Fungi of the Beauveria spp., typified by Beauveria bassiana ATCC 7159, are among the most frequently used whole cell biocatalysts. They have been reported to catalyse many different reactions, including oxidative, reductive and hydrolytic transformations, of a wide range of substrates. This review covers the range and application of biocatalytic reactions of Beauveria spp., with emphasis on the scope and utility of Beauveria-catalysed reactions for preparative biotransformations.

The biocatalytic reactions of Beauveria spp.

The knowledge about these Baeyer-Villiger monooxygenases has grown tremendously since the first discovery and fundamen- tal progress in the understanding of structure, function, substrate specificities and other enzyme properties has been facilitated by the de- velopment of recombinant biocatalysts. Nature uses these biocatalysts in aerobic biodegradation pathways of cyclic and acyclic ketones and in the biosynthetic pathways of natural products. The excellent performance of Baeyer-Villiger monooxygenases in nature for the ca- talysis of Baeyer-Villiger oxidations with high chemo-, regio- and enantioselectivity is a role model for sustainable catalytic Baeyer- Villiger oxidations in organic synthesis. A broad range of biocatalytic conversions of cyclic ketones to lactones, linear ketones to esters, sulfoxidations and other oxidations is described. Applications in dynamic kinetic resolution as well as process and scale-up issues have been important in making this reaction platform attractive to industrial scale Baeyer-Villiger oxidations. New discoveries of Baeyer- Villiger monooxygenases in biosynthesis are promising for highly selective oxidations. The Baeyer-Villiger oxidation (1) has become a highly useful synthetic tool for a variety of oxidative transformations like cyclic ketones to lactones, linear ketones to esters, aldehydes to alcohols and aromatic aldehydes to phenols (2,3). The central role of Baeyer-Villiger oxidation in organic synthesis is based on tolerance of other functional groups, predictable regioselective conversions and retention of configuration for the � -carbon of the migrating group. As many synthetic sequences rely on this selective introduction of oxygen into the carbon backbone, it is of high interest for the synthetic chemist to focus on improvements of this classical oxida- tion methodology in organic synthesis. Three main directions are thereby of central importance. First, non-specific side-reactions, over-oxidation and rearrangements can introduce major hurdles. Therefore, an increase in chemo-, regio- and enantioselectivity is a priority and will simplify product workup. Second, the combination of safety issues of the traditionally used peroxy acids as oxidants with flammable solvents become increasingly challenging, if the reaction is scaled up in reactor volume. The replacement of tradi- tionally used organic solvents and peroxy acids is a major goal. Third, the impact on the environment needs to be minimized, as the peroxy acids have to be used in stoichiometric amounts in the clas- sical Baeyer-Villiger oxidation. The amount of waste accumulated during the reaction also increases stoichiometrically with the scale- up of the reaction. Strategies to avoid the use of peroxy acids as oxidants have focused on hydrogen peroxide, which will yield wa- ter as the stoichiometric waste product. Since hydrogen peroxide does not react directly with the ketones, homogeneous and hetero- geneous catalysts have been developed for catalytic Baeyer-Villiger oxidations with high chemoselectivity. It has so far however been

Applications of Baeyer-Villiger Monooxygenases in Organic Synthesis

Microbial Baeyer-Villiger oxidation of steroidal ketones using Beauveria bassiana: Presence of an 11α-hydroxyl group essential to generation of D-homo lactones.

A series of chalcones (1–9) was synthesized by base catalyzed aldol condensation with 50–94% yields. These α,β-unsaturated carbonyl compounds were used as substrates in biotransformation reactions mediated by three industrial Saccharomyces cerevisiae strains in biphasic systems. Several reaction parameters were evaluated, such as yeast concentration, temperature, pH, substrate concentration, organic solvent, volume of aqueous and organic phases and the influence of substituent groups on chalcones 1–9. The biotransformation was chemoselective and formed only the corresponding saturated ketones. The highest conversion (>99%) to the dihydrochalcone was obtained at 30–45 °C and pH above 5.5, while the cellular and substrate concentrations also showed a strong influence on the biohydrogenation reaction. Organic solvents with log P >3.2 (hexane or heptane) were the most appropriate, and 40–80% of aqueous phase allowed the highest conversions probably by maintaining the yeast enzymes catalytically active. The influence of substituents on rings A and B of chalcones 1–9 was low and no correlation between the donor or withdrawing electron groups was observed.

Efficient chemoselective biohydrogenation of 1,3-diaryl-2-propen-1-ones catalyzed by Saccharomyces cerevisiae yeasts in biphasic system

The ene-reductases mediated bioreduction of a selection of open-chain α-alkyl-β-aryl enones afforded the corresponding saturated α-chiral ketones in high yield and optical purity in several cases. The stereo-electronic requirements of the reaction have been investigated, considering the nature and location of substituents on the aromatic ring as well as the steric hindrance at the α-position and adjacent to the carbonyl functionality. The general considerations drawn allow us to guide the design of α,β-unsaturated ketones to be employed as substrates of ene-reductases in future preparative applications. An interesting case of orthogonality between enzyme-based and substrate-based stereocontrol within the highly homologous ene-reductases from Saccharomyces species (OYE1–3) has been reported and rationalized with the help of computational docking studies. Furthermore, to demonstrate the synthetic versatility of the reaction, the key chiral precursors of biologically active compounds such as (2′R)-stenusines and (S)-iopanoic acid were obtained. The very robust protocol allowed us to run the reactions on preparative scale in quantitative yields, with a simple work-up and no chromatographic purification steps.

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Substrate scope and synthetic applications of the enantioselective reduction of α-alkyl-β-arylenones mediated by Old Yellow Enzymes

α,β-unsaturated aryl ketones, 1a–12 , have been submitted to the action of the fungus Beauveria bassiana (ATCC 7159) in growing conditions. The saturation of the double bond strictly depends from the substituent α to the carbonyl group. The saturated ketone is then oxidised in a Baeyer-Villiger type reaction. This new oxidative capacity of the fungus has been studied and the adaptability of the micro organism towards structural modifications has been investigated.

Jose Minut

Papers

On the microbial transformation of α,β-unsaturated aryl ketones by the fungus Beauveria bassiana