Edzard M. Geertsema

Bridging between Organocatalysis and Biocatalysis: Asymmetric Addition of Acetaldehyde to β‐Nitrostyrenes Catalyzed by a Promiscuous Proline‐Based Tautomerase

In recent years, organocatalysis has become one of the main areas in asymmetric catalysis of carbon–carbon bond-forming reactions. The fast evolution of the organocatalysis field has been particularly fueled by aminocatalysis, in which secondary and primary amines react with carbonyl compounds to give enamine and iminium ion intermediates. The field was completely transformed during the last two decades by the seminal contributions of List, MacMillan, Yamaguchi, and co-workers. The natural chiral amino acid proline and derivatives thereof were found to be powerful organocatalysts. These secondary amines are applied in substoichiometric quantities and afford high product yields and enantioselectivities in fundamental carbon–carbon bond-forming reactions such as aldolizations, 2,3b] Michael additions, 4, 5] Mannich reactions, 6] and Knoevenagel condensations. Inspired by the versatile success of proline and its derivatives as organocatalysts, we examined whether the enzyme 4-oxalocrotonate tautomerase (4-OT), which carries a catalytic amino-terminal proline (Pro = P), might be suitable to promiscuously catalyze carbon–carbon bondforming reactions. Herein, we describe the discovery and characterization of two 4-OT-catalyzed asymmetric carbon– carbon bond-forming Michael-type addition reactions. Considering our reported 4-OT-catalyzed aldolizations, this work is a pivotal step forward towards our aim to bridge organocatalysis and biocatalysis by developing a new class of biocatalysts that use the powerful proline-based enamine mechanism of organocatalysts but that take advantage of the water solubility and relatively high catalytic rates available with enzymes. A few elegant studies on promiscuous enzyme-catalyzed carbon–carbon bond-forming Michael additions have been reported, but most of these reactions proceed in organic solvents and with moderate stereocontrol. 4-OT is a stable enzyme composed of six identical subunits of only 62 amino acid residues each. It belongs to the tautomerase superfamily, a group of homologous proteins that share a conserved catalytic amino-terminal proline and a characteristic b-a-b structural fold. 12] 4-OT takes part in a degradation pathway for aromatic hydrocarbons in Pseudomonas putida mt-2, where it catalyzes the tautomerization of 2-hydroxy-2,4-hexadienedioate (1) into 2-oxo-3-hexenedioate (2, Scheme 1). The key catalytic residues of 4-OTare Pro-1,

Recent developments in enzyme promiscuity for carbon-carbon bond-forming reactions

Numerous enzymes have been found to catalyze additional and completely different types of reactions relative to the natural activity they evolved for. This phenomenon, called catalytic promiscuity, has proven to be a fruitful guide for the development of novel biocatalysts for organic synthesis purposes. As such, enzymes have been identified with promiscuous catalytic activity for, one or more, eminent types of carbon-carbon bond-forming reactions like aldol couplings, Michael(-type) additions, Mannich reactions, Henry reactions, and Knoevenagel condensations. This review focuses on enzymes that promiscuously catalyze these reaction types and exhibit high enantioselectivities (in case chiral products are obtained).

Optimizing rotary processes in synthetic molecular motors

We deal with the issue of quantifying and optimizing the rotation dynamics of synthetic molecular motors. For this purpose, the continuous four-stage rotation behavior of a typical light-activated molecular motor was measured in detail. All reaction constants were determined empirically. Next, we developed a Markov model that describes the full motor dynamics mathematically. We derived expressions for a set of characteristic quantities, i.e., the average rate of quarter rotations or “velocity,” V, the spread in the average number of quarter rotations, D, and the dimensionless Péclet number, Pe = V/D. Furthermore, we determined the rate of full, four-step rotations (Ωeff), from which we derived another dimensionless quantity, the “rotational excess,” r.e. This quantity, defined as the relative difference between total forward (Ω+) and backward (Ω−) full rotations, is a good measure of the unidirectionality of the rotation process. Our model provides a pragmatic tool to optimize motor performance. We demonstrate this by calculating V, D, Pe, Ωeff, and r.e. for different rates of thermal versus photochemical energy input. We find that for a given light intensity, an optimal temperature range exists in which the motor exhibits excellent efficiency and unidirectional behavior, above or below which motor performance decreases.

Biocatalytic Michael-Type Additions of Acetaldehyde to Nitroolefins with the Proline-Based Enzyme 4-Oxalocrotonate Tautomerase Yielding Enantioenriched γ-Nitroaldehydes

Call me Michaelase: The enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes the Michael-type addition of acetaldehyde to a collection of aromatic and aliphatic nitroolefins with high stereoselectivity producing precursors of γ-aminobutyric acid (GABA) analogues.

Promiscuous Catalysis of Asymmetric Michael-Type Additions of Linear Aldehydes to β-Nitrostyrene by the Proline-Based Enzyme 4-Oxalocrotonate Tautomerase

Exploiting catalytic promiscuity: The proline-based enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes asymmetric Michael-type additions of linear aldehydes--ranging from acetaldehyde to octanal--to trans-β-nitrostyrene in aqueous solvent. The presence of 1.4 mol% of 4-OT effected formation of the anticipated γ-nitroaldehydes in fair to good yields with dr values of up to 93:7 and ee values of up to 81 %.

Recent Advances in the Study of Enzyme Promiscuity in the Tautomerase Superfamily

Catalytic promiscuity and evolution: Many enzymes exhibit catalytic promiscuity--the ability to catalyze reactions other than their biologically relevant one. These reactions can serve as starting points for both natural and laboratory evolution of new enzymatic functions. Recent advances in the study of enzyme promiscuity in the tautomerase superfamily are discussed.

Enhancement of the Promiscuous Aldolase and Dehydration Activities of 4-Oxalocrotonate Tautomerase by Protein Engineering

Double play: The enzyme 4-oxalocrotonate tautomerase (4-OT) catalyzes not only the initial cross-coupling of acetaldehyde and benzaldehyde to yield 3-hydroxy-3-phenylpropanal, but also the subsequent dehydration of this aldol compound to yield cinnamaldehyde as the final product. Mechanism-inspired engineering provided an active site mutant (F50A) with strongly enhanced aldol condensation activity.

New Mechanistic Insight in the Thermal Helix Inversion of Second‐Generation Molecular Motors

The introduction of dibenzocyclohepten-5-ylidene as part of a unidirectional light-driven molecular motor allows a more complete picture of the pathway of thermal helix inversion to be developed. The most stable conformation is similar to that found in related motors in that it has, overall, an anti-folded structure with the substituent at the stereogenic centre adopting an axial orientation. Photochemical cis/trans isomerisation at -40 degrees C results in the formation of an isomer in a syn-folded conformation with the methyl group in an axial orientation. This contrasts with previous studies on related molecular rotary motors. The conformation of the higher energy intermediate typically observed for this class of compound is the anti-folded conformation, in which the methyl group is in an equatorial orientation. This conformation is available through an energetically uphill upper half ring inversion of the observed photochemical product. However, this pathway competes with a second process that leads to the more stable anti-folded conformation in which the methyl group is oriented axially. It has been shown that the conformations and pathways available for second-generation molecular motors can be described by using similar overall geometries. Differences in the metastable high-energy species are attributable to the relative energy and position on the reaction coordinate of the transition states. Kinetic studies on these new molecular motors thus provide important insights into the conformational dynamics of the rotation cycle.

Asymmetric synthesis of overcrowded alkenes by transfer of axial single bond chirality to axial double bond chirality

Optically active overcrowded alkenes were synthesized by employing bis-beta-naphthol as a chiral template during an intramolecular coupling reaction. The major isomer 2 has a unique helical structure with twisted and folded structural moieties. Removal of the chiral template afforded overcrowded thioxanthylidene 3 with 96 % ee, which indicates that no racemization or isomerization of the enantiomers took place.

An Unexpected Promiscuous Activity of 4-Oxalocrotonate Tautomerase : The cis-trans Isomerisation of Nitrostyrene

Serendipitous switch: While exploring cis-nitrostyrene as a potential electrophile in Michael-type addition reactions catalysed by the enzyme 4-oxalocrotonate tautomerase (4-OT), it was unexpectedly found that 4-OT catalyses the isomerisation of cis-nitrostyrene to trans-nitrostyrene (k(cat) /K(m) = 1.9×10(3)  M(-1)  s(-1) ).