Chemo-enzymatic deracemization methods for the preparation of enantiopure non-natural α-amino acids

The mechanism of efficient asymmetric transfer hydrogenation of acetophenone using an iron(II) complex containing an (S,S)-Ph2PCH2CH═NCHPhCHPhN═CHCH2PPh2 ligand: partial ligand reduction is the key

On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of a highly active system generated from the precatalyst trans-[Fe(CO)(Br)(Ph(2)PCH(2)CH═N-((S,S)-C(Ph)H-C(Ph)H)-N═CHCH(2)PPh(2))][BPh(4)] (2) for the asymmetric transfer hydrogenation of acetophenone in basic isopropanol. An induction period for catalyst activation is observed before the catalytic production of 1-phenethanol. The activation step is proposed to involve a rapid reaction of 2 with excess base to give an ene-amido complex [Fe(CO)(Ph(2)PCH(2)CH═N-((S,S)-C(Ph)H-C(Ph)H)-NCH═CHPPh(2))](+) (Fe(p)) and a bis(enamido) complex Fe(CO)(Ph(2)PCH═CH-N-(S,S-CH(Ph)CH(Ph))-N-CH═CHPPh(2)) (5); 5 was partially characterized. The slow step in the catalyst activation is thought to be the reaction of Fe(p) with isopropoxide to give the catalytically active amido-(ene-amido) complex Fe(a) with a half-reduced, deprotonated PNNP ligand. This can be trapped by reaction with HCl in ether to give, after isolation with NaBPh(4), [Fe(CO)(Cl)(Ph(2)PCH(2)CH(2)N(H)-((S,S)-CH(Ph)CH(Ph))-N═CHCH(2)PPh(2))][BPh(4)] (7) which was characterized using multinuclear NMR and high-resolution mass spectrometry. When compound 7 is treated with base, it directly enters the catalytic cycle with no induction period. A precatalyst with the fully reduced P-NH-NH-P ligand was prepared and characterized by single crystal X-ray diffraction. It was found to be much less active than 2 or 7. Reaction profiles obtained by varying the initial concentrations of acetophenone, precatalyst, base, and acetone and by varying the temperature were fit to the kinetic model corresponding to the proposed mechanism by numerical simulation to obtain a unique set of rate constants and thermodynamic parameters.

One-pot chemoenzymatic synthesis of chiral 1,3-diols using an enantioselective aldol reaction with chiral Zn2+ complex catalysts and enzymatic reduction using oxidoreductases with cofactor regeneration

We previously reported on enantioselective aldol reactions of acetone and some aldehydes catalyzed by chiral Zn(2+) complexes of L-prolyl-pendant [12]aneN(4) (L-ZnL(1)) and L-valyl-pendant [12]aneN(4) (L-ZnL(2)) in aqueous solution. Here, we report on the one-pot chemoenzymatic synthesis of chiral 1,3-diols in an aqueous solvent system at room temperature by a combination of enantioselective aldol reactions catalyzed by Zn(2+) complexes of L- and D-phenylalanyl-pendant [12]aneN(4) (L-ZnL(3) and D-ZnL(3) ) and the successive enantioselective reduction of the aldol products using oxidoreductases with the regeneration of the NADH (reduced form of nicotinamine adenine dinucleotide) cofactor. The findings indicate that all four stereoisomers of 1,3-diols can be produced by appropriate selection of a chiral Zn(2+)-complex and an oxidoreductase commercially available from the "Chiralscreen OH" kit.

Unusual carbamate-directed CH-activation at an annulated ferrocenophane framework

The tetrahydroazepine-annulated [3]ferrocenophane carbamate (4) was synthesized by two different linear routes starting from the readily available α-dimethylamino[3]ferrocenophane-ortho-carbaldehyde rac-6. The carbamate directed lithiation of 4 resulted in a selective attack at a (Cp)C-H bond at the higher substituted "lower" [3]ferrocenophane Cp-ring to eventually yield the respective ester (18) after treatment with ClCO(2)Me.

Recent biocatalytic oxidation-reduction cascades

The combination of an oxidation and a reduction in a cascade allows performing transformations in a very economic and efficient fashion. The challenge is how to combine an oxidation with a reduction in one pot, either by running the two reactions simultaneously or in a stepwise fashion without isolation of intermediates. The broader availability of various redox enzymes nowadays has triggered the recent investigation of various oxidation–reduction cascades.

Simultaneous iridium catalysed oxidation and enzymatic reduction employing orthogonal reagents

An iridium catalysed oxidation was coupled concurrently to an asymmetric biocatalytic reduction in one-pot; thus it was shown for the first time that iridium- and alcohol dehydrogenase-catalysed redox reactions are compatible. As a model system racemic chlorohydrins were transformed to enantioenriched chlorohydrins via an oxidation-asymmetric reduction sequence.

Deracemization of mexiletine biocatalyzed by omega-transaminases

(S)- as well as (R)-mexiletine [1-(2,6-dimethylphenoxy)-2-propanamine], a chiral orally effective antiarrhythmic agent, was prepared by deracemization starting from the commercially available racemic amine using omega-transaminases in up to >99% ee and conversion with 97% isolated yield by a one-pot two-step procedure. The absolute configuration could be easily switched to the other enantiomer, just by switching the order of the applied transaminases. The cosubstrate pyruvate needed in the first oxidative step was recycled by using an amino acid oxidase.