Syntheses of the P-methylase substrates of the bialaphos biosynthetic pathway

Towards Enantiomerically Pure Unnatural α-Amino Acids via Photoredox Catalytic 1,4-Additions to a Chiral Dehydroalanine

Chemo- and diastereoselective 1,4-conjugate additions of anionic and radical C-nucleophiles to a chiral bicyclic dehydroalanine (Dha) are described. Of particular importance, radical carbon photolysis by a catalytic photoredox process using a simple method with a metal-free photocatalyst provides exceptional yields and selectivities at room temperature. Moreover, these 1,4-conjugate additions offer an excellent starting point for synthesizing enantiomerically pure carbon-β-substituted unnatural α-amino acids (UAAs), which could have a high potential for applications in chemical biology.

Enzymatic kinetic resolution of desmethylphosphinothricin indicates that phosphinic group is a bioisostere of carboxyl group

Escherichia coli glutamate decarboxylase (EcGadB), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, is highly specific for L-glutamate and was demonstrated to be effectively immobilised for the production of γ-aminobutyric acid (GABA), its decarboxylation product. Herein we show that EcGadB quantitatively decarboxylates the L-isomer of D,L-2-amino-4-(hydroxyphosphinyl)butyric acid (D,L-Glu-γ-P_H), a phosphinic analogue of glutamate containing C-P-H bonds. This yields 3-aminopropylphosphinic acid (GABA-P_H), a known GABA_B receptor agonist and provides previously unknown D-Glu-γ-P_H, allowing us to demonstrate that L-Glu-γ-P_H, but not D-Glu-γ-P_H, is responsible for D,L-Glu-γ-P_H antibacterial activity. Furthermore, using GABase, a preparation of GABA-transaminase and succinic semialdehyde dehydrogenase, we show that GABA-P_H is converted to 3-(hydroxyphosphinyl)propionic acid (Succinate-P_H). Hence, PLP-dependent and NADP⁺-dependent enzymes are herein shown to recognise and metabolise phosphinic compounds, leaving unaffected the P-H bond. We therefore suggest that the phosphinic group is a bioisostere of the carboxyl group and the metabolic transformations of phosphinic compounds may offer a ground for prodrug design.

Enantioselective synthesis of α-alkenyl α-amino acids via N-H insertion reactions

A new highly enantioselective route to α-alkenyl α-amino acid derivatives, which are important naturally occurring compounds with attractive bioactivity and synthetic utility, was developed using a N-H insertion reaction of vinyldiazoacetates and tert-butyl carbamate cooperatively catalyzed by achiral dirhodium(ii) carboxylates and chiral spiro phosphoric acids under mild, neutral conditions. This reaction has a broad substrate scope, a fast reaction rate (turnover frequency > 6000 h^-1), a high yield (61-99%), and excellent enantioselectivity (83-98% ee). The chiral spiro phosphoric acid, which is proposed to realize the enantioselectivity of the insertion reaction by promoting the proton transfer of a ylide intermediate by acting as a chiral proton shuttle catalyst, can suppress several usual side reactions of vinyldiazoacetates and broaden the applications of these versatile carbene precursors in organic synthesis. To our knowledge, it is the first highly enantioselective carbene insertion reaction of vinyldiazoacetates with heteroatom-hydrogen bonds in which the heteroatom has lone-pair electrons.

Investigation of enzymatic C-P bond formation using multiple quantum HCP nuclear magnetic resonance spectroscopy

The biochemical mechanism for the formation of the C-P-C bond sequence found in l-phosphinothricin, a natural product with antibiotic and herbicidal activity, remains unclear. To obtain further insight into the catalytic mechanism of PhpK, the P-methyltransferase responsible for the formation of the second C-P bond in l-phosphinothricin, we utilized a combination of stable isotopes and two-dimensional nuclear magnetic resonance spectroscopy. Exploiting the newly emerged Bruker QCI probe (Bruker Corp.), we specifically designed and ran a (13) C-(31) P multiple quantum (1) H-(13) C-(31) P (HCP) experiment in (1) H-(31) P two-dimensional mode directly on a PhpK-catalyzed reaction mixture using (13) CH3 -labeled methylcobalamin as the methyl group donor. This method is particularly advantageous because minimal sample purification is needed to maximize product visualization. The observed 3:1:1:3 multiplet specifically and unequivocally illustrates direct bond formation between (13) CH3 and (31) P. Related nuclear magnetic resonance experiments based upon these principles may be designed for the study of enzymatic and/or synthetic chemical reaction mechanisms.

Peptide-tethered monodentate and chelating histidylidene metal complexes: synthesis and application in catalytic hydrosilylation

The Nδ,Nε-dimethylated histidinium salt (His*) was tethered to oligopeptides and metallated to form Ir(III) and Rh(I) NHC complexes. Peptide-based histidylidene complexes containing only alanine, Ala-Ala-His*-[M] and Ala-Ala-Ala-His*-[M] were synthesised ([M] = Rh(cod)Cl, Ir(Cp*)Cl2), as well as oligopeptide complexes featuring a potentially chelating methionine and tyrosine residue, Met-Ala-Ala-His*-Rh(cod)Cl and Tyr-Ala-Ala-His*-Rh(cod)Cl. Chelation of the methionine-containing histidylidene ligand was induced by halide abstraction from the rhodium centre, while tyrosine remained non-coordinating under identical conditions. High catalytic activities in hydrosilylation were achieved with all peptide-based rhodium complexes. The cationic S(Met),C(His*)-bidentate peptide rhodium catalyst outperformed the monodentate neutral peptide complexes and constitutes one of the most efficient rhodium carbene catalysts for hydrosilylation, providing new opportunities for the use of peptides as N-heterocyclic carbene ligands in catalysis.

A chelating tetrapeptide rhodium complex comprised of a histidylidene residue: biochemical tailoring of an NHC-Rh hydrosilylation catalyst

Coupling of a histidinium salt with a MetAlaAla amino acid sequence followed by metallation with [RhCl(cod)](2) yields a rhodium(I) NHC complex with a pending peptide residue. Methionine chelation, induced by chloride abstraction from the metal coordination sphere, affords an efficient hydrosilylation catalyst precursor comprised of a peptidic macrocyclic chelate backbone.

In vitro characterization of a heterologously expressed nonribosomal Peptide synthetase involved in phosphinothricin tripeptide biosynthesis

The late stages of biosynthesis of phosphinothricin tripeptide (PTT) involve peptide formation and methylation on phosphorus. The exact timing of these transformations is not known. To provide insight into this question, we developed a heterologous expression system for PhsA, one of three NRPS proteins in PTT biosynthesis. The apparent k(cat)/K(m) value for ATP-pyrophosphate exchange activity for d,l-N-acetylphosphinothricin was 3.5 muM(-1) min(-1), whereas the k(cat)/K(m,app) for l-N-acetyldemethylphosphinothricin was 0.5 microM(-1) min(-1), suggesting the former might be the physiological substrate. Each substrate could be loaded onto the phosphopantetheine arm of the thiolation domain as observed by Fourier transform mass spectrometry (FTMS).