Organocatalytic Conjugate Addition Research Articles

Peptide Catalyzed Conjugate Additions to β-Nitroacrylates - Steric Bulk Increases the Reaction Rate.

The organocatalytic conjugate addition of aldehydes to β-nitroacrylates provides direct access to β-ester-γ-nitroaldehydes and, thereby, common structural motifs of many bioactive compounds. However, the deactivation of amine-based catalysts by alkylation with the highly electrophilic nitroacrylates hampers this reaction. Here, we show that the peptide H-Mep-dPro-dGlu-NH2, which is reluctant to undergo alkylation, catalyzes this reaction at low catalyst loading (0.5-1 mol %) within short reaction times (15-60 min) to yield a broad range of β-ester-γ-nitroaldehydes with high stereoselectivity. Kinetic studies revealed that increased steric bulk on the β-nitroacrylate enhances the reaction rate by hindering catalyst alkylation.

Overcoming Deactivation of Amine-Based Catalysts: Access to Fluoroalkylated γ-Nitroaldehydes.

Organocatalytic conjugate addition reactions of aldehydes to fluoroalkylated nitroolefins with chiral amine catalysts offer a straightforward stereoselective path to fluoroalkylated γ-nitroaldehydes and downstream derivatives. However, amine-based catalysts suffer from deactivation by reaction with electron-poor fluoroalkylated nitroolefin. Here, we show that catalyst deactivation can be overcome by catalysts that bear an intramolecular acid for protonation and release of the alkylated catalyst through ß-elimination of the nitroolefin. NMR spectroscopic, kinetic, and molecular modeling studies provided detailed structural and mechanistic insights into the factors that control reversible catalyst alkylation and facilitate efficient catalysis.

Enantioselective Organocatalytic Conjugate Addition in a Tandem Synthesis of δ-Substituted Cyclohexenones and Four-Step Total Synthesis of Penienone.

A bisperfluorotoluyl-BINOL catalyzed conjugate addition of trifluoroborate salts to doubly vinylogous esters and aldol condensation synthesized chiral δ-substituted cyclohexenones with high yields and enantioselectivities (10 examples, up to 89% yield, 89-98% ee). Stepwise and single-pot sequences were developed, with the former also providing β-substituted masked ketoaldehydes containing a vinyl ether. The transformation was used in a four-step total synthesis of penienone (24% overall yield), ≤ half the steps as in previous syntheses.

Re-visiting the diastereoselectivity of organocatalytic conjugate addition of 2-trimethylsiloxyfuran to trans-crotonaldehyde

We describe the re-assignment of configuration previously ascribed to product diastereomers resultant from imidazolidinone-catalyzed conjugate addition of 2-trimethylsiloxyfuran to trans-crotonaldehyde. A modified procedure that uses a diphenylprolinol catalyst was subsequently developed to selectively provide the ‘syn’ diastereomeric product in high enantiomeric excess on decagram scales.

Asymmetric Michael Reaction of Aldehydes and α‐Cyano α,β‐Unsaturated Esters Catalyzed by Diphenylprolinol Silyl Ether; a Facile Asymmetric Route to 3,4,5‐Trisubstituted Piperidines

AbstractA highly enantioselective diphenylprolinol silyl ether‐mediated asymmetric Michael reaction of a variety of aldehydes and β‐substituted α‐cyano α,β‐unsaturated esters was developed. The organocatalytic conjugate addition provided synthetically useful chiral adducts possessing three consecutive stereocenters and suitably oriented functional handles that were readily transformed via a sequence of reductive‐cyclization and subsequent epimerization to furnish single isomers of 3,4,5‐trisubstituted piperidines with high levels of yield and excellent enantiopurity.

Relay Catalysis To Synthesize β-Substituted Enones: Organocatalytic Substitution of Vinylogous Esters and Amides with Organoboronates

Organocatalysis was shown to facilitate conjugate additions to vinylogous esters and amides for the first time. Subsequent elimination of a β-alcohol or amine provided π-conjugated β-substituted enones. Remarkably, nucleophile addition to the electron-rich vinylogous substrates is more rapid than classical enones, forming monosubstituted products. A doubly organocatalytic (organic diol and methyl aniline) conjugate addition synthesized the products directly from alkynyl ketones. Both of these catalytic transformations are orthogonal to transition metal catalysis, allowing for good yields, easily accessible or commercially available reagents, high selectivity, reagent recovery and recyclability, facile scalability, and exceptional functional group tolerance.

One-Pot Enantioselective Synthesis of 2-Pyrrolidinone Derivatives Bearing a Trifluoromethylated All-Carbon Quaternary Stereocenter.

A new methodology for the one-pot enantioselective construction of 2-pyrrolidinone derivatives bearing a trifluoromethylated all-carbon quaternary stereocenter at the 4-position has been described. This strategy combines an organocatalytic conjugate addition of nitroalkanes to isatin-derived α-trifluoromethyl acrylates and a reduction/lactamization process, affording the corresponding products in moderate to high yields (50-95%) with generally excellent stereoselectivities (up to 96% ee and >20:1 dr).

Multigram-scale flow synthesis of the chiral key intermediate of (-)-paroxetine enabled by solvent-free heterogeneous organocatalysis.

The catalytic enantioselective synthesis of the chiral key intermediate of the antidepressant (-)-paroxetine is demonstrated as a continuous flow process on multi-gram scale. The critical step is a solvent-free organocatalytic conjugate addition followed by a telescoped reductive amination-lactamization-amide/ester reduction sequence. Due to the efficient heterogeneous catalysts and the solvent-free or highly concentrated conditions applied, the flow method offers key advances in terms of productivity and sustainability compared to earlier batch approaches.

Organocatalytic enantioselective conjugate addition of pyrazolin-5-ones to arylomethylidene malonates.

An efficient asymmetric organocatalytic conjugate addition reaction of pyrazolin-5-ones with arylomethylidene malonates has been successfully developed by employing bifunctional Cinchona derived thiourea organocatalysts. The protocol provides straightforward access to pyrazole substituted scaffolds in good yields with high stereocontrol.

High pressure-assisted low-loading asymmetric organocatalytic conjugate addition of nitroalkanes to chalcones.

The application of high pressure (up to 9 kbar) allows for relatively fast (1-5 h) and highly enantioselective 1,4-addition of nitromethane and 2-nitropropane to chalcones at room temperature with substantial reduction of catalyst loading (0.2-1 mol% of cinchona alkaloid-based thioureas and squaramides). Various γ-nitroketones were obtained at 9 kbar with high yield and enantioselectivity (up to 98%), whereas in control experiments at atmospheric pressure usually only a small amount (<10%) of products were formed after 20 h.

Organocatalytic Enantioselective Synthesis of Trifluoromethyl‐Containing Tetralin Derivatives by Sequential (Hetero)Michael Reaction–Intramolecular Nitrone Cycloaddition

AbstractThe enantioselective synthesis of tetralin derivatives bearing a trifluoromethylated all‐carbon quaternary stereocenter has been accomplished through a synthetic sequence comprising an organocatalytic β‐functionalization of ortho‐1‐trifluoromethylvinyl‐(hetero)aromatic conjugated aldehydes followed by the intramolecular nitrone 1,3‐cycloaddition reaction (INCR). Both nitromethane and N‐Cbz‐hydroxylamine were employed as nucleophiles in the initial organocatalytic conjugate addition step, which provided the chiral information required for the subsequent diastereoselective INCR. Although diastereoselectivity was moderate, good yields and enantioselectivities were, in general, obtained. Interestingly, an inversion of the selectivity in the intramolecular cycloaddition step was observed when either nitromethane or N‐Cbz‐hydroxylamine was employed. This outcome was studied by means of theoretical calculations, which were in agreement with the experimental results. In addition, the ring opening of the isoxazolidine moiety furnished the corresponding fluorinated diamino alcohols in a very efficient manner.magnified image

Enantio- and Diastereoselective Organocatalytic Conjugate Additions of Nitroalkanes to Enone Diesters.

Enantio- and diastereoselective conjugate addition reactions between nitroethane or nitropropane and enone diesters are reported. A bifunctional triaryliminophosphorane catalyzed the addition reaction with consistently excellent stereoselectivities and yields across a wide range of substrates. By the use of the gem-diester functional handle present in the adducts, local desymmetrization via diastereotopic group discrimination was demonstrated, and a polyfunctionalized lactam with three contiguous stereocenters was synthesized.

Asymmetric Organocatalytic Conjugate Addition of Thiocarboxylic Acids to In Situ‐Generated ortho‐Quinomethanes in Oil‐Water Phases

AbstractAn asymmetric conjugate addition of thiocarboxylic acids to in situ‐generated ortho‐quinomethanes (o‐QMs) catalyzed by bifunctional squaramide catalysts has been developed. The transformation proceeds with high yields (up to 96%) and enantioselectivities (up to 96% ee) in the oil‐water phases. The resulting thioester could be converted facilely into the chiral benzyl mercaptan in high yield without loss of enantioselectivity. The study also showed that the oil‐water biphase contributed significantly to the efficiency of the catalytic system.magnified image

Recent Advances in Asymmetric Organocatalyzed Conjugate Additions to Nitroalkenes.

The asymmetric conjugate addition of carbon and heteroatom nucleophiles to nitroalkenes is a very interesting tool for the construction of highly functionalized synthetic building blocks. Thanks to the rapid development of asymmetric organocatalysis, significant progress has been made during the last years in achieving efficiently this process, concerning chiral organocatalysts, substrates and reaction conditions. This review surveys the advances in asymmetric organocatalytic conjugate addition reactions to α,β-unsaturated nitroalkenes developed between 2013 and early 2017.

有機触媒による共役付加反応を鍵反応とした不斉四級炭素構築法

有機触媒による共役付加反応を鍵反応とした不斉四級炭素構築法

Organocatalytic Enantioselective Conjugate Addition of Azlactones to Enolizable Linear and Cyclic Enones.

Highly diastereo- and enantioselective conjugate additions of azlactones to enolizable cyclic and linear enones were conducted by employing proline aryl sulfonamide as the organocatalyst in trifluorotoluene. The conjugate adducts bearing contiguous quaternary and tertiary stereocenters were obtained in moderate to good yields with excellent diastereoselectivities and moderate to good enantioselectivities. This developed protocol filled in the substrate gap for the organocatalytic conjugate addition of azlactone to enones.

Asymmetric Assembly of All‐Carbon Tertiary/Quaternary Nonadjacent Stereocenters through Organocatalytic Conjugate Addition of α‐Cyanoacetates to a Methacrylate Equivalent

An efficient, highly diastereo- and enantioselective assembly of acyclic carbonyl fragments possessing nonadjacent all-carbon tertiary/quaternary stereoarrays is reported based on a Brønsted base catalyzed Michael addition/α-protonation sequence involving α-cyanoacetates and 2,4-dimethyl-4-hydroxypenten-3-one as novel methacrylate equivalent.

Multicomponent Synthesis of Cyclic Depsipeptide Mimics by Ugi Reaction Including Cyclic Hemiacetals Derived from Asymmetric Organocatalysis.

The synthesis of novel cyclic depsipeptide mimics by means of an organocatalytic conjugate addition, leading to chiral cyclic hemiacetals, followed by a multicomponent reaction with α-amino acids and isocyanides, is described. The initial organocatalytic step is employed for the asymmetric derivatization of α,β-unsaturated aldehydes to 4,5-disubstituted 2-hydroxytetrahydropyrans, which are next used as chiral bifunctional substrates on the Ugi five-center three-component reaction, giving rise to nine-membered-ring lactones. This sequential approach proved to be suitable for the rapid generation of molecular complexity through the combination of aliphatic, dipeptidic, glucosidic, and lipidic isocyanides with several amino acids, thus giving access to amido-, glyco-, and lipo-depsipeptide scaffolds featuring natural product-like structures.

Asymmetric Organocatalytic Conjugate Addition of Trifluoroborates

Asymmetric Organocatalytic Conjugate Addition of Trifluoroborates

ChemInform Abstract: Stereoselective Reaction of 2‐Carboxythioesters‐1,3‐dithiane with Nitroalkenes: An Organocatalytic Strategy for the Asymmetric Addition of a Glyoxylate Anion Equivalent.

AbstractThe enantioselective organocatalyzed conjugate addition of a newly activated thioester acting as an acyl anion mimic is presented using a chiral bifunctional catalyst to control the stereochemistry.