Copper(I)-Catalyzed Kinetic Resolution of exo-3-Oxodicyclopentadienes and endo-3-Oxodicyclopentadiene

Copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of 1,3-enynes and azomethine ylides

Herein, we report a copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides and 1,3-enynes, which provides a series of chiral poly-substituted pyrrolidines in high regio-, diastereo-, and enantioselectivities. Both 4-aryl-1,3-enynes and 4-silyl-1,3-enynes serve as suitable dipolarophiles while 4-alkyl-1,3-enynes are inert. Moreover, the method is successfully applied in the construction of both tetrasubstituted stereogenic carbon centers and chiral spiro pyrrolidines. The DFT calculations are also conducted, which imply a concerted mechanism rather than a stepwise mechanism. Finally, various transformations started from the pyrrolidine bearing a triethylsilylethynyl group and centered on the alkyne group are achieved, which compensates for the inertness of 4-alkyl-1,3-enynes in the present reaction.

Electron-rich benzofulvenes as effective dipolarophiles in copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

A series of benzofulvenes without any electron-withdrawing substituents were employed as 2π-type dipolarophiles for the first time to participate in Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition (1,3-DC) reactions of azomethine ylides. An intrinsic non-benzenoid aromatic characteristic from benzofulvenes serves as a key driving force for activation of the electron-rich benzofulvenes. Utilizing the current methodology, a wide range of multi-substituted chiral spiro-pyrrolidine derivatives containing two contiguous all-carbon quaternary centers were formed in good yield with exclusive chemo-/regioselectivity and high to excellent stereoselectivity. Computational mechanistic studies elucidate the origin of the stereochemical outcome and the chemoselectivity, in which the thermostability of these cycloaddition products is the major factor.

Stereodivergent synthesis of enantioenriched azepino[3,4,5-cd]-indoles via cooperative Cu/Ir-catalyzed asymmetric allylic alkylation and intramolecular Friedel-Crafts reaction

The development of enantioselective annulation reactions using readily available substrates for the construction of structurally and stereochemically diverse heterocycles is a compelling topic in diversity-oriented synthesis. Herein, we report efficient catalytic asymmetric formal 1,3-dipolar (3 + 4) cycloadditions of azomethine ylides with 4-indolyl allylic carbonates for the construction of azepino[3,4,5-cd]-indoles fused with a challenging seven-membered N-heterocycle, a frequently occurring tricyclic indole scaffold in bioactive compounds and pharmaceuticals. Through cooperative Cu/Ir-catalyzed asymmetric allylic alkylation followed by intramolecular Friedel-Crafts reaction, an array of azepino[3,4,5-cd]-indoles were obtained in good yields with excellent diastereo-/enantioselective control. More importantly, the full stereodivergence of this transformation was established via synergistic catalysis followed by acid-promoted epimerization, and up to eight stereoisomers of the cycloadducts bearing three stereogenic centers could be predictably achieved from the same set of starting materials for the first time. Quantum mechanical computations established a plausible mechanism for the synergistic Cu/Ir catalysis to stereodivergently introduce two vicinal stereocenters whose stereochemical information is remotely delivered across the fused azepine ring to control the third chiral center. Epimerization of the last center involves protonation-enabled reversal of the thermodynamically controlled relative configuration.

Current applications of kinetic resolution in the asymmetric synthesis of substituted pyrrolidines

Chiral substituted pyrrolidines are key elements in various biologically active molecules and are therefore valuable synthetic targets. One traditional method towards enantiomerically pure compounds is the application of kinetic resolution. In this review, current KR methodology used in the synthesis of substituted pyrrolidines is surveyed, including enzymatic methods, cycloadditions and reduction of ketones.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 3] Cycloaddition of 4-Indolyl Allylic Alcohols with Azomethine Ylides

An unprecedented iridium-catalyzed asymmetric [4 + 3] cycloaddition of racemic 4-indolyl allylic alcohols with azomethine ylides is reported. The ability of acid promoter zinc triflate to perform multiple roles is the key factor for the success of this strategy. This method provides scalable and efficient access to biologically important azepino[3,4,5-cd] indoles in good yields with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Mild reaction conditions, easily accessible substrates and chiral catalyst, and broad substrate scope highlight the practicality of this methodology.

Catalytic Asymmetric Reactions with N-Metallated Azomethine Ylides

Optically active nitrogen-containing compounds have attracted substantial attention due to their ubiquity in the cores of natural products and bioactive molecules. Among the various synthetic approaches to nitrogenous frameworks, catalytic asymmetric 1,3-dipolar cycloadditions are one of the most attractive methods because of their powerful ability to rapidly construct various chiral N-heterocycles. In particular, N-metallated azomethine ylides, common and readily available 1,3-dipoles, have been extensively applied in dipolar cycloaddition reactions. Despite the fact that asymmetric transformations of azomethine ylides have been investigated for decades, most of the efforts have been directed toward the preparation of pyrrolidines using glycinate-derived α-unsubstituted aldimine esters as the precursors of the azomethine ylides. While α-substituted azomethine ylides derived from amino esters other than glycinate have seldom been harnessed, the construction of non-five-membered chiral N-heterocycles via 1,3-dipolar cycloadditions remains underexplored. In addition, the asymmetric α-functionalization of aldimine esters to prepare acyclic nitrogenous compounds such as α-amino acids, in which an in situ-generated N-metallated azomethine ylide serves as the nucleophile, has not been sufficiently described.In this Account, we mainly discuss the achievements we have made in the past decade toward broadening the applications of N-metallated azomethine ylides for the preparation of nitrogen-containing compounds. We began our investigation with the design and synthesis of a new type of chiral ligand, TF-BiphamPhos, which not only coordinates with Lewis acids to activate dipolar species but also serves as an H-bond donor to increase the reactivity of dipolarophiles with significantly enhanced stereochemical control. Using the Cu(I) or Ag(I)/TF-BiphamPhos complex as the catalyst, we achieved highly stereoselective (3+2) cycloadditions of glycinate and non-glycinate-derived azomethine ylides with diverse dipolarophiles, producing a variety of enantioenriched pyrrolidines with multiple stereocenters in a single step. To further expand the synthetic utility of N-metallated azomethine ylides, we successfully developed higher order cycloadditions with fulvenes, tropone, 2-acyl cycloheptatrienes, and pyrazolidinium ylides serving as the reaction partner, and this reaction provides straightforward access to enantioenriched fused piperidines, bridged azabicyclic frameworks, and triazines via (3+6)- and (3+3)-type cycloadditions. Using N-metallated azomethine ylides as the nucleophile, we realized Cu(I)-catalyzed asymmetric 1,4-Michael additions with α,β-unsaturated bisphosphates/Morita-Baylis-Hillman products, furnishing an array of structurally diverse unnatural α-amino acids. Based on the strategy of synergistic activation, we achieved highly efficient dual Cu/Pd and Cu/Ir catalysis for the α-functionalization of aldimine esters via the asymmetric allylic/allenylic alkylation of N-metallated azomethine ylides. Notably, Cu/Ir catalysis allowed the stereodivergent synthesis of α,α-disubstituted α-amino acids via a branched allylic alkylation reaction, in which the two distinct chiral metal catalysts independently have full stereochemical control over the corresponding nucleophile and electrophile. Furthermore, an expedient and stereodivergent preparation of biologically important tetrahydro-γ-carbolines was realized through a Cu/Ir-catalyzed cascade allylation/iso-Pictet-Spengler cyclization. In addition, when the steric congestion in the allylation intermediates was increased, the combined Cu/Ir catalysts provided an asymmetric cascade allylation/2-aza-Cope rearrangement, producing various optically active homoallylic amines with impressive results.

Catalytic asymmetric synthesis of diazabicyclo[3.1.0]hexanes by 1,3-dipolar cycloaddition of azomethine ylides with azirines

Substituted 1,3-diazabicyclo[3.1.0]hexanes with two contiguous quaternary stereocentres are readily prepared by catalytic asymmetric [3+2] cycloaddition of α-substituted iminoesters with azirines. High diastereoselectivities and enantioselectivities (up to 98% ee) are achieved using Cu^I/(R)-Fesulphos as the catalytic system.

Enantioselective Synthesis of 15-Deoxy-Δ12,14-Prostaglandin J2

An enantioselective synthesis of 15-deoxy-Δ^12,14-prostaglandin J₂ is reported. The synthesis begins with the preparation of enantiopure 3-oxodicyclopentadiene by a lipase-mediated kinetic resolution. A three-component coupling followed by a retro-Diels-Alder reaction provides the C8 stereochemistry of the prostaglandin skeleton with high enantioselectivity. Stereoretentive olefin metathesis followed by a Pinnick oxidation affords 15-deoxy-Δ^12,14-prostaglandin J₂ in high enantiopurity.

Ligand-controlled switch in diastereoselectivities: catalytic asymmetric construction of spirocyclic pyrrolidine-azetidine/oxe(thie)tane derivatives

An asymmetric [3+2] cycloaddition of azomethine ylides with four-membered ring-containing dipolarophiles was developed, and either exo or endo spirocyclic pyrrolidine-azetidine/oxe(thie)tanes were obtained.