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Structure, dynamics and assembly of folded helical nanoarchitectures

Biocatalytic Oxygenations in
In this contribution our recent studies in the field Bioactive Compound Synthesis
intramolecular Diels-Alder cyclizations are presented. Utilizing microwave irradiation as sustainable energy source, polycyclic scaffolds were accessed in enantioselective manner with high control over several chiral centers in a minimum number of Institute for Applied Synthetic Chemistry Getreidemarkt 9/163-OC, A-1060 Vienna, Austria 1,2-dioxygenases
Introduction

Enzyme mediated oxygenations represent a highly environmentally benign class of transformations, which cannot be carried out enantioselectively using conventional synthetic approaches in many cases. However, wide-spread application among the community of synthetic chemists has so far been hampered be several obstacles. Our approach aims at the development of a biocatalyst based toolbox for enantioselective oxidation reactions. It is based on the utilization of whole-cells to overcome the problems of cofactor recycling and protein isolation and takes advantage of wild- type, mutant, and recombinant strains of living microorganisms. Scheme 1. Dihydroxylation of aryl carboxylates by mutant
whole-cells and subsequent synthetic elaboration 1,2-Dihydroxylation of Aryl Carboxylates
implementing intramolecular cycloaddition reactions under While the potential of microbial 2,3-dihydroxylation has been demonstrated in recent years, the related enzymatic 1,2- dioxygenation of aryl carboxylates has been neglected to a Microbial Baeyer-Villiger Oxidation
greater extent. This biotransformation utilizes planar aromatic systems to generate quaternary and tertiary chiral centers. The Baeyer-Villiger-type oxidation reactions using biotransformation is performed with mutant microorganisms biocatalysts represent a powerful methodology for the one-step with a deficient metabolic pathway enabling the accumulation asymmetric synthesis of chiral lactones. Flavin dependent of highly functionalized intermediates in enantiomerically pure monooxygenases have been established as versatile chiral form. The obtained compounds represent versatile platforms catalysts for this biooxidation and these enzymes are able to that offer a great variety of options for subsequent functional transform a large spectrum of non-natural substrates in high Scheme 2. Microbial Baeyer-Villiger oxidation of prochiral ketones using recombinant whole-cells and subsequent synthetic elaboration
towards the preparation of natural products and bioactive compounds.
The utilization of recombinant living cells circumvents the obstacle to recycle non-covalently bound cofactors such as NAD(P)H and provides easy to handle catalytic entities. According to recent reports, the stability of Baeyer-Villiger monooxygenases in isolated form is limited due to loss of cofactors and concomitant decrease of structural integrity. Hence, application of such enzymes in their natural environment inside living cells give superior results with respect to biocatalyst performance. In addition, the required proteins are produced in high concentration by strong promoters and become the dominant fraction in the cell's proteome. As a consequence, such high performance overexpression systems minimize the chance of unwanted side reactions caused by competing enzymes.2 Figure 2. Phylogenetic tree representation of (putative) BVMO
sequences, indicating their relative relationship based on
sequence homology. Sequences were retrieved with the PHI-
BLAST option using the sequence of HAPMO as seed and the BVMO-“fingerprint” sequence FXGXXXHXXXW as pattern. Characterized BVMOs are labeled with established acronyms. Taking advantage of the natural diversity of Baeyer-Villiger monooxygenases available in our lab subcloned in recombinant overexpression systems (Figure 2), we are Figure 1. Phylogenetic tree of BVMOs originating from
currently assessing substrate profiles in order to identify Acinetobacter, Arthrobacter, Brachymonas, Brevibacterium, unique properties of this enzyme family. In this context we Comamonas, and Rhodococcus species using the N4- have developed a rapid parallel screening format, which diaminopropane monoxygenase from Sinorhizobium meliloti provides information on the stereopreference and catalytic performance of recombinant microbial strains. In addition, Sino) as outgroup (1000 bootstraps). modifications of the enzymes by random and knowledge- Recently, our group has discovered the enantiodivergent based mutagenesis and the effect on biotransformation biooxidation of a number of prochiral ketones by various efficiency and selectivity are currently investigated. Baeyer-Villigerases. In combination with novel data on the regiodivergent transformation of racemic substrates, the Acknowledgements
classification of a small library of 8 enzymes - available as The author thanks numerous co-workers during the recent recombinant whole-cell expression systems - into clusters is years. Financial support for these projects by the Austrian presented. These results represent the first connection of Science Fund FWF (P16373 & I19-B10), by the European protein sequence information with stereopreference of such Union within the Mobility Programme of FP5 (Marie-Curie biocatalysts and suggests the existence of two distinct enzyme Training Site GEMCAT, HPMT-CT-2001-00243), and by Vienna University of Technology is gratefully acknowledged. This work was carried out in context with COST Action D25 – Applications of stereoselective Baeyer-Villiger biooxidations for the synthesis of natural products and bioactive compounds will be outlined on structurally diverse lactones starting from carbo- References
and heterocyclic precursor ketones using the above enzyme [1] Mihovilovic, M.D.; Leisch, H.G.; Mereiter, K. Tetrahedron library (Scheme 2). A number of selected case studies will be Lett. 2004, 45, 7087-7090.
[2] Mihovilovic, M.D.; Müller, B.; Stanetty, P. Eur. J. Org. presented to demonstrate the specific properties of some Chem. 2002, 3711-3730. Mihovilovic, M.D.; Rudroff, F.;
representatives of the library. The desymmetrization of Grötzl, B. Curr. Org. Chem. 2004, 8, 1057-1069.
prochiral ketone substrates will be emphasized as most [3] Mihovilovic, M.D.; Rudroff, F.; Grötzl, B.; Kapitan, P.; efficient method to introduce chirality. With this approach up to Snajdrova, R.; Rydz, J.; Mach, R. Angew. Chem. 2005,
four novel chiral centers could be generated in stereoselective 117, 3675-3679; Angew. Chem. Int. Ed. 2005, 44, 3609-
Baeyer-Villiger biooxidations, moreover, allowing access to both antipodal lactone products in high optical purity.

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