact that the reduction of nitro compounds is one of the most classical reactions in organic synthesis.Using plant cells as a reducing agent in organic sythesis was first carried out. Various plant cells were able to reduce 4-nitro-l, 8-naphthalic anhydride. Cells from a grape (Vitis vinifera L.) exhibited the unprecedented chemoselectivity for reduction of conjugated aromatic nitro compounds (hydroxylamine/amine = 96/4 ~ 100/0). In addition, this chemoselective reaction has unique advantages, which did not require strict control of reaction time and amount of catalyst.Although the enantioselective reduction of prochiral ketones is the most important and powerful method for the preparation of enantiomerically pure alcohols, the reduction of fluorenones by this means is still extremely challenging. It may be due to the following two reasons: 1) High steric hindrance caused by rigid structure of substituted fluorenone preventsthe approach of the chiral reductor;2) The prochiral center is, moreover, linked to two coplanar aromatic rings, which due to strong conjugation are highly electronically similar and thus it becomes extremely difficult to distinguish between them.Highly enantioselective reduction of substituted fluorenones was first carried out. In the presence of DMSO as co-solvent and under vigorous agitation, baker's yeast in water was found to reduce substituted fluorenones to the corresponding fluorenols in good to excellent enantioselectivies (52% ~ 100% ee). The experimental results clearly revealed that when mass transfer limitations are overcome, baker's yeast is able to reduce even highly sterically hindered ketones, such as substituted fluorenones. Agitation efficiency, which has been almost neglected in enzyme-catalysed reactions, was proven to be crucial towhether or not the reaction could proceed.