Catalytic Asymmetric Carbon−Carbon Bond Formations: Their Evolution from Biocatalysis to Organocatalysis over the Millennium

TiCl4-promoted condensation of methyl acetoacetate, isobutyraldehyde, and indole: a theoretical and experimental study

A combined DFT and experimental study of the title reaction points to a three-step mechanism: (1) titanium enolate formation, (2) Knoevenagel condensation of enolate and aldehyde, and (3) Michael addition of indole.

C3-Symmetric chiral trisimidazoline-catalyzed Friedel–Crafts (FC)-type reaction

The first imidazoline-catalyzed enantioselective Friedel–Crafts (FC)-type reaction was established using C₃-symmetric chiral trisimidazolines.

A highly productive, whole-cell DERA chemoenzymatic process for production of key lactonized side-chain intermediates in statin synthesis

Employing DERA (2-deoxyribose-5-phosphate aldolase), we developed the first whole-cell biotransformation process for production of chiral lactol intermediates useful for synthesis of optically pure super-statins such as rosuvastatin and pitavastatin. Herein, we report the development of a fed-batch, high-density fermentation with Escherichia coli BL21 (DE3) overexpressing the native E. coli deoC gene. High activity of this biomass allows direct utilization of the fermentation broth as a whole-cell DERA biocatalyst. We further show a highly productive bioconversion processes with this biocatalyst for conversion of 2-substituted acetaldehydes to the corresponding lactols. The process is evaluated in detail for conversion of acetyloxy-acetaldehyde with the first insight into the dynamics of reaction intermediates, side products and enzyme activity, allowing optimization of the feeding strategy of the aldehyde substrates for improved productivities, yields and purities. The resulting process for production of ((2S,4R)-4,6-dihydroxytetrahydro-2H-pyran-2-yl)methyl acetate (acetyloxymethylene-lactol) has a volumetric productivity exceeding 40 g L⁻¹ h⁻¹ (up to 50 g L⁻¹ h⁻¹) with >80% yield and >80% chromatographic purity with titers reaching 100 g L⁻¹. Stereochemical selectivity of DERA allows excellent enantiomeric purities (ee >99.9%), which were demonstrated on downstream advanced intermediates. The presented process is highly cost effective and environmentally friendly. To our knowledge, this is the first asymmetric aldol condensation process achieved with whole-cell DERA catalysis and it simplifies and extends previously developed DERA-catalyzed approaches based on the isolated enzyme. Finally, applicability of the presented process is demonstrated by efficient preparation of a key lactol precursor, which fits directly into the lactone pathway to optically pure super-statins.

Carbene catalysts

The use of N-heterocyclic carbenes as catalysts for organic transformations has received increased attention in the past 10 years. A discussion of catalyst development and nucleophilic characteristics precedes a description of recent advancements and new reactions using N-heterocyclic carbenes in catalysis.

Dinuclear chiral vanadium catalysts for oxidative coupling of 2-naphthols via a dual activation mechanism

This review describes our recent efforts in the development of chiral dinuclear vanadium complexes that work as dual activation catalysts for the oxidative coupling of 2-naphthols. The dinuclear vanadium(iv) complex (R(a),S,S)- was prepared by complexation of VOSO(4) with the Schiff base derived from (R)-3,3'-diformyl-2,2'-dihydroxy-1,1'-binaphthyl () and (S)-tert-leucine. Since the dinuclear vanadium(iv) complex was found to be readily oxidized to afford a corresponding vanadium(v) species during preparation in air, a new synthetic procedure using VOCl(3) has been applied towards dinuclear vanadium(v) complexes (R(a),S,S)- and (R(a),S,S)-. To the best of our knowledge, (R(a),S,S)-, and show considerably higher catalytic activity than previously reported vanadium complexes for the oxidative coupling of 2-naphthols.

Analysis of an asymmetric addition with a 2:1 mixed lithium amide/n-butyllithium aggregate

A 2:1 lithium amide/ n-butyllithium aggregate 1 is investigated as an asymmetric addition template in hydrocarbon solvents. Several different chiral lithium amides were synthesized from l-valine and tested in the asymmetric addition of n-BuLi to various aldehydes. Enantiomeric excesses up to 83% were obtained in the case of the addition of n-BuLi to pivaldehyde at -116 degrees C in pentane. (1)H and (13)C INEPT DOSY were utilized to characterize a new trimeric complex 12 between 2 equiv of lithium amide and 1 equiv of lithium alkoxide. This mixed aggregate strongly indicates the possibility of product-induced chirality inhibition that is detrimental to the enantioselectivity of asymmetric addition reaction.

Kinetic plasticity and the determination of product ratios for kinetic schemes leading to multiple products without rate laws — New methods based on directed graphs

This paper presents two new and fast methods of determining product ratios for kinetic schemes leading to more than one product on which the Acree–Curtin–Hammett (ACH) principle is based. The methods involve rewriting a given kinetic scheme as a directed graph with nodes and arrows connecting the nodes and takes advantage of the directionality of the kinetic arrows and the enumeration of paths to the various target product nodes. The first, based on path divergent trees, is computationally simpler but works under a specific set of conditions, whereas the second, based on an adapted version of Chou’s graphical method, works for all cases. By means of illustrated examples, both methods are shown to be completely verifiable with conventional more tedious treatments based on rate law determinations. The directed graph concept also works for kinetic schemes that involve entirely equilibrated species. In addition, the paper extends these ideas to variants of the basic ACH scheme, thereby testing the validity of the ACH principle and bringing about a deeper understanding of it. Generalization of the results yields a new parameter, called degree of kinetic plasticity, which completely describes the dynamics of kinetic resolution between the boundary limits of ACH behaviour (100% kinetic plasticity) and anti-ACH behaviour (100% kinetic rigidity). It is shown that this parameter is a good descriptor of all possible scenarios between and including these limits and can be determined experimentally by conducting a new kind of product study that tracks the behaviour of final product excesses as a function of initial substrate excesses. The resulting plot is always linear with a positive slope. The degree of kinetic plasticity is found by simply subtracting the slope from unity. These ideas are tested on complex kinetic schemes exhibiting dynamic kinetic resolution (DKR) by means of organocatalysis.Key words: physical organic chemistry, kinetics, mechanism, directed graph, Chou digraph, Chou graphical rule, Acree-Curtin-Hammett principle, product ratio, dynamic kinetic resolution, organocatalysis.

Stereoselective Synthesis of Quaternary Proline Analogues

This review describes available methods for the diastereoselective and asymmetric synthesis of quaternary prolines. The focus is on the preparation of alpha-functionalized prolines with the pyrrolidine moiety not embedded in a polycyclic frame. The diverse synthetic approaches are classified according to the bond which is formed to complete the quaternary skeleton.