Enantioselective Decarboxylative Protonation Research Articles

Organocatalytic enantioselective decarboxylative protonation of α-alkyl-α-aryl malonate monoesters.

In contrast to the well-established enzymatic enantioselective decarboxylative protonation (EDP), the corresponding chemocatalytic reactions of acyclic malonic acid derivatives remain challenging. Herein, we developed a biomimetic EDP of α-alkyl-α-aryl malonate monoesters using a chiral 1,2-trans-diaminocyclohexane-based N-sulfonamide as an organocatalyst. The method demonstrates excellent chemical yields, good enantioselectivity, mild reaction conditions, and the generation of only CO2 as waste.

Modular and diverse synthesis of amino acids via asymmetric decarboxylative protonation of aminomalonic acids.

Stereoselective protonation is a challenge in asymmetric catalysis. The small size and high rate of transfer of protons mean that face-selective delivery to planar intermediates is hard to control, but it can unlock previously obscure asymmetric transformations. Particularly, when coupled with a preceding decarboxylation, enantioselective protonation can convert the abundant acid feedstocks into structurally diverse chiral molecules. Here an anchoring group strategy is demonstrated as a potential alternative and supplement to the conventional structural modification of catalysts by creating additional catalyst-substrate interactions. We show that a tailored benzamide group in aminomalonic acids can help build a coordinated network of non-covalent interactions, including hydrogen bonds, π-π interactions and dispersion forces, with a chiral acid catalyst. This allows enantioselective decarboxylative protonation to give α-amino acids. The malonate-based synthesis introduces side chains via a facile substitution of aminomalonic esters and thus can access structurally and functionally diverse amino acids.

Polymer-supported thioamide-derived cinchona alkaloids as efficient organocatalysts for the enantioselective decarboxylative protonation

A new family of copolymers based on styrene and a polymerizable cinchonidine thioamide derivative was synthesized by conventional radical polymerization. The copolymers were tested as homogeneous supported organocatalysts in the enantioselective decarboxylative protonation (EDP) of α-aminomalonate hemiesters giving access to α-amino acids under very mild conditions. These macromolecular catalysts could be prepared on a large scale and outperformed their unsupported analogues providing the desired products in high yield and enantioselectivity. Furthermore, recycling experiments were conducted which showed good enantioselectivities.

Enantioselective decarboxylative protonation and deuteration of β-ketocarboxylic acids.

Enantioselective decarboxylative protonation of tetralone-derived β-ketocarboxylic acids was achieved with up to 89% enantiomeric excess (ee)-in the presence of a chiral primary amine catalyst. Furthermore, this method was applied to enantioselective deuteration to afford the corresponding α-deuterioketones with up to 88% ee.

Organocatalytic Enantioselective Decarboxylative Protonation Reaction of Meldrum's Acid Derivatives under PTC Conditions

An original organocatalyzed enantioselective protonation sequence of a transient quaternary ammonium enolate species has been developed by starting from readily available disubstituted Meldrum's acid derivatives and phenols. Under phase‐transfer‐catalytic (PTC) conditions, chiral nonracemic 2‐aryl propionic ester derivatives were obtained in good isolated yields with enantioselectivities up to 70 % ee. The usefulness of the approach was demonstrated by the synthesis of enantioenriched (S)‐ibuprofen.

Ultrasound‐Promoted Enantioselective Decarboxylative Protonation of α‐Aminomalonate Hemiesters by Chiral Squaramides: A Practical Approach to Both Enantiomers of α‐Amino Esters

Herein, we report an ultrasound‐promoted enantioselective decarboxylative protonation reaction of α‐aminomalonate hemiesters 1 in the presence of chiral cinchona‐derived squaramide Brønsted bases under mild conditions, which afforded both the (S)‐ and (R)‐enantiomers of α‐amino acid derivatives 2 in excellent yields (> 90 %) and excellent enantioselectivities (up to 98 % ee).

Towards a universal organocatalyst for the synthesis of enantioenriched phenylalanine derivatives by enantioselective decarboxylative protonation

Access to enantioenriched non-proteogenic phenylalanine derivatives is described using the enantioselective decarboxylative protonation reaction of amidohemimalonate esters catalysed by various cinchona-based compounds. This study compares the catalytic efficiency as well as the enantioselectivity induced by three types of common organocatalysts, namely thioureas, squaramides and bis-cinchona squaramides. One of the main outcome of this work is the observation of a significant influence of the N-protecting group of the hemimalonate on its interaction with the catalyst. This methodology carried out under mild conditions exhibits good substrate scope and functional group tolerance. A substoichiometric amount of catalyst can also be used in certain cases while affording good yields and selectivities.

The Enantioselective Dakin–West Reaction

AbstractHere we report the development of the first enantioselective Dakin–West reaction, yielding α‐acetamido methylketones with up to 58 % ee with good yields. Two of the obtained products were recrystallized once to achieve up to 84 % ee. The employed methylimidazole‐containing oligopeptides catalyze both the acetylation of the azlactone intermediate and the terminal enantioselective decarboxylative protonation. We propose a dispersion‐controlled reaction path that determines the asymmetric reprotonation of the intermediate enolate after the decarboxylation.

The Enantioselective Dakin-West Reaction.

Here we report the development of the first enantioselective Dakin-West reaction, yielding α-acetamido methylketones with up to 58 % ee with good yields. Two of the obtained products were recrystallized once to achieve up to 84 % ee. The employed methylimidazole-containing oligopeptides catalyze both the acetylation of the azlactone intermediate and the terminal enantioselective decarboxylative protonation. We propose a dispersion-controlled reaction path that determines the asymmetric reprotonation of the intermediate enolate after the decarboxylation.

Enzymatic enantioselective decarboxylative protonation of heteroaryl malonates.

The enzyme aryl/alkenyl malonate decarboxylase (AMDase) catalyses the enantioselective decarboxylative protonation (EDP) of a range of disubstituted malonic acids to give homochiral carboxylic acids that are valuable synthetic intermediates. AMDase exhibits a number of advantages over the non-enzymatic EDP methods developed to date including higher enantioselectivity and more environmentally benign reaction conditions. In this report, AMDase and engineered variants have been used to produce a range of enantioenriched heteroaromatic α-hydroxycarboxylic acids, including pharmaceutical precursors, from readily accessible α-hydroxymalonates. The enzymatic method described here represents an improvement upon existing synthetic chemistry methods that have been used to produce similar compounds. The relationship between the structural features of these new substrates and the kinetics associated with their enzymatic decarboxylation is explored, which offers further insight into the mechanism of AMDase.

Mechanistic Insights on Organocatalytic Enantioselective Decarboxylative Protonation by Epicinchona-Thiourea Hybrid Derivatives

Mechanism and the origin of enantioselectivity in the decarboxylative protonation of α-amino malonate hemiester promoted by epicinchona-thiourea hybrid organocatalyst is established by using the DFT(M06-2X/6-311+G**//ONIOM2) computational methods. The origin of stereoselectivity rendered by this hybrid bifunctional catalyst in asymmetric protonation is investigated for the first time using suitable transition-state models. A detailed conformational analysis of N-[3,5-bis(trifluoromethyl)]phenylthiourea-based epicinchonidine reveals the potential for a bifunctional mode of activation of the substrate α-amino malonate hemiester through hydrogen bonding. Six different conformer families differing in characteristic dihedral angles are identified within a range of 16 kcal/mol with respect to the lowest energy conformer. Different likely mechanistic pathways obtained through detailed analysis of the transition states and intermediates are compared. It is identified that in the preferred pathway, the decarboxylation is followed by a direct proton transfer from the chiral quinuclidinium moiety to the enolate carbon as opposed to a conventional protonation at the enolate oxygen followed by a keto-enol tautomerization. The factors responsible for high levels of observed stereoselectivity are traced to interesting hydrogen-bonding interactions offered by the thiourea-cinchona bifunctional framework. The predicted stereoselectivities using computed Gibbs free energies of diastereomeric transition states are in fair agreement with the experimental stereoselectivities.

Asymmetric Malonic and Acetoacetic Acid Syntheses – A Century of Enantioselective Decarboxylative Protonations

AbstractThe enantioselective decarboxylative protonation (EDP) of malonic or acetoacetic acid derivatives is a synthetic methodology by which the chirality of the product is generated during the enol/enolate protonation step. Although EDP is a century‐old reaction, it has not received much attention until recently. This review focuses on the EDP as an alternative to the strong‐base‐mediated deprotonation/asymmetric reprotonation for the stereocontrol of C–H bond formation. The diverse synthetic approaches are classified according to the type of catalysis used, which are organic, metallic or enzymatic.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Highly Enantioselective Decarboxylative Protonation of α‐Aminomalonates Mediated by Thiourea Cinchona Alkaloid Derivatives: Access to Both Enantiomers of Cyclic and Acyclic α‐Aminoacids.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Enantioselective Decarboxylative Protonation

A base-mediated enantioselective decarboxylative protonation of acyclic, cyclic, and bicyclic α-aminomalonate hemiesters is described. Presumably, the tertiary amine moiety of the thiourea-containing cinchona alkaloid acts as a base, promoting the formation of an enolate via decarboxylation. While binding the enolate through urea-mediated hydrogen bonds, the chiral catalyst may enantioselectively protonate this intermediate. Enantioenriched aminoesters were obtained in high yields and enantioselectivities. Both enan­tiomeric forms could be produced by using either base A or pseudoenantiomeric base B. Interestingly, a wide range of polar as well as apolar aprotic solvents afforded high selectivities.

Highly Enantioselective Decarboxylative Protonation of α-Aminomalonates Mediated by Thiourea Cinchona Alkaloid Derivatives: Access to Both Enantiomers of Cyclic and Acyclic α-Aminoacids

Thiourea derived cinchona alkaloids promote the asymmetric decarboxylative protonation of cyclic, acyclic, or bicyclic alpha-aminomalonate hemiesters under mild and metal-free conditions to afford enantioenriched aminoesters in high yields and enantioselectivities up to 93%. Both enantiomers of the aminoesters have been synthesized with the same selectivity when using organic base 3 and its pseudoenantiomer 6 derived from quinine.

Catalytic Enantioselective Decarboxylative Protonation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Catalytic Enantioselective Decarboxylative Protonation

We report a highly enantioselective, general catalytic system for the facile synthesis of tertiary stereocenters by protonation adjacent to cyclic ketones. The method relies on catalytic decarboxylative protonation of readily accessible racemic quaternary beta-ketoesters. A range of substituted cycloalkanone compounds can be accessed through this process with high levels of enantioselectivity.

New challenges in quantum chemistry: Quests for accurate calculations for large molecular systems.

As quantum chemistry plays a more and more central role in many complicated chemical problems, it has become necessary to obtain accurate results for large molecular systems. Conventional quantum chemistry methods are either too expensive to apply to large systems or too approximate for the results to be reliable, and they fail to satisfy this requirement. A variety of different approaches is being developed with the aim of achieving this goal: local correlation methods; divide-and-conquer methods; linear-scaling density functional methods based on the fast multipole and other approximations; effective potential methods; and hybrid methods. ONIOM (our N-layered integrated molecular orbital plus molecular mechanics method), developed by the authors, is a hybrid method in which a large molecular system is divided into onion-skin-like layers, and different quantum chemistry/molecular mechanics methods are used for different parts of the system; the results are combined to extrapolatively estimate the results of high-level calculation for the real system. Several applications of ONIOM will be discussed.