Einhundert Jahre Vitamine - eine naturwissenschaftliche Erfolgsgeschichte

Modulating the Coupling Efficiency of P450 BM3 by Controlling Water Diffusion through Access Tunnel Engineering

Cytochromes P450 have gained much interest for their broad substrate scope in the catalysis of oxidation reactions for pharmaceuticals, plastics, and hormones. However, achieving high coupling efficiency by the engineering of P450s is still a big challenge. The presence of extra water around the active site is deemed to be related to uncoupling. In this study, the access tunnels of P450 BM3 from Bacillus megaterium are engineered to control water access from bulk solvent to the active site. Nine residues located in tunnels are investigated by site-saturation mutagenesis to reduce water diffusion, thereby improving the coupling efficiency. The recombined variant N319L/T411V/T436A shows improved coupling efficiency (from 31.2 % to 52.6 %). Tunnel polarity analysis and molecular dynamics simulation further indicate that reduced water molecules around the active site lead to higher coupling efficiency. Overall, this study provides valuable insight on improving coupling efficiency by controlling water diffusion through tunnel engineering.

Enzymatic Asymmetric Reduction of Unfunctionalized C=C Bonds with Archaeal Geranylgeranyl Reductases

The asymmetric reduction of activated C=C bonds such as enones is well established for non-enzymatic methods as well as in biocatalysis. However, the asymmetric reduction of unfunctionalized C=C bonds is mainly performed with transition metal catalysts whereas biocatalytic approaches are lacking. We have tested two FAD-dependent archaeal geranylgeranyl reductases (GGR) for the asymmetric reduction of isolated C=C bonds. The reduction of up to four double bonds in terpene chains with different chain lengths and head groups was confirmed. Methyl-branched E-alkenes were chemoselectively reduced in the presence of cyclic, terminal or activated alkenes. Using a removable succinate "spacer", farnesol and geraniol could be quantitatively reduced (>99 %). The reduction is strictly (R)-selective (enantiomeric excess >99 %). Hence, GGRs are promising biocatalysts for the asymmetric reduction of unactivated isolated C=C bonds, opening new opportunities for the synthesis of enantiopure branched alkyl chains.

Organophotocatalytic Aerobic Oxygenation of Phenols in a Visible-Light Continuous-Flow Photoreactor

A mild photocatalytic phenol oxygenation enabled by a continuous-flow photoreactor using visible light and pressurized air is described herein. Products for wide-ranging applications, including the synthesis of vitamins, were obtained in high yields by precisely controlling principal process parameters. The reactor design permits low organophotocatalyst loadings to generate singlet oxygen. It is anticipated that the efficient aerobic phenol oxygenation to benzoquinones and p-quinols contributes to sustainable synthesis.

A Systems Approach to a One-Pot Electrochemical Wittig Olefination Avoiding the Use of Chemical Reductant or Sacrificial Electrode

An unprecedented one‐pot fully electrochemically driven Wittig olefination reaction system without employing a chemical reductant or sacrificial electrode material to regenerate triphenylphosphine (TPP) from triphenylphosphine oxide (TPPO) and base‐free in situ formation of Wittig ylides, is reported. Starting from TPPO, the initial step of the phosphoryl P=O bond activation proceeds through alkylation with RX (R=Me, Et; X=OSO₂CF₃ (OTf)), affording the corresponding [Ph₃POR]⁺X⁻ salts which undergo efficient electroreduction to TPP in the presence of a substoichiometric amount of the Sc(OTf)₃ Lewis acid on a Ag‐electrode. Subsequent alkylation of TPP affords Ph₃PR⁺ which enables a facile and efficient electrochemical in situ formation of the corresponding Wittig ylide under base‐free condition and their direct use for the olefination of various carbonyl compounds. The mechanism and, in particular, the intriguing role of Sc³⁺ as mediator in the TPPO electroreduction been uncovered by density functional theory calculations.

Total Synthesis of α-Tocopherol through Enantioselective Iridium-Catalyzed Fragmentation of a Spiro-Cyclobutanol Intermediate

A conceptionally new strategy for the asymmetric (2R-selective) synthesis of α-tocopherol (vitamin E) was developed. In the stereocontrolled key step, a prochiral spiro[chromane-2,3'-cyclobutanol] unit is effectively desymmetrized under C-C bond activation in an unprecedented iridium-catalyzed transformation using (S)-DTBM-SegPhos as a chiral ligand (e.r. 97:3). To complete the synthesis, the side chain was attached through Ru-catalyzed cross-metathesis employing an alkene derived from (R,R)-hexahydrofarnesol. To suppress epimerization during the final hydrogenation, PtO₂ had to be used as a catalyst instead of Pd/C. In an alternative approach (employing a propargyl-substituted spiro-cyclobutanol), the side chain was constructed prior to the Ir-catalyzed ring fragmentation (>99:1 d.r.) through enyne cross-metathesis (using an alkene derived from (R)-dihydrocitronellal) followed by Cr-catalyzed 1,4-hydrogenation and (diastereoselective) Pfaltz hydrogenation of the resulting triple-substituted olefin. The work demonstrates the potential of iridium catalysis for enantioselective C-C bond activation.

Reduction of Phosphine Oxide by Using Chlorination Reagents and Dihydrogen: DFT Mechanistic Insights

Extensive DFT calculations provide detailed mechanistic insights into the metal-free reduction of phosphine oxide Ph₃ P=O by using chlorination reagents O=CClX (X=COCl, Cl, OCCl₃ and Ph) and H₂ . Fast electrophilic attack to the P=O group oxygen atom is favored by exergonic CO₂ release to form phosphonium Ph₃ PCl⁺ and chloride Cl^- , which may slowly cleave H₂ by an unstable HPh₃ PCl complex yielding Ph₃ PH⁺ and Cl^- ions in solution. Moderate heating is required to accelerate the slow H₂ -activation step and to eliminate HCl to form phosphine Ph₃ P instead of Ph₃ PH⁺ Cl^- salt as the desired product. Though partially quenched by Ph₃ P (and reactant Ph₃ P=O if present), borane B(2,6-F₂ C₆ H₃ )₃ can be still combined with Cl^- and Ph₃ P as reactive frustrated Lewis pair (FLP) catalysts.

Antivitamins B12--A Structure- and Reactivity-Based Concept

B12 -antimetabolites are compounds that counteract the physiological effects of vitamin B12 and related natural cobalamins. Presented here is a structure- and reactivity-based concept of the specific 'antivitamins B12 ': it refers to analogues of vitamin B12 that display high structural similarity to the vitamin and are 'locked chemically' to prevent their metabolic conversion into the crucial organometallic B12 -cofactors. Application of antivitamins B12 to healthy laboratory animals is, thus, expected to induce symptoms of B12 -deficiency. Antivitamins B12 may, hence, be helpful in elucidating still largely puzzling pathophysiological phenomena associated with B12 -deficiency, and also in recognizing physiological roles of B12 that probably still remain to be discovered.

Total synthesis of (R,R,R)-γ-tocopherol through Cu-catalyzed asymmetric 1,2-addition

Based on the asymmetric copper-catalyzed 1,2-addition of Grignard reagents to ketones, (R,R,R)-γ-tocopherol has been synthesized in 36 % yield over 12 steps (longest linear sequence). The chiral center in the chroman ring was constructed with 73 % ee by the 1,2-addition of a phytol-derived Grignard reagent to an α-bromo enone prepared from 2,3-dimethylquinone.

Total synthesis of (R,R,R)-α-tocopherol through asymmetric Cu-catalyzed 1,4-addition

By introducing a disposable activating substituent at C-3, the asymmetric 1,4-addition to a notoriously unreactive 2-substituted chromenone was achieved with high levels of (2R)-stereoselectivity in the presence of a chiral Cu(I)-phosphoramidite complex as a catalyst. This paved the way for an efficient and conceptually novel synthesis of (R,R,R)-α-tocopherol from readily available starting materials.