Bifunctional N ‐Acyl‐Aminophosphine‐Catalyzed Asymmetric [4+2] Cycloadditions of Allenoates and Imines

Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon-carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon's [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and ³¹P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, ³¹P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe.

Synthesis of heterocyclic compounds through nucleophilic phosphine catalysis

Nucleophilic phosphine catalysis is a practical and powerful tool for the synthesis of various heterocyclic compounds with the advantages of environmentally friendly, metal-free, and mild reaction conditions. The present report summarizes the construction of four to eight-membered heterocyclic compounds containing nitrogen, oxygen and sulphur atoms through phosphine-catalyzed intramolecular annulations and intermolecular [2+2], [3+2], [4+1], [3+1+1], [5+1], [4+2], [2+2+2], [3+3], [4+3] and [3+2+3] annulations of electron-deficient alkenes, allenes, alkynes and Morita-Baylis-Hillman carbonates.

Organocatalytic Synthesis of Highly Functionalized Heterocycles by Enantioselective aza-Morita-Baylis-Hillman-Type Domino Reactions

Organocatalytic enantioselective domino reactions are an extremely attractive methodology, as their use enables the construction of complex chiral skeletons from readily available starting materials in two or more steps by a single operation under mild reaction conditions. Thus, these reactions can save both the quantity of chemicals and length of time typically required for the isolation and/or purification of synthetic intermediates. Additionally, no metal contamination of the products occurs, given that organocatalysts include no expensive or toxic metals. The aza-Morita-Baylis-Hillman (aza-MBH) reaction is an atom-economical carbon-carbon bond-forming reaction between α,β-unsaturated carbonyl compounds and imines mediated by Lewis base (LB) catalysts, such as nucleophilic phosphines and amines. aza-MBH products are functionalized chiral β-amino acid derivatives that are highly valuable as pharmaceutical raw materials. Although various enantioselective aza-MBH processes have been investigated, very few studies of aza-MBH-type domino reactions have been reported due to the complexity of the aza-MBH process, which involves a Michael/Mannich/H-transfer/β-elimination sequence. Accordingly, in this review article, our recent efforts in the development of enantioselective domino reactions initiated by MBH processes are described. In the domino reactions, chiral organocatalysts bearing Brønsted acid (BA) and/or LB units impart synergistic activation to substrates, leading to the easy synthesis of highly functionalized heterocycles (some of which have tetrasubstituted and/or quaternary carbon stereocenters) in high yield and enantioselectivity.

Phosphine-Catalyzed Cascade Michael Addition/[4+2] Cycloaddition Reaction of Allenoates and 2-Arylidene-1,3-indanediones

The phosphine-catalyzed cascade Michael addition/[4+2] cycloaddition reaction of tetrahydrobenzofuranone-derived allenoates and 2-arylidene-1,3-indanediones has been reported, affording spirocyclic 1,3-indanedione derivatives in moderate to high yields with moderate to good diastereoselectivities. A scaled-up reaction worked well under mild conditions, and a plausible mechanism is proposed.

Enantioselective [4+2] Annulation to the Concise Synthesis of Chiral Dihydrocarbazoles

A highly efficient phosphine-catalyzed enantioselective [4 + 2] annulation of allenoates with 3-nitroindoles or 3-nitrobenzothiophenes has been developed. The protocol represents a unique dearomatization–aromatization process to access functionalized dihydrocarbazoles or dihydrodibenzothiophenes with high optical purity (up to 97% ee) under mild reaction conditions. The synthetic utility of the highly enantioselective [4 + 2] annulation enables a concise synthesis of analgesic agent.

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High regio-, chemo-, and enantioselectivity

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Broad substrate scope

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Dearomatization/aromatization steps proceed under mild conditions

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Concise synthesis of chiral analgesic agent

Phosphine-Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines, Enamines, and a Primary Amine

Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C-C bond or two C-N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by ³¹ P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively.

Chiral aminophosphines derived from hydroxyproline and their application in allene-imine [4 + 2] annulation

A robust synthetic route from L-hydroxyproline (L-Hyp) to phosphines has established an expandable library of six chiral aminophosphines, which were then applied to the phosphine-catalyzed [4 + 2] allene-imine annulation. The enantioinduction in the annulations-induced by a purely steric effect-were moderate (up to 57% ee). A switch of the reaction site from the γ- to the β'-carbon atom of the allenoate was observed during the annulations performed using sterically demanding chiral phosphines.

Organophosphine-Catalyzed [4C+X] Annulations

In recent years, there have been extraordinary developments of organophosphine-catalyzed reactions. This includes progress in the area of [4C+X] annulations, which are of particular interest due to their potential for the rapid construction of 5–8-membered cyclic products. In this short overview, we summarize the remarkable progress, emphasizing reaction mechanisms and key intermediates involved in the processes. The discussion is classified according to the type of electrophilic reactants that acted as C₄ synthons in the annulation process, in the order of α-alkyl allenoates, γ-alkyl allenoates, α-methyl allene ketones, β′-OAc allenoate, δ-OAc allenoate, activated dienes and cyclobutenones.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Catalytic Enantioselective Synthesis of Guvacine Derivatives through [4 + 2] Annulations of Imines with α-Methylallenoates

P-Chiral [2.2.1] bicyclic phosphines (HypPhos catalysts) have been applied to reactions between α-alkylallenoates and imines, producing guvacine derivatives. These HypPhos catalysts were assembled from trans-4-hydroxyproline, with the modular nature of the synthesis allowing variations of the exocyclic P and N substituents. Among them, exo-( p-anisyl)-HypPhos was most efficacious for [4 + 2] annulations between ethyl α-methylallenoate and imines. Through this method, ( R)-aplexone was identified as being responsible for the decrease in the cellular levels of cholesterol.

Phosphine-catalyzed [5+1] annulation of δ-sulfonamido-substituted enones with N-sulfonylimines: a facile synthesis of tetrahydropyridines

The first phosphine-catalyzed [5+1] annulation of enones with N-sulfonylimines works efficiently to give tetrahydropyridines.

Phosphine-catalyzed [5+1] annulation of δ-sulfonamido-substituted enones with N-sulfonylimines for the synthesis of 1,2,3,6-tetrahydropyridines is developed. The reaction proceeds smoothly under mild reaction conditions to give the annulation products in moderate to excellent yields. Mechanistic exploration of this new annulation shows that the δ-sulfonamido-substituted enone and the N-sulfonylimine serve as C5 and C1 synthons to furnish the annulation, respectively. Using chiral phosphine as the catalyst, an asymmetric variant of the model reaction gave the chiral product in up to 73% ee.

Catalyst-controlled regioselectivity in phosphine catalysis: the synthesis of spirocyclic benzofuranones via regiodivergent [3 + 2] annulations of aurones and an allenoate

Catalyst-controlled regiodivergent [3 + 2] annulations of aurones and allenoates have been developed. When a dipeptide phosphine catalyst with an l-d- configuration was employed, α-selective [3 + 2] annulation products could be obtained with good regioselectivities and enantioselectivities. With the employment of l-l- dipeptide phosphines, γ-selective annulation products could be selectively obtained with excellent enantioselectivities. By simply tuning the catalyst configurations, a wide range of α-selective or γ-selective spirocyclic benzofuranones with either aryl or alkyl substitutions could be readily prepared. DFT calculations suggest that the conformation of the dipeptide phosphines influences the hydrogen bonding interactions or the distortion energy, resulting in delicate energy differentiation in the transition states, and accounting for the observed regioselectivity.

Lewis base-catalyzed diastereoselective [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes: an access to isoxazolidine derivatives

A Lewis base-catalyzed [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes has been achieved under mild reaction conditions, affording functionalized isoxazolidines as single diastereomers in moderate to excellent yields.

Highly enantioselective synthesis of dihydrocoumarin-fused dihydropyrans via the phosphine-catalyzed [4 + 2] annulation of allenones with 3-aroylcoumarins

Phosphine-catalyzed [4 + 2] annulation between 3-aroylcoumarins and allenones has been developed. In the presence of a dipeptide phosphine catalyst 7, dihydrocoumarin-fused dihydropyrans were prepared in high yields and with excellent enantioselectivities.

Enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines catalyzed by a bifunctional dipeptide phosphine

The first enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines is described. In the presence of a dipeptide phosphine catalyst, a wide range of highly functionalized cyclopentenes bearing an all-carbon quaternary center were obtained in moderate to good yields and with good to excellent enantioselectivities.

Understanding the mechanisms, regioselectivies and enantioselectivities of the DMAP-catalyzed [2 + 4] cycloaddition of γ-methyl allenoate and phenyl(phenyldiazenyl)methanone

DMAP-catalyzed [2 + 4] cycloaddition reaction.