Organocatalytic Asymmetric Synthesis of α,α-Disubstituted α-Amino Acids and Derivatives

Direct, Stereodivergent, and Catalytic Michael Additions of Thioimides to α,β-Unsaturated Aldehydes - Total Synthesis of Tapentadol

Direct and stereodivergent Michael additions of N-acyl 1,3-thiazinane-2-thiones to α,β-unsaturated aldehydes catalyzed by chiral nickel(II) complexes are reported. The reactions proceed with a remarkable regio-, diastereo-, and enantioselectivity, so access to any of the four potential Michael stereoisomers is granted through the appropriate choice of the chiral ligand of the nickel(II) complex. Simple removal of the heterocyclic scaffold furnishes a wide array of either syn or anti enantiomerically pure derivatives, which can be exploited for the asymmetric synthesis of biologically active compounds, as demonstrated in a new approach to tapentadol. In turn, a mechanism, based on theoretical calculations, is proposed to account for the stereochemical outcome of these transformations.

Picolinaldehyde-Zinc(II)-Palladium(0) Catalytic System for the Asymmetric α-Allylation of N-Unprotected Amino Esters

Reported in this work is a synergistic ternary achiral picolinaldehyde-Zn(II)-chiral palladium complex system for the highly enantioselective α-allylation of N-unprotected amino esters. By utilizing a variety of allylic carbonates or vinyl benzoxazinanones as substrates, α-allyl α-amino esters were obtained in high yields (up to 96 %) with high enantioselectivities (up to 98 % ee). Control experiments suggest that the coordination of Zn(II) with the Schiff base intermediate enhances the acidity of the α-C-H bonds of amino esters, thereby favoring α-allylation over intrinsic N-allylation. Furthermore, NMR studies reveal an interaction between the chiral palladium complex and the Zn(II)-Schiff base intermediate, leading to the formation of a picolinaldehyde-Zn(II)-Pd(0) catalytic system.

Highly diastereo- and enantioselective C2 addition of 5H-oxazol-4-ones to γ-keto-α,β-unsaturated esters

The direct C2-addition of 5H-oxazol-4-ones to γ-keto-α,β-unsaturated esters catalyzed by a chiral squaramide has been achieved. Diverse highly functionalized γ-keto esters bearing a C2-oxazolone at the α-position were afforded in high yields with excellent stereoselectivities (d.r. > 20 : 1 and up to 98% ee).

One-Pot Friedel-Crafts/Robinson-Gabriel Synthesis of the Indole-Oxazole Scaffold and Its Application to the Synthesis of Breitfussins C, G, and H

The indole-oxazole scaffold is found in a range of biologically active natural products, including the breitfussin family. Divergent methods that provide access to the indole-oxazole template are relatively scarce, which impedes the wider exploration of these natural products and their exciting biological activity. Herein, we describe a highly divergent synthesis of the indole-oxazole scaffold via a one-pot Friedel-Crafts/Robinson-Gabriel synthesis and the application of this methodology to the synthesis of breitfussins C, G, and H.

Iridium-catalyzed asymmetric double allylic alkylation of azlactone: efficient access to chiral α-amino acid derivatives

An unprecedented Ir-catalyzed enantioselective double allylic alkylation of less bulky cyclic imine glycinate (azlactone) was rationally designed and developed, providing various bisallylated chiral amino acid derivatives. Control experiments revealed that this transformation proceeds in a sequential manner featuring quasi-dynamic kinetic resolution of the initially-formed monoallylation intermediates.

A step-economic and one-pot access to chiral Cα-tetrasubstituted α-amino acid derivatives via a bicyclic imidazole-catalyzed direct enantioselective C-acylation

C^α-Tetrasubstituted α-amino acids are ubiquitous and unique structural units in bioactive natural products and pharmaceutical compounds. The asymmetric synthesis of these molecules has attracted a lot of attention, but a more efficient method is still greatly desired. Here we describe the first sequential four-step acylation reaction for the efficient synthesis of chiral C^α-tetrasubstituted α-amino acid derivatives from simple N-acylated amino acids via an auto-tandem catalysis using a single nucleophilic catalyst. The synthetic efficiency is improved via a direct enantioselective C-acylation; the methodology affords the corresponding C^α-tetrasubstituted α-amino acid derivatives with excellent enantioselectivities (up to 99% ee). This step-economic, one-pot, and auto-tandem strategy provides facile access to important chiral building blocks, such as peptides, serines, and oxazolines, which are often used in medicinal and synthetic chemistry.

The first four-step sequential reaction for the synthesis of C^α-tetrasubstituted chiral α-amino acid derivatives via auto-tandem catalysis has been developed.

Pd-Catalyzed Asymmetric Hydroalkylation of 1,3-Dienes: Access to Unnatural α-Amino Acid Derivatives Containing Vicinal Quaternary and Tertiary Stereogenic Centers

Pd-phosphinooxazoline (Pd-PHOX)-catalyzed asymmetric hydroalkylation of 1,3-dienes with azlactones was successfully developed for the first time, affording various enantioenriched α-quaternary α-amino acid derivatives bearing contiguous quaternary and tertiary stereogenic centers in good yields with exclusive regioselectivity and excellent stereoselective control (up to 92% yield, >20:1 dr, and >99% ee). The scale-up catalytic asymmetric hydroalkylation was performed well without loss of reactivity and stereoselectivities, which exhibited great potential application. The synthetic utility of the current methodology was demonstrated through product transformations to access other biologically important compounds such as chiral β-amino alcohol and α-quaternary cyclic α-amino acid derivatives.

The Quinary Catalyst-Substrate Complex Induced Construction of Spiro-Bridged or Cagelike Polyheterocyclic Compounds via a Substrate-Controlled Cascade Process

The asymmetric organocatalytic cascade reaction of cyclic β-oxo aldehydes to 2-hydroxycinnamaldehydes is developed. The bifunctional tertiary amine-thiourea catalyst was used in a rationally designed multiple catalysis where the asymmetric iminium catalysis and thiourea anion-binding catalysis were combined by carboxylate anion as a ternary catalytic system to form a quinary catalyst-substrate complex, providing an efficient protocol for the construction of enantioenriched spiro-bridged or cagelike polyheterocyclic compounds. The reuse of catalysts was also successfully realized.

Enantioselective Synthesis of Chromanones Bearing an α,α-Disubstituted α-Amino Acid Moiety via Decarboxylative Michael Reaction

In this manuscript, a novel, decarboxylative Michael reaction between α-substituted azlactones and chromone-3-carboxylic acids is described. The reaction proceeds in a sequence Michael addition followed by decarboxylative deprotonation, and it results in the formation of chromanones bearing an azlactone structural unit. The possibility of transforming an azlactone moiety into a protected α,α-disubstituted α-amino acid derivative is also demonstrated.

Asymmetric Organocatalyzed Reaction Sequence To Synthesize Chiral Bridged and Spiro-Bridged Benzofused Aminals via Divergent Pathways

A highly efficient asymmetric organocatalysis-triggered reaction sequence is developed. 2-Hydroxy cinnamaldehydes and cyclic N-sulfonyl ketimines were both used as multisite substrates (more than two reactive sites) to access structurally diverse chiral bridged and spiro-bridged benzofused aminal derivatives, where an inseparable equilibrating mixture of isomers can be regioselectively converted into bridged benzofused aminals with different ring connectivities via divergent pathways. Several stereoselective transformations of the resulted bridged aminals are presented.

Synthesis of HDAC Substrate Peptidomimetic Inhibitors Using Fmoc Amino Acids Incorporating Zinc-Binding Groups

Syntheses of Fmoc amino acids having zinc-binding groups were prepared and incorporated into substrate inhibitor H3K27 peptides using Fmoc/^tBu solid-phase peptide synthesis (SPPS). Peptide 11, prepared using Fmoc-Asu(NHO^tBu)-OH, is a potent inhibitor (IC₅₀ = 390 nM) of the core NuRD corepressor complex (HDAC1–MTA1–RBBP4). The Fmoc amino acids have the potential to facilitate the rapid preparation of substrate peptidomimetic inhibitor (SPI) libraries in the search for selective HDAC inhibitors.

Bifunctional Brønsted base catalyzed inverse-electron-demand aza-Diels–Alder reactions of saccharin-derived 1-azadienes with azlactones

An enantioselective inverse-electron-demand aza-Diels–Alder reaction of saccharin-derived 1-azadienes and azlactones was developed using a phenylalanine-derived bifunctional Brønsted base-squaramide catalyst.

Enantioselective synthesis of pyrano[2,3-c]pyrrole via an organocatalytic [4 + 2] cyclization reaction of dioxopyrrolidines and azlactones

The present work provides a simple and efficient access to chiral pyrano[2,3-c]pyrrole via an asymmetric [4 + 2] cyclization reaction catalyzed by a cinchona-squaramide catalyst.

Two Competitive but Switchable Organocatalytic Cascade Reaction Pathways: The Diversified Synthesis of Chiral Acetal-Containing Bridged Cyclic Compounds

The organocatalytic enantioselective synthesis of methanobenzodioxepine derivatives bearing a 6,6,5-bridged ring system is presented. The m-CPBA-triggered in situ α-oxidation of β-oxoesters to provide the required but unstable α-hydroxy-β-dicarbonyl substrates is the key to this three-step sequence, providing the desired cyclic acetals with excellent stereoselectivities containing two bridgehead and one fully substituted stereocenters. It is noteworthy that the absence of m-CPBA furnished the acetal products bearing a 6,6,6-bridged ring system with similar good results from the same starting materials.

Access to α,γ-Diamino Diacid Derivatives via Organocatalytic Asymmetric 1,4-Addition of Azlactones and Dehydroalanines

A convenient and functional-group-tolerant organocatalytic asymmetric 1,4-addition of azlactones and dehydroalanine is disclosed. The reaction is used for the first synthesis of chiral α,γ-diamino diacid derivatives with nonadjacent stereogenic centers in moderate to high yields, with excellent diastereo- and enantioselectivities, under the catalysis of a chiral thiourea catalyst. In addition, the reaction could be conducted in gram-scale, and the products of the reaction could be readily converted to various α,γ-diamino diacid derivatives, α,γ-diamino dialcohols, and modified peptides with nonproteinogenic amino acid residues.

Organocatalytic Asymmetric Synthesis of Spiro-Bridged and Spiro-Fused Heterocyclic Compounds Containing Chromane, Indole, and Oxindole Moieties

Following the reactivity inversion strategy, two different two-step sequences were designed and successfully applied to the asymmetric synthesis of spiro-bridged and spiro-fused heterocyclic compounds, which combined chromane, indole, and oxindole, three potential pharmacophores, in one molecule. The power of these two organocatalytic pathways is underscored by mild reaction conditions and high efficiency in the production of synthetically challenging, but biologically important heterocyclic products, which could be transformed into more biologically interesting heterocyclic structures.

In Situ-Generated Glycinyl Chloroaminals for a One-Pot Synthesis of Non-proteinogenic α-Amino Esters

An acetyl chloride-mediated cascade transformation involving a primary carbamate, ethyl glyoxylate, and various types of nucleophiles is reported for the synthesis of orthogonally protected α-amino esters. These reactions proceeded rapidly to afford the pivotal α-chloroglycine intermediate in excellent yields, which can be directly functionalized in situ with various types of nucleophiles. A mild and unique AcOH(cat.)/AcCl system was found to promote an autocatalytic-like condensation and facilitate the multicomponent assembly of non-proteinogenic α-amino esters. To better understand this one-pot transformation and the orchestration of the components' condensations, the investigation of a broader scope of nucleophiles and some kinetic studies are presented. Our findings suggest that the halogenation step toward the formation of α-chloroglycine is the rate-determining step likely proceeding through the formation of N-carbamoyl iminium. Also, the initial kinetic profiling for the nucleophilic substitution supports an S_N1-like (S_N2C+) mechanism in which nucleophiles add to the iminium-chloride tight ionic pair. These results lead ultimately to the design of a new protocol in which an achiral hydrogen bond donor thiourea catalyst was utilized to enhance the reaction scope and enable silylated nucleophiles to be efficiently exploited to synthesize novel non-proteinogenic α-amino esters.

Solid Phase Synthesis of (Benzannelated) Six-Membered Heterocycles via Cyclative Cleavage of Resin-Bound Pseudo-Oxazolones

A solid supported procedure for the synthesis of benzoxazinones, dihydropyrazinones, quinoxalinones, and dihydrooxazinones using immobilized oxazolones in combination with difunctional nucleophiles as cleavage agent is presented. The scope of the novel method has been demonstrated through subsequent modification of the parent oxazolone scaffold on solid supports using conversions with electrophiles or CuAAC reactions to give functionalized pyrazin-2-ones. The described method allows the synthesis of the target heterocycles in good yields via three to five steps on solid phases with only one chromatographic purification step.

Catalytic Asymmetric Inverse-Electron-Demand 1,3-Dipolar Cycloaddition of C,N-Cyclic Azomethine Imines with Azlactones: Access to Chiral Tricyclic Tetrahydroisoquinolines

Reported herein is a bifunctional-organocatalyst-mediated enantioselective inverse-electron-demand 1,3-dipolar cycloaddition of C,N-cyclic azomethine imines with azlactones. The strategy provides concise access to enantioenriched C1-substituted tetrahydroisoquinolines featuring a pyrazolidinone scaffold. Moreover, the scalability and practical utility of this protocol was well demonstrated by employing a gram-scale reaction and some representative transformations.

Enantioselective organocatalytic Michael addition of isorhodanines to α,β-unsaturated aldehydes

The Michael reaction of substituted isorhodanines with α,β-unsaturated aldehydes in the presence of a catalytic amount of a chiral secondary amine is presented. This transformation proceeds in good to high yields furnishing the corresponding 4,5-disubstituted isorhodanine adducts in good to excellent enantioselectivities.

An innovation for development of Erlenmeyer–Plöchl reaction and synthesis of AT-130 analogous: a new application of continuous-flow method

The first micro-flow Erlenmeyer–Plöchl azlactone reaction and synthesis of N-benzoylglycine carbamide were established.

Organocatalytic enantio- and diastereoselective synthesis of 3,5-disubstituted prolines

The asymmetric synthesis of substituted pyrrolidines has been accomplished using a novel organocatalytic cyclization reaction promoted by aCinchonaalkaloid base primary amine.

Stereoselective Synthesis of 1,3-Diaminotruxillic Acid Derivatives: An Advantageous Combination of C-H-ortho-Palladation and On-Flow [2+2]-Photocycloaddition in Microreactors

The stereoselective synthesis of ε-isomers of dimethyl esters of 1,3-diaminotruxillic acid in three steps is reported. The first step is the ortho-palladation of (Z)-2-aryl-4-aryliden-5(4H)-oxazolones 1 to give dinuclear complexes 2 with bridging carboxylates. The reaction occurs through regioselective activation of the ortho-CH bond of the 4-arylidene ring in carboxylic acids. The second step is the [2+2]-photocycloaddition of the CC exocyclic bonds of the oxazolone skeleton in 2 to afford the corresponding dinuclear ortho-palladated cyclobutanes 3. This key step was performed very efficiently by using LED light sources with different wavelengths (465, 525 or 625 nm) in flow microreactors. The final step involved the depalladation of 3 by hydrogenation in methanol to afford the ε-1,3-diaminotruxillic acid derivatives as single isomers.

2-Vinyl Threoninol Derivatives via Acid-Catalyzed Allylic Substitution of Bisimidates

A diastereoselective synthesis of 4-vinyl oxazolines syn-2 was developed based on an acid-catalyzed cyclization of bistrichloroacetimidates (E)-1. The reaction likely involves an allyl carbenium ion intermediate in which the adjacent stereocenter directs the stereoselectivity for C-N bond formation. Oxazolines syn-2 were transformed to C-quaternary threoninol, threoninal, and threonine derivatives which can be further incorporated into complex natural compounds.

Organocatalytic regioselective asymmetric Michael addition of azlactones to o-hydroxy chalcone derivatives

The regioselective and enantioselective Michael addition between azlactones and o-hydroxy chalcone derivatives is reported. Enantiomerically enriched N,O-aminals with two continuous stereogenic centers are exclusively obtained in moderate to good yields with excellent diastereoselectivities and good to excellent enantioselectivities. The experimental results show that an o-hydroxy group on the cinnamenyl motif of chalcone derivatives plays a crucial role at the reaction sites for the regioselective Michael addition. In addition, circular dichroism (CD) spectroscopy and density functional theory (DFT) are used to investigate the absolute configuration of N,O-aminals and the corresponding transition-state structures.

Racemic total synthesis of dactyloidin and demethyldactyloidin through the dl-proline-catalyzed Knoevenagel condensation/[4 + 2] cycloaddition cascade

An efficient approach towards the first racemic total synthesis of dactyloidin (2) and demethyldactyloidin (3) is described.

Organocatalytic enantioselective formal arylation of azlactones using quinones as the aromatic partner

Catalytic enantioselective formal arylation of azlactones using quinones as the aromatic partner was developed.