A Facile Cu(I)/TF-BiphamPhos-Catalyzed Asymmetric Approach to Unnatural α-Amino Acid Derivatives Containing gem-Bisphosphonates

Copper/ruthenium relay catalysis enables 1,6-double chiral inductions with stereodivergence

The dual catalysis strategy is an efficient and powerful tool to fulfill the stereodivergent synthesis of stereoisomeric products from the same set of starting materials. Great attention has been given to the construction of chiral compounds with two contiguous stereocenters. However, the synthesis of two remote noncontiguous stereocenters is more challenging and is less developed, despite the high demand for synthetic tactics. We herein developed an unprecedented example of the stereodivergent preparation of synthetically useful and biologically important chiral ζ-hydroxy amino ester derivatives containing remote 1,6-noncontiguous stereocenters and a unique β,γ-unsaturation moiety. This cascade dehydrogenation/1,6-Michael addition/hydrogenation protocol between readily-available ketoimine esters and racemic branched dienyl carbinols was rationally realized with bimetallic copper/ruthenium relay catalysis. The key features of the process were atom economy, step economy, and redox-neutrality. All four stereoisomers of chiral ζ-hydroxy amino ester derivatives were easily achieved by the orthogonal permutations of a chiral copper catalyst and chiral ruthenium catalyst. Importantly, a much more challenging stereodivergent synthesis of all eight stereoisomers of chiral peptide products containing three remote stereocenters was accomplished with excellent results through the cooperation of two chiral catalyst pairs and substrate enantiomers.

Organocatalyzed Asymmetric Michael Addition of 3-Fluorooxindole to Vinylidene Bisphosphonates

Both the 3-fluorooxindole and germinal bisphosphonate structural motifs are prevalent in bioactive molecules because of their associated biological activities. We describe an approach to accessing 3,3-disubstituted 3-fluorooxindoles bearing a geminal bisphosphate fragment through a highly enantioselective Michael addition reaction between 3-fluorooxindoles and vinylidene bisphosphonates. These reactions are catalyzed by a commercially available cinchona alkaloid catalyst, have a broad substrate scope concerning 3-fluorooxindoles, and provide the corresponding addition products in a yield of up to 95% with an enantiomeric excess of up to 95%. A reasonable reaction pathway to explain the observed stereochemistry is also proposed.

Copper/Ruthenium Relay Catalysis for Stereodivergent Access to δ-Hydroxy α-Amino Acids and Small Peptides

An atom- and step-economical and redox-neutral cascade reaction enabled by asymmetric bimetallic relay catalysis by merging a ruthenium-catalyzed asymmetric borrowing-hydrogen reaction with copper-catalyzed asymmetric Michael addition has been realized. A variety of highly functionalized 2-amino-5-hydroxyvaleric acid esters or peptides bearing 1,4-non-adjacent stereogenic centers have been prepared in high yields with excellent enantio- and diastereoselectivity. Judicious selection and rational modification of the Ru catalysts with careful tuning of the reaction conditions played a pivotal role in stereoselectivity control as well as attenuating undesired α-epimerization, thus enabling a full complement of all four stereoisomers that were otherwise inaccessible in previous work. Concise asymmetric stereodivergent synthesis of the key intermediates for biologically important chiral molecules further showcases the synthetic utility of this methodology.

Modular access to chiral bridged piperidine-γ-butyrolactones via catalytic asymmetric allylation/aza-Prins cyclization/lactonization sequences

Chiral functionalized piperidine and lactone heterocycles are widely spread in natural products and drug candidates with promising pharmacological properties. However, there remains no general asymmetric methodologies that enable rapid assemble both critical biologically important units into one three-dimensional chiral molecule. Herein, we describe a straightforward relay strategy for the construction of enantioenriched bridged piperidine-γ-butyrolactone skeletons incorporating three skipped stereocenters via asymmetric allylic alkylation and aza-Prins cyclization/lactonization sequences. The excellent enantioselectivity control in asymmetric allylation with the simplest allylic precursor is enabled by the synergistic Cu/Ir-catalyzed protocol; the success of aza-Prins cyclization/lactonization can be attributed to the pivotal role of the ester substituent, which acts as a preferential intramolecular nucleophile to terminate the aza-Prins intermediacy of piperid-4-yl cation species. The resulting chiral piperidine-γ-butyrolactone bridged-heterocyclic products show impressive preliminary biological activities against a panel of cancer cell lines.

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.

Enantio- and diastereodivergent synthesis of fused indolizines enabled by synergistic Cu/Ir catalysis

Highly diastereo-/enantioselective assembly of 2,3-fused indolizine derivatives could be easily available through a cascade allylation/Friedel-Crafts type reaction enabled by a synergistic Cu/Ir catalysis. This designed protocol provides an unprecedented and facile route to enantioenriched indolizines bearing three stereogenic centers in moderate to high yields with excellent stereoselective control, which also featured broad substrate generality. Remarkably, four stereoisomers of the 2,3-fused indolizine products could be efficiently constructed in a predictable manner through the pairwise combination of copper and iridium catalysts. The synthetic utility of this method was readily elaborated by a gram-scale reaction, and synthetic transformations to other important chiral indolizine derivatives. Quantum mechanical explorations constructed a plausible synergetic catalytic cycle, revealed the origins of stereodivergence, and rationalized the protonation-stimulated stereoselective Friedel-Crafts type cyclization to form the indolizine products.

Conjugate reduction of vinyl bisphosphonates

Vinyl bisphosphonates can be readily prepared by condensation of an aromatic aldehyde with the tetraester of a methylenebisphosphonate, and reduction of the resulting olefin is an attractive strategy for the preparation of monoalkyl geminal bisphosphonates. Conjugate reduction through use of variations on the Stryker approach has proven to be an efficient method for that reduction, even in the presence of aromatic substituents that also could be reduced. Furthermore, remote olefins in an isoprenoid chain survive this conjugate reduction unaffected, allowing access to isoprenoid-substituted triazole bisphosphonates of interest as potential inhibitors of terpenoid biosynthesis.

Stereodivergent Synthesis of Enantioenriched α-Deuterated α-Amino Acids via Cascade Cu(I)-Catalyzed H-D Exchange and Dual Cu- and Ir-Catalyzed Allylation

A one-pot Cu-mediated H-D exchange with inexpensive heavy water as the deuterium source, followed by Cu- and Ir-catalyzed stereodivergent allylic alkylation, has been developed, providing efficient access to enantioenriched α-deuterium-labeled α-amino acids from readily available glycine imine esters in a high yield with excellent stereoselectivity. High deuterium enrichment, exquisite regioselectivity, precise stereoselectivity control, and operationally convenient procedures make this protocol appealing for the preparation of highly synthetically useful α-deuterated α-amino acids.

Stereodivergent silver-catalyzed synthesis of pyroglutamic acid esters

We report here a silver-catalyzed method for the enantio- and diastereodivergent synthesis of chiral pyroglutamic acid esters with multiple stereocenters. This process proceeds through asymmetric conjugate addition of glycine imine esters to a broad range of β-substituted α,β-unsaturated perfluorophenyl esters followed by lactamization. By leveraging catalyst control and stereospecificity of the 1,4-addition process, all four product stereoisomers containing two adjacent stereocenters are accessible with high stereoselectivity.

Recent advances in catalytic asymmetric aza-Cope rearrangement

Aza-Cope rearrangement, as one of the fundamental reactions for C-C and C-N bond formation, has been extensively utilized for the rapid construction of synthetically challenging organic molecules. Despite significant achievements having been made in the past 80 years, catalytic enantioselective versions still remain a challenge, mainly due to the inherent nature of the reversibility of aza-Cope rearrangement. Recently, owing to the intensive development of asymmetric catalysis strategies, various chiral organocatalysts and transition-metal catalysts have been successfully applied to control the stereoselectivity of aza-Cope rearrangement, and remarkable advances have been achieved. This review highlights recent progress relating to catalytic asymmetric aza-Cope rearrangement and covers important features of these studies, including catalytic system design, mechanistic insights, stereochemistry analysis, and synthetic applications.

Enantioselective Synthesis of Pyroglutamic Acid Esters from Glycinate via Carbonyl Catalysis

Direct α-functionalization of NH₂ -free glycinates with relatively weak electrophiles such as α,β-unsaturated esters still remains a big challenge in organic synthesis. With chiral pyridoxal 5 d as a carbonyl catalyst, direct asymmetric conjugated addition at the α-C of glycinate 1 a with α,β-unsaturated esters 2 has been successfully realized, to produce various chiral pyroglutamic acid esters 4 in 14-96 % yields with 81-97 % ee's after in situ lactamization. The trans and cis diastereomers can be obtained at the same time by chromatography and both of them can be easily converted into chiral 4-substituted pyrrolidin-2-ones such as Alzheimer's drug Rolipram (11) with the same absolute configuration via tert-butyl group removal and subsequent Barton decarboxylation.

Catalytic asymmetric synthesis of quaternary trifluoromethyl α- to ε-amino acid derivatives via umpolung allylation/2-aza-Cope rearrangement

In this study, we developed an efficient Ir-catalyzed cascade umpolung allylation/2-aza-Cope rearrangement of tertiary α-trifluoromethyl α-amino acid derivatives for the preparation of a variety of quaternary α-trifluoromethyl α-amino acids in high yields with excellent enantioselectivities. The umpolung reactivity empowered by the activation of the key isatin-ketoimine moiety obviates the intractable enantioselectivity control in Pd-catalyzed asymmetric linear α-allylation. In combination with quasi parallel kinetic resolution or kinetic resolution, the generality of this method is further demonstrated by the first preparation of enantioenriched quaternary trifluoromethyl β-, γ-, δ- and ε-amino acid derivatives.

A new entry to highly functionalized pyrroles via a cascade reaction of α-amino esters and alkynals

Here, we developed an expedient access route to highly functionalized pyrroles from readily available α-amino acid ester hydrochlorides and alkynals via a cascade condensation/intramolecular cyclization followed by a unique C-N ester migration process. A variety of 1,2,3-trisubstituted pyrroles, which were difficult to acquire with the common methodologies, were successfully prepared in good yields under mild conditions.

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.

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.

Asymmetric synthesis of quaternary α-trifluoromethyl α-amino acids by Ir-catalyzed allylation followed by kinetic resolution

Facile access to quaternary α-trifluoromethyl α-amino acids has been developed. This sequential reaction involves an Ir-catalyzed asymmetric allylation of α-trifluoromethyl aldimine esters followed by an unprecedented kinetic resolution.

Synergistic catalysis for cascade allylation and 2-aza-cope rearrangement of azomethine ylides

The efficient construction of enantiomerically enriched molecules from simple starting materials via catalytic asymmetric synthesis strategies is a key challenge in synthetic chemistry. Metallated azomethine ylides are commonly-used synthons for the preparation of N-heterocycles and α-amino acids. Remarkably, to date, the utilization of azomethine ylides for the facile access to chiral amines has proven elusive. Here, we report that a synergistic Cu/Ir-catalytic system combined with careful tuning of the steric congestion can be used to convert aldimine esters to a variety of chiral homoallylic amines via a cascade allylation/2-aza-Cope rearrangement. The elucidation of the distinct effects of each stereogenic center of the allylation intermediates on the stereochemical outcome and chirality transfer in the rearrangement further guided the selection of catalysts combination.

Metallated azomethine ylides are commonly used for the construction of N-heterocycles and α-amino acids. Here, the authors report a synergistic Cu/Ir-catalytic system that converts aldimine esters to a variety of chiral homoallylic amines via a cascade allylation/2-aza-Cope rearrangement.

Asymmetric Vinylogous Aldol-type Reactions of Aldehydes with Allyl Phosphonate and Sulfone

Two catalytic asymmetric vinylogous aldol-type reactions of aldehydes with allyl phosphonate and allyl sulfone have been uncovered in good to high yields for the first time. The bulky ligand-(R)-DTBM-SEGPHOS-was found to be the key to perfectly control both regio- and enantioselectivities. Transformations of the vinylogous products (including Horner-Wadsworth-Emmons and Julia olefinations) were successfully realized by virtue of the phosphonate and sulfone moieties. Moreover, the present methodology was successfully applied in the asymmetric synthesis of natural products.

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

The first example of highly efficient kinetic resolution of exo-3-oxodicyclopentadienes and endo-3-oxodicyclopentadiene has been developed by means of Cu(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylide. Compared with the existing methodologies for those synthetically important optically active convex molecules, the current protocol provides an alternative but more practical approach from the readily available racemic starting materials, which is free from the repetitive reduction/oxidation steps in the enzymatic resolution or the indispensable stoichiometric amount of chirality-induction reagents.

Palladium-catalyzed oxidative cross-coupling for the synthesis of α-amino ketones

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed. This transformation was achieved by direct C–H oxidation of α-aminocarbonyl compounds and arylation. The mild reaction has a broad reaction scope and gives desired α-aryl α-amino ketones in moderate to excellent yields.

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed.

Functional bisphosphonate synthesis for the development of new anti-resorption bone drug candidates

Widening the bisphosphonate toolbox: new bisphosphonate scaffolds enable new functionalizations.

A AgOAc/quinine-derived aminophosphine complex as an efficient catalyst for diastereo- and enantioselective 1,3-dipolar cycloaddition of α,β-unsaturated 7-azaindoline amides and azomethine ylides

The Dixon’s catalyst system is effective for the asymmetric 1,3-dipolar cycloaddition.

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Fluorous-tag assisted synthesis of bile acid-bisphosphonate conjugates via orthogonal click reactions: an access to potential anti-resorption bone drugs

The synthesis of a small collection of novel bile acid-bisphosphonate (BA-BP) conjugates as potential drug candidates is reported. The disclosed methodology relied on the installation of azide and thiol functionalities at the head and tail positions, respectively, of the BA scaffold and its subsequent decoration by orthogonal click reactions (copper-catalyzed azide-alkyne cycloaddition, thiol-ene or thiol-yne coupling) to introduce BP units and a fluorophore. Because of the troublesome isolation of the target conjugates by standard procedures, the methodology culminated with the functionalization of the BA scaffold with a light fluorous tag to rapidly and efficiently purify intermediates and final products by fluorous solid-phase extraction.

Chiral ligands designed in China

Asymmetric catalysis has become an indispensable and productive field within the Chinese organic chemistry society. The design of chiral ligands is one of the most prominent research areas in this field. Since the late 1990s, Chinese organic chemists have developed numerous chiral ligands possessing novel chiral skeletons and design concepts. Some of these ligands have been widely adopted and can be regarded as ‘privileged ligand’, which have shown excellent performance in many asymmetric catalytic reactions. In this review, we provide an overview of the chiral ligands designed by Chinese scientists with the aim of promoting the development of this area in China and with the hope of encouraging more scientists across the world to use these ligands when designing asymmetric reactions.

Oxidative cross-coupling reaction by scandium catalysis for synthesis of α-alkyl α-amino acid ester derivatives

A novel oxidative cross-coupling reaction between N-arylglycine esters and alkyl boronic acid esters was developed by scandium catalysis.

Copper(i)/TF-BiphamPhos catalyzed asymmetric nitroso Diels–Alder reaction

A highly efficient enantioselective nitroso Diels–Alder reaction of 6-methyl-2-nitroso pyridine with various 1,3-dienes was successfully developed using a copper(i)/(S)–TF-BiphamPhos complex as the catalyst. For most of the cyclic dienes, the cycloadducts were obtained in high yields with excellent regio-, and stereoselectivities. Acyclic 2-silyloxy-1,3-diene also worked well in the reaction.

Enantioselective Silver-Catalyzed Transformations

This review collects the major progress in the field of enantioselective transformations promoted by chiral silver catalysts, covering the literature since 2008, well illustrating the power of these especially mild Lewis acid catalysts to provide novel asymmetric reactions.