The Construction of All-Carbon Quaternary Stereocenters by Use of Pd-Catalyzed Asymmetric Allylic Alkylation Reactions in Total Synthesis

Asymmetric synthesis of enantioenriched α-allyl esters through Pd(BINAPHANE)-catalysed allylation of disubstituted ketenes

Pd2dba3·CHCl3 (2.5 mol%)-BINAPHANE (5 mol%) was used to promote the first catalytic enantioselective allylation of disubstituted ketenes to give α-allyl esters. The ester products were formed in good to excellent yields (61-93% yield for 13 examples, 16 examples in all), with moderate to good enantioselectivity (68-80% ee for 7 examples).

Brønsted Base-Catalyzed Enantioselective α-Functionalization of Carbonyl Compounds Involving π-Extended Enolates

Chiral Brønsted base (BB) catalyzed asymmetric transformations constitute an important tool for synthesis. A meaningful fraction of these transformations proceeds through transiently generated enolate intermediates, which display quite versatile reactivity against a variety of electrophiles. Some years ago, our group became interested in developing BB-catalyzed asymmetric reactions of enolizable carbonyl substrates that involve π-extended enolates in which, besides control of reaction diastereo and enantioselectivity, the site-selectivity control is an additional issue in most cases. In the examples covered in this account the opportunities deployed, and the challenges posed, by these methods are illustrated, with a focus on the generation of quaternary carbon stereocenters. In the way, new bifunctional BB catalysts as well as achiral templates were developed that may find further applications.

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.

Catalytic asymmetric defluorinative allylation of silyl enol ethers

The stereocontrolled installation of alkyl fragments at the alpha position of ketones is a fundamental yet unresolved transformation in organic chemistry. Herein we report a new catalytic methodology able to construct α-allyl ketones via defluorinative allylation of silyl enol ethers in a regio-, diastereo- and enantioselective manner. The protocol leverages the unique features of the fluorine atom to simultaneously act as a leaving group and to activate the fluorophilic nucleophile via a Si-F interaction. A series of spectroscopic, electroanalytic and kinetic experiments demonstrate the crucial interplay of the Si-F interaction for successful reactivity and selectivity. The generality of the transformation is demonstrated by synthesising a wide set of structurally diverse α-allylated ketones bearing two contiguous stereocenters. Remarkably, the catalytic protocol is amenable for the allylation of biologically significant natural products.

Tandem Dearomatization/Enantioselective Allylic Alkylation of Pyridines

Herein, we report a multistep one-pot reaction of substituted pyridines leading to N-protected tetrahydropyridines with outstanding enantioselectivity (up to 97% ee). An iridium(I)-catalyzed dearomative 1,2-hydrosilylation of pyridines enables the use of N-silyl enamines as a new type of nucleophile in a subsequent palladium-catalyzed asymmetric allylic alkylation. This telescoped process overcomes the intrinsic nucleophilic selectivity of pyridines to synthesize enantioenriched, C-3-substituted tetrahydropyridine products that have been otherwise challenging to access.

Catalytic Asymmetric α-Functionalization of α-Branched Aldehydes

Aldehydes constitute a main class of organic compounds widely applied in synthesis. As such, catalyst-controlled enantioselective α-functionalization of aldehydes has attracted great interest over the years. In this context, α-branched aldehydes are especially challenging substrates because of reactivity and selectivity issues. Firstly, the transient trisubstituted enamines and enolates resulting upon treatment with an aminocatalyst or a base, respectively, would exhibit attenuated reactivity; secondly, mixtures of E- and Z-configured enamines/enolates may be formed; and third, effective face-discrimination on such trisubstituted sp2 carbon intermediates by the incoming electrophilic reagent is not trivial. Despite these issues, in the last 15 years, several catalytic approaches for the α-functionalization of prostereogenic α-branched aldehydes that proceed in useful yields and diastereo- and enantioselectivity have been uncovered. Developments include both organocatalytic and metal-catalyzed approaches as well as dual catalysis strategies for forging new carbon–carbon and carbon–heteroatom (C-O, N, S, F, Cl, Br, …) bond formation at Cα of the starting aldehyde. In this review, some key early contributions to the field are presented, but focus is on the most recent methods, mainly covering the literature from year 2014 onward.

Asymmetric Synthesis of a Quaternary Carbon Stereogenic Center by Organocatalysis Using a Primary Amino Acid and Its Salt

In this personal account, our recent developments on the asymmetric synthesis of a quaternary carbon stereogenic center by organocatalysis using a primary amino acid and its salt as a catalyst are described in three chapters: (1) conjugate addition to nitroalkenes and vinyl ketones, (2) nucleophilic addition to π-allyl palladium complexes, and (3) nucleophilic substitution reactions with allyl and propargyl halides. By these methods, asymmetric α-allylation of α-branched aldehydes and ketones smoothly proceeded to give γ-nitroaldehydes, ketoaldehydes, α-allylated aldehydes, and α-allylated β-ketoesters possessing a quaternary carbon stereogenic center in good yields with high enantioselectivities.

Stereocontrolled α-Allylation of α-Branched N-tert-Butanesulfinyl Ketimines via a Michael-Wittig Cascade for the Construction of Acyclic Quaternary Stereocenters

A single-flask cascade of Michael addition and Wittig olefination was developed to allow the stereoselective α-allylic alkylation of α-branched N-tert-butanesulfinyl ketimines for the construction of acyclic quaternary stereocenters bearing two sterically and electronically similar substituents. In this process, tBuOK-promoted stereoselective α-deprotonation gives fully substituted aza-enolates with a stereodefined geometry, Michael addition with α,β-unsaturated phosphonates generates C-C bonds with exceptional stereocontrol, and finally paraformaldehyde trapping of the conjugate addition intermediate generates functionalized α-allylated imines.

Highly Stereospecific Cyclizations of Homoallylic Silanols

We demonstrate that di-tert-butylsilanols are competent nucleophiles for the intramolecular interception of palladium π-allyl species. In these reactions, allyl ethyl carbonates are the best precursors for the formation of palladium π-allyl intermediates, and [(Cinnamyl)PdCl]₂/BINAP is superior to other Pd salt/ligand framework combinations. Our optimized protocol is compatible with a variety of silanol substrates. Importantly, the cyclization is perfectly stereospecific, proceeding via an anti-syn mechanism, which stands in contrast to reported analogous reactions of alcohols and phenols, known to proceed via an anti-anti mechanism. The alkenes in the product dioxasilinanes serve as blank slates for further functionalization.

Divergent Total Syntheses of Napelline-Type C20-Diterpenoid Alkaloids: (-)-Napelline, (+)-Dehydronapelline, (-)-Songorine, (-)-Songoramine, (-)-Acoapetaldine D, and (-)-Liangshanone

The napelline-type alkaloids possess an azabicyclo[3.2.1]octane moiety and an ent-kaurane-type tetracyclic skeleton (6/6/6/5) along with varied oxidation patterns embedded in the compact hexacyclic framework. Herein, we disclose a divergent entry to napelline-type alkaloids that hinges on convergent assembly of the ent-kaurane core using a diastereoselective intermolecular Cu-mediated conjugate addition and subsequent intramolecular Michael addition reaction as well as rapid construction of the azabicyclo[3.2.1]octane motif via an intramolecular Mannich cyclization. The power of this strategy has been demonstrated through efficient asymmetric total syntheses of eight napelline-type alkaloids, including (-)-napelline, (-)-12-epi-napelline, (+)-dehydronapelline, (+)-12-epi-dehydronapelline, (-)-songorine, (-)-songoramine, (-)-acoapetaldine D, and (-)-liangshanone.

Organocatalyzed Asymmetric Allylic Alkylation Enables Synthesis of Chiral γ-Lactones Bearing Vicinal Tertiary and Quaternary Stereocenters

The synthesis of enantioenriched α-aryl-α'-allyl-γ-butyrolactones bearing vicinal tertiary and quaternary stereocenters through organocatalyzed asymmetric allylic alkylation is reported. The process demonstrated that weakly stabilized enolates derived from α-aryl-γ-butyrolactones can undergo regio-, diastereo-, and enantioselective allylation using the proper activation of Morita-Baylis-Hillman (MBH) carbonates.

Catalytic Enantioselective Synthesis of α-Difunctionalized Cyclic Sulfones

As saturated heterocyclic building blocks become increasingly popular in medicinal chemistry and drug discovery programs, expansion of the synthetic toolkit to novel stereofunctionalized heterocycles is a priority. Herein, we report the development of a palladium-catalyzed decarboxylative asymmetric allylic alkylation reaction to access a broad range of enantioenriched α-difunctionalized 5- and 6-membered sulfones from easily accessible racemic starting materials. The allylic alkylation step was found to occur with high levels of enantioselectivity as a result of a palladium-mediated dynamic kinetic resolution of E/Z enolate intermediates. This methodology paves the way to hitherto unexplored stereodefined cyclic sulfones for medicinal chemistry applications.

Synthesis of Highly Congested Tertiary Alcohols via the [3,3] Radical Deconstruction of Breslow Intermediates

Pericyclic processes such as [3,3]-sigmatropic rearrangements leading to the rapid generation of molecular complexity constitute highly valuable tools in organic synthesis. Herein, we report the formation of particularly hindered tertiary alcohols via rearrangement of Breslow intermediates formed in situ from readily available N-allyl thiazolium salts and benzaldehyde derivatives. Experimental mechanistic studies performed suggest that the reaction proceeds via a close radical pair which recombine in a regio- and diastereoselective manner, formally leading to [3,3]-rearranged products.

Decarboxylative Allylic Alkylation of Phthalides: Stabilized Benzylic Nucleophiles for sp3-sp3 Coupling

A new family of stabilized benzylic nucleophiles for the palladium-catalyzed decarboxylative allylic alkylation reaction has been developed. Allyl esters derived from 3-carboxyphthalides were found to undergo palladium-catalyzed deallylation and decarboxylation under mild reaction conditions, a process facilitated by the formation of a stabilized aromatic anion. The regioselective allylic coupling of this intermediate afforded a variety of functionalized phthalides in 73-96% yields.

Stereospecific Construction of Quaternary Carbon Stereocenters from Quaternary Carbon Stereocenters

Organoaluminum species promote a smooth nucleophilic substitution at the quaternary carbon stereocenter of stereodefined polysubstituted cyclopropyl methyl phosphate with a complete inversion of configuration, even when more reactive functional groups are present. The regio- and diastereoselectivity of the substitution is attributed to the existence of a bicyclobutonium intermediate.

Rhodium-Catalyzed Desymmetric Arylation of γ,γ-Disubsituted Cyclohexadienones: Asymmetric Synthesis of Chiral All-Carbon Quaternary Centers

The desymmetric arylation of prochiral cyclohexadienones with ArZnCl in the presence of an (R)-segphos-rhodium catalyst gave high yields of the corresponding cyclohexenones, which contain a chiral arylated carbon center at the β-position and a chiral all-carbon quaternary center at the γ-position, with high diastereo- and enantioselectivities. This catalytic system was also applied to the arylation of spirocarbocyclic cyclohexadienones and afforded the corresponding cyclohexenones bearing a chiral spiro quaternary carbon with high dr and ee.

Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation of Thietane 1,1-Dioxides

A palladium-catalyzed decarboxylative asymmetric allylic alkylation of thietane 1,1-dioxides via linear enolate intermediates from racemic starting materials has been developed. This process installs an α-sulfonyl tetrasubstituted stereogenic center with high enantioselectivity. The potential to transform the alkylated products to novel types of enantioenriched spirocycles for medicinal chemistry applications has also been demonstrated.

Recent Advances in the Construction of Quaternary Stereocenters via Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation

Palladium-catalyzed decarboxylative asymmetric allylic alkylation (DAAA) provides an efficient and powerful strategy to construct quaternary stereocenters, which are widely present in biologically active natural products and approved drugs. In this short review, we summarize recent developments (since 2018) in the facile synthesis of quaternary stereocenters via DAAA methods. Several representative examples of the use of DAAA strategies for the total synthesis of complex natural products further demonstrate its synthetic potential in the realm of organic and medicinal chemistry.

1 Introduction

2 Construction of Quaternary Stereocenters via Palladium Catalyzed DAAA

3 Construction of Quaternary Stereocenters via Pd-Catalyzed Interceptive DAAA

4 Application of DAAA in Natural Product Synthesis

5 Conclusion

Palladium-Catalyzed Asymmetric Decarboxylative Addition of β-Keto Acids to Heteroatom-Substituted Allenes

The Pd-catalyzed asymmetric addition reaction of β-keto acids to heteroatom-substituted allene is reported. This reaction generates β-substituted ketones in an asymmetric manner through a branch-selective decarboxylative allylation pathway. The reaction accommodates various alkoxyallenes as well as amidoallenes.

Construction of Acyclic All-Carbon Quaternary Stereocenter Based on Asymmetric Michael Addition of Chiral Amine

Acyclic asymmetric quaternary stereocenters, which are composed of four carbon-carbon bonds, were finely constructed by utilizing a face-selective alkylation of enolate intermediates derived from an asymmetric Michael addition reaction of a chiral lithium amide with trisubstituted (E)-α,β-unsaturated esters. The present face-selective alkylation was able to employ diverse alkyl halides as an electrophile to afford various Michael adducts having an all-carbon quaternary stereocenter. With regard to the deprotection of the chiral auxiliary, N-iodosuccinimide used in our previous study did not work in the present cases; however, we found that pyridine iodine monochloride in the presence of H₂O was effective to remove the bornyl group and the benzyl group on the amino group to provide the β-amino ester derivative.

Catalytic Enantioselective Alkylation of Prochiral Enolates

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Leveraging Fleeting Strained Intermediates to Access Complex Scaffolds

Arynes, strained cyclic alkynes, and strained cyclic allenes were validated as plausible intermediates in the 1950s and 1960s. Despite initially being considered mere scientific curiosities, these transient and highly reactive species have now become valuable synthetic building blocks. This Perspective highlights recent advances in the field that have allowed access to structural and stereochemical complexity, including recent breakthroughs in asymmetric catalysis.

Chemically modified and conjugated antimicrobial peptides against superbugs

Antimicrobial resistance (AMR) is one of the greatest threats to human health that, by 2050, will lead to more deaths from bacterial infections than cancer. New antimicrobial agents, both broad-spectrum and selective, that do not induce AMR are urgently required. Antimicrobial peptides (AMPs) are a novel class of alternatives that possess potent activity against a wide range of Gram-negative and positive bacteria with little or no capacity to induce AMR. This has stimulated substantial chemical development of novel peptide-based antibiotics possessing improved therapeutic index. This review summarises recent synthetic efforts and their impact on analogue design as well as their various applications in AMP development. It includes modifications that have been reported to enhance antimicrobial activity including lipidation, glycosylation and multimerization through to the broad application of novel bio-orthogonal chemistry, as well as perspectives on the direction of future research. The subject area is primarily the development of next-generation antimicrobial agents through selective, rational chemical modification of AMPs. The review further serves as a guide toward the most promising directions in this field to stimulate broad scientific attention, and will lead to new, effective and selective solutions for the several biomedical challenges to which antimicrobial peptidomimetics are being applied.

Recent advances in the synthesis of ent-kaurane diterpenoids

Covering: 2015 to 2020The ent-kaurane diterpenoids are integral parts of tetracyclic natural products that are widely distributed in terrestrial plants. These compounds have been found to possess interesting bioactivities, ranging from antitumor, antifungal and antibacterial to anti-inflammatory activities. Structurally, the different tetracyclic moieties of ent-kauranes can be seen as the results of intramolecular cyclizations, oxidations, C-C bond cleavages, degradation, or rearrangements, starting from their parent skeleton. During the past decade, great efforts have been made to develop novel strategies for synthesizing these natural products. The purpose of this review is to describe the recent advances in the total synthesis of ent-kaurane diterpenoids covering the period from 2015 to date.

Pd/Cu Dual-Catalyzed Asymmetric Synthesis of Highly Functional All-Carbon Quaternary Stereocenters from Vinyl Carbonates

The challenging metal-catalyzed asymmetric synthesis of highly functional quaternary carbon centers using decarboxylative C(sp³ )-C(sp³ ) bond formation reactions is reported. The key substrate, a vinyl cyclic carbonate, is activated to provide concomitantly both the requisite nucleophile (by formal umpolung) and electrophile reaction partner preceding the asymmetric cross-coupling process. A wide screening of reaction conditions, additives and catalyst precursors afforded a protocol that gave access to a series of compounds featuring densely functionalized, elusive quaternary carbon stereocenters in appreciable yield and with enantiomeric ratios (er's) of up to 90 : 10.

Enantio- and Diastereodivergent Sequential Catalysis Featuring Two Transition-Metal-Catalyzed Asymmetric Reactions

This study demonstrates the feasibility and inherent benefits of combining two distinct asymmetric transition-metal-catalyzed reactions in one pot. The reported transformation features a Pd-catalyzed asymmetric allylic alkylation and a Rh-catalyzed enantioselective 1,4-conjugate addition, effectively converting simple allyl enol carbonate precursors into enantioenriched cyclic ketones with two remote stereocenters. Despite the anticipated challenges associated with controlling stereoselectivity in such a complex system, the products are obtained in enantiomeric excesses ranging up to >99 % ee, exceeding those obtained from either of the individual asymmetric reactions. In addition, since the stereoselectivity of both steps is under catalyst control, this one-pot reaction is enantio- and diastereodivergent, enabling facile access to all stereoisomers from the same set of starting materials.

Overcoming the Naphthyl Requirement in Stereospecific Cross-Couplings to Form Quaternary Stereocenters

The use of a simple stilbene ligand has enabled a stereospecific Suzuki-Miyaura cross-coupling of tertiary benzylic carboxylates, including those lacking naphthyl substituents. This method installs challenging all-carbon diaryl quaternary stereocenters in good yield and ee and represents an important breakthrough in the "naphthyl requirement" that pervades stereospecific cross-couplings involving enantioenriched electrophiles.

Total Synthesis of (-)-Arborisidine

An asymmetric total synthesis of cage-like indole alkaloid arborisidine is presented. The new synthetic strategy features a catalytic parallel kinetic resolution based on ambident nucleophilicity (C3/N) of indole to set the absolute configurations of the two quaternary chiral centers, and a 5-exo-trig radical cyclization to form the bridged nitrogen-containing five-membered ring.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Enantioselective formation of quaternary carbon stereocenters in natural product synthesis: a recent update

Covering: 2013-2018 Natural products bearing quaternary carbon stereocenters have attracted tremendous interest from the synthetic community due to their diverse biological activities and fascinating molecular architectures. However, the construction of these molecules in an enantioselective fashion remains a long-standing challenge because of the lack of efficient asymmetric catalytic methods for installing these motifs. The rapid progress in the development of new-generation efficient chiral catalysts has opened the door for several asymmetric reactions, such as Michael addition, dearomative cyclization, polyene cyclization, α-arylation, cycloaddition, allylation, for the construction of quaternary carbon stereocenters in a highly enantioselective fashion. These asymmetric catalytic methods have greatly facilitated the synthesis of complex natural products with improved output and overall efficiency. In this concise review, we highlight the progress in the last six years in complex natural product synthesis, in which at least one quaternary carbon stereocenter has been constructed via asymmetric catalytic technologies, with particular emphasis on the analysis of the stereochemical model of each enantioselective transformation.

Highly enantioselective one-pot sequential synthesis of valerolactones and pyrazolones bearing all-carbon quaternary stereocentres

Highly enantiopure and bioactive δ-valerolactones and pyrazolones, bearing α-all-carbon quaternary stereocentres, were successfully and sequentially prepared via a one-pot procedure starting from readily available, inexpensive materials, catalysed by a new chiral squaramide under mild reaction conditions. An organocatalytic Michael reaction afforded the valerolactones, while a one-pot Michael-hydrazinolysis-imidization cascade yielded the pyrazolones. This procedure is economically efficient and environmentally benign.

Synthesis of quaternary carbon-centered indolo[1,2-a]quinazolinones and indazolo[1,2-a]indazolones via C-H functionalization

An unprecedented Ru(ii)-catalyzed Csp2-H bond activation and annulation reaction of phenylindazolones with diaryl-substituted alkynes and dialkyl-substituted alkynes provided efficient routes for the construction of all-carbon quaternary-centered indolo[1,2-a]quinazolinones and quaternary carbon-centered indazolo[1,2-a]indazolones, respectively. The indolo[1,2-a]quinazolinones were fomed via Csp2-H activation, alkyne insertion and a 1,2-phenyl shift. Indazolo[1,2-a]indazolones were formed through a cascade reaction via the formation of exocyclic double bonds containing indolo[1,2-a]quinazolinones.

Nickel-Catalyzed, Stereospecific C-C and C-B Cross-Couplings via C-N and C-O Bond Activation

Highly enantioenriched benzylic and allylic amines and alcohols are readily available via asymmetric synthesis and in complex natural products. The development of mild, nickel-catalyzed cross-couplings of their derivatives has advanced the tools available for the preparation of a range of highly enantioenriched products, including those with quaternary stereocenters. This perspective focuses on cross-couplings with convenient and functional group-tolerant organoboron reagents and highlights the discoveries of activating groups and conditions that have led to high-yielding and highly stereospecific reactions. Emphasis is placed on mechanistic understanding, particularly with regards to controlling inversion vs. retention pathways. Limitations and opportunities for future developments are also highlighted.