Hans E. Schoemaker

Veratryl alcohol as a mediator and the role of radical cations in lignin biodegradation by Phanerochaete chrysosporium

The oxidative capacity of the ligninase from Phanerochaete chrysosporium toward monomethoxylated and dimethoxylated aromatic compounds was investigated. Phenylacetic acid derivatives were shown to be decarboxylated by the ligninase, via C‐C bond cleavage. Dimethoxylated substrates were much more readily oxidised by the ligninase than were monomethoxylated derivatives, but oxidation of the latter could be stimulated by catalytic amounts of 1,4‐dimethoxybenzene or veratryl alcohol. A mechanism based upon the ability of radical cations to function as one‐electron oxidants is described. The role of redox mediators in lignin degradation, and the biological significance of veratryl alcohol as a secondary metabolite of P. chrysosporium are discussed.

On the mechanism of enzymatic lignin breakdown

The recent discovery of an extracellular enzyme from Phanerochaete chrysosporium capable of degrading lignin model compounds has countered the view that reactive diffusible oxygen species are responsible for lignin biodegradation. In this paper we propose a mechanism which accounts for both the results obtained in enzymatic lignin degradation studies and in studies involving active oxygen species. The quintessence of the mechanism is initial one‐electron oxidation of the lignin model compounds or of a specific lignin subunit followed by subsequent breakdown reactions via radical cation intermediates. The implication of this type of mechanism on the oxidative biodegradation of the natural lignin polymer is discussed.

Chiral Induction Effects in Ruthenium(II) Amino Alcohol Catalysed Asymmetric Transfer Hydrogenation of Ketones: An Experimental and Theoretical Approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.

A regioselective reduction of gem-disubstituted succinimides

Abstract The NaBH4 reduction of mono- and disubstituted succinimides in the presence of hydrochloric acid leading to ω-carbinol-lactams shows a remarkable regio- and/or stereo-selectivity. The reduction takes place at the most substituted CO in the succinimides. Possible explanations are reviewed. The preparative value of the method is amply illustrated.

Single‐electron transfer processes and the reaction mechanism of enzymic degradation of lignin

Lignin degradation Single‐electron transfer Cα‐Cβ bond cleavage Phanerochaete chrysosporium Peroxidase compound I

Oxidation of veratryl alcohol by the lignin peroxidase of Phanerochaete chrysosporium Involvement of activated oxygen

The oxidation of veratryl alcohol by the lignin peroxidase of Phanerochaete chrysosporium was studied. Five products were identified: veratraldehyde, two quinones and two aromatic ring cleavage lactones. A similar product pattern was obtained with the 1‐electron oxidant cerium(IV). Under anaerobic reaction conditions or in the presence of Mn(II) only traces of quinones or lactones were detected besides veratraldehyde. This indicates the involvement of activated oxygen species in the enzyme reaction. Possible mechanisms for the formation of the primary oxidation products from veratryl alcohol are discussed.

On the mechanism of oxidation of non-phenolic lignin model compounds by the laccase-ABTS couple

On the Mechanism of Oxidation of Non-Phenolic Lignin Model Compounds by the Laccase-ABTS Couple By Andreas Muheim, Armin Fiechter, Patricia J. Harvey and Hans E. Schoemaker 1 Eidgen ssische Technische Hochschule Z rich, Institut f r Biotechnologie, ETH-H nggerberg, CH-8093 Z rich, Switzerland 2 Imperial College of Science,Technology and Medicine, Department of Biology, Prince Consort Road, London SW7 2BB, United Kingdom 3 DSM Research, Bio-organic Chemistry Section, P.O. Box 18, NL-6160 MD Geleen,The Netherlands

Enzymatic resolution of α,α-disubstituted α-amino acid esters and amides

The scope and limitations of the enzymatic resolution of α,α-disubstituted α-amino acid amides by an amino acid amidase from Mycobacterium neoaurum and of the corresponding ethyl esteis with Pig liver estetase (PLE) have been studied. Moderate enantiomeric excesses were obtained with PLE, with only a narrow substrate specificity. Mycobacterium neoaurum on the contrary yields a broad range of S-α,α-disubstituted α-amino acids 1 and the corresponding R-amides 2.

Biomimetic α-acylimmonium cyclisations of unactivated olefin

Cyclisation of olefinic ω-ethoxylactams 9-16 leads to ringclosed products in excellent yields. The reaction is weakly acid-catalyzed, in several cases also stereospecific and proceeds at ambient temperature. The structure and configuration of the products are discussed. Possible relationships with other immonium cyclisations and with cationic carbocyclic olefincyclisations are reviewed. The cyclisation of one example—13b—deserves special attention because of the concomitant operation of synchronous and stepwise cyclisation mechanisms.

Asymmetric reduction of ketones via whole cell bioconversions and transfer hydrogenation: complementary approaches

Abstract Prochiral aryl and dialkyl ketones were enantioselectively reduced to the corresponding alcohols using whole cells of the white-rot fungus Merulius tremellosus ono991 as a biocatalytic reduction system and ruthenium(II)–amino alcohol and iridium(I)–amino sulfide complexes as metal catalysts in asymmetric transfer hydrogenation. Comparison of the results showed that the corresponding chiral alcohols could be obtained with moderate to high enantioselectivities (e.e.s of up to 98%). The biocatalytic and transfer hydrogenation approaches appear to be complementary. The biocatalytic approach is the most suitable for the enantioselective reduction of chloro-substituted (aryl) ketones, whereas in the reduction of α,β-unsaturated compounds excellent results were obtained using the catalytic hydrogenation protocol.