Stereoselective Total Synthesis of KAE609 via Direct Catalytic Asymmetric Alkynylation to Ketimine

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Modular Total Synthesis of the 5/5-Spirocyclic Spiroindimicins

Total syntheses of the 5/5-spirocyclic indoline alkaloids (±)-spiroindimicins B, C, D, E, F, and G have been achieved via a modular approach. Our route features direct coupling of halogenated pyrrolemetal and isatin partners, Suzuki coupling to append the indole unit, Lewis acid-mediated spirocyclization, and divergent functionalization to give various family members. These syntheses are concise (six or seven steps from commercial materials) and highly amenable to analogue synthesis.

Enantioselective Alkynylation of 2-Aryl-3H-indol-3-ones via Cooperative Catalysis of Copper/Chiral Phosphoric Acid

A facile enantioselective alkynylation of cyclic ketimines attached to a neutral functional group utilizing the dual Cu(I)-CPA catalysis is described. The strategy of the alkynylation of 2-aryl-3H-indol-3-one directly to chiral propargylic amines containing indolin-3-one moiety in good yields and enantioselectivities. Moreover, gram-scale synthesis of chiral propargylamines based C2-quaternary indolin-3-ones was performed. The synthetic applications were confirmed by transformations of the products with no decrease in the yield and enantioselectivity.

Enantioselective synthesis of 3-hydroxy- and 3-amino-3-alkynyl-2-oxindoles by the dimethylzinc-mediated addition of terminal alkynes to isatins and isatin-derived ketimines

A common protocol for enantioselective alkynylation of isatins and isatin-derived ketimines using terminal alkynes and Me2Zn in the presence of a catalytic amount of a chiral perhydro-1,3-benzoxazine with moderate to excellent enantioselectivity under mild reaction conditions is described. The additions to ketimines present a novel approach to chiral amines being derivatives of oxindoles. The reaction is broad in scope with respect to aryl- and alkyl-substituted terminal alkynes and isatin derivatives. In isatins, the alkynylation occurs at the Si face of the carbonyl group, whereas in the ketimine derivatives it occurs at the Re face of the imine.

Direct Enantioselective Addition of Alkynes to Imines by a Highly Efficient Palladacycle Catalyst

Enantiopure propargylic amines are highly valuable synthetic building blocks. Much effort has been devoted to develop methods for their preparation. The arguably most important strategy is the 1,2-addition of alkynes to imines. Despite remarkable progress, the known methods using Zn and Cu catalysts suffer from the need for high catalyst loadings, typically ranging from 2-60 mol % for neutral aldimine substrates. Here we report a planar chiral Pd complex acting as very efficient catalyst for direct asymmetric alkyne additions to imines, requiring very low catalyst loadings. Turnover numbers of up to 8700 were accomplished. Our investigation suggests that a Pd-acetylide complex is generated as a catalytically relevant intermediate by the aid of an acac ligand acting as internal catalytic base. It is shown that the catalyst is quite stable under the reaction conditions and that product inhibition is not an issue. A total of 39 examples is shown which all yielded almost enantiopure products.

Mechanistic Study of Asymmetric Alkynylation of Isatin-Derived Ketimine Mediated by a Copper/Guanidine Catalyst

In this work, we performed a mechanistic study of asymmetric alkynylation of isatin-derived N-Boc ketimine that was first reported by Feng, Liu, and co-workers (Chem. Commun. 2018, 54, 678-681). Guanidine-amide promoted the formation of highly nucleophilic copper acetylene species by abstracting the terminal proton of phenylacetylene with an imine moiety. The guanidinium salt-Cu(I) complex was the most active species in the addition of the C═N bond, in which copper acetylene coordinated to the O atom of the amide moiety, and the isatin-derived ketimine substrate was activated by hydrogen bonding as well as tert-butoxycarbonyl···Cu(I) coordination. Due to weak interaction between Cu(I) and the Ph group in the amide of guanidine, as well as the repulsion between the tert-butyl group in ketimine and the cyclohexyl group in guanidine, the copper acetylene preferred to attack isatin-derived ketimine from the re-face, leading to the S-configuration product with excellent stereoselectivity. The affinity of the counterion for the Cu(I) center in the copper salt affected the deprotonation of phenylacetylene and the formation of guanidinium salt active species. In contrast to CuBr and CuCl, the combination of CuI with aniline-derived guanidine-amide exhibited high catalytic activity and a chiral induction effect, contributing to a high turnover frequency (9.70 × 10^-4 s^-1) in catalysis and ee%.

Amide phosphonium salt catalyzed enantioselective Mannich addition of isoxazole-based nucleophiles to β,γ-alkynyl-α-ketimino esters

An enantioselective Mannich addition of 3,5-disubstituted 4-nitroisoxazoles to β,γ-alkynyl-α-ketimino esters promoted by an amide phosphonium salt-based catalyst has been developed. N-Cbz-protected ketimino esters with various aryl substituents attached to the alkyne unit were reacted with a series of isoxazoles with different substitution patterns. Chiral tertiary propargylic amine products were obtained with moderate to good yields and enantioselectivities. TIPS- and cyclopropyl-substituted alkynyl ketimines were also examined in the current system and the desired products were obtained with moderate yields and enantioselectivities. The potential scalability and utility of the current protocol were demonstrated by carrying out a relatively larger scale reaction followed by further transformations.

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Asymmetric catalytic alkynylation of thiazolones and azlactones for synthesis of quaternary α-amino acid precursors

Asymmetric alkynylation of thiazolones and azlactones with alkynylbenziodoxolones as the electrophilic alkyne source catalyzed by thiourea phosphonium salt is described. By using thiazolones as nucleophiles, the desired alkyne functionalized thiazolones were obtained in 55-89% yields with 31-86% ee. Azlactones gave the desired products in comparable yields with lower enantioselectivities. Ring-opening of the alkynylation products led to α,α-disubstituted α-amino acid derivatives efficiently without loss of enantioselectivity.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Direct Conversion of N-Alkylamines to N-Propargylamines through C-H Activation Promoted by Lewis Acid/Organocopper Catalysis: Application to Late-Stage Functionalization of Bioactive Molecules

An efficient catalytic method to convert an α-C-H bond of N-alkylamines into an α-C-alkynyl bond was developed. In the past, such transformations were carried out under oxidative conditions, and the enantioselective variants were confined to tetrahydroisoquinoline derivatives. Here, we disclose a method for the union of N-alkylamines and trimethylsilyl alkynes, without the presence of an external oxidant and promoted through cooperative actions of two Lewis acids, B(C6F5)3 and a Cu-based complex. A variety of propargylamines can be synthesized in high diastereo- and enantioselectivity. The utility of the approach is demonstrated by the late-stage site-selective modification of bioactive amines. Kinetic investigations that shed light on various mechanistic nuances of the catalytic process are presented.

Syntheses of 2-Iminoindolin-3-ones and 2-Alknyl-2,3-dihydroquinazolin-4(1H)-ones from 3-Diazoindolin-2-imines

3-Diazoindolin-2-imines reacted with nitrones to furnish 2-iminoindolin-3-ones through a Au(I)-catalyzed cascade oxygen transfer/imine exchange process. The prepared 2-iminoindolin-3-ones could be further transformed into 2-alknyl-2,3-dihydroquinazolin-4(1H)-ones through a Ag(I)-catalyzed reaction with terminal alkynes. A MeOH-triggered ring expansion mechanism involving cyclic iminium formation and nucleophilic addition is proposed for this novel alkynylation reaction. This two-step procedure provides a general and convenient approach to 2-alknyl-2,3-dihydroquinazolin-4(1H)-ones, which are privileged structures in medicinal chemistry.

The early preclinical and clinical development of cipargamin (KAE609), a novel antimalarial compound

BACKGROUND

Cipargamin (KAE609) is a novel spiroindolone class drug for the treatment of malaria, currently undergoing phase 2 clinical development. This review provides an overview and interpretation of the pre-clinical and clinical data of this possible next-generation antimalarial drug published to date.

METHODS

We systematically searched the literature for studies on the preclinical and clinical development of cipargamin. PubMed and Google Scholar databases were searched using the terms 'cipargamin', 'KAE609' or 'NITD609' in the English language; one additional article was identified during revision. Nineteen of these in total 43 papers identified reported original studies; 13 of those articles were on pre-clinical studies and 6 reported clinical trials.

RESULTS

A total of 20 studies addressing its preclinical and clinical development have been published on this compound at the time of writing. Cipargamin acts on the PfATP4, which is a P-type Na + ATPase disrupting the Na + homeostasis in the parasite. Cipargamin is a very fast-acting antimalarial, it is active against all intra-erythrocytic stages of the malaria parasite and exerts gametocytocidal activity, with transmission-blocking potential. It is currently undergoing phase 2 clinical trial to assess safety and efficacy, with a special focus on hepatic safety.

CONCLUSION

In the search for novel antimalarial drugs, cipargamin exhibits promising properties, exerting activity against multiple intra-erythrocytic stages of plasmodia, including gametocytes. It exhibits a favourable pharmacokinetic profile, possibly allowing for single-dose treatment with a suitable combination partner. According to the clinical results of the first studies in Asian malaria patients, a possible safety concern is hepatotoxicity.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

One-Pot Synthesis of Enantiopure Spiro[3,4-dihydrobenzo[b][1,4]oxazine-2,3'-oxindole] via Regio- and Stereoselective Tandem Ring Opening/Cyclization of Spiroaziridine Oxindoles with Bromophenols

A highly efficient regio- and stereoselective spiroaziridine ring opening with 2-bromophenols and a subsequent tandem cyclization reaction was developed for the one-pot synthesis of enantiopure 3,4-dihydrospiro[benzo[b][1,4]oxazine-2,3'-oxindole] with excellent enantiopurity (ee up to >99%). It is further extended to asymmetric synthesis of NH-free 3,4-dihydrospiro[benzo[b][1,4]oxazine-2,3'-xindole] retaining the optical activity.

Photochemical benzylic bromination in continuous flow using BrCCl3 and its application to telescoped p-methoxybenzyl protection

Photochemical benzylic bromination in flow using BrCCl₃, which is compatible with electron-rich aromatics, allowing in situ p-methoxybenzyl bromide formation and PMB-protection.

Synthesis of Chiral Tryptamines via a Regioselective Indole Alkylation

A practical synthesis of chiral tryptamines from simple, unprotected indoles has been developed. Indole nucleophiles prepared with MeMgCl in the presence of CuCl reacted with chiral cyclic sulfamidates almost exclusively at the C³-position of indole to form a variety of α- and/or β-substituted chiral tryptamines in good yield with excellent regioselectivity. The utility of this simple alkylation process has been demonstrated with the practical synthesis of two biologically active targets, cipargamin and TIK-301, which were completed in three steps, starting from the corresponding indole starting materials.

Catalytic stereoselective total synthesis of a spiro-oxindole alkaloid and the pentacyclic core of tryptoquivalines

An expedient route to the pentacyclic core of the tryptoquivaline alkaloids and the total synthesis of natural product (+)-3′-(4-oxoquinazolin-3-yl)spiro[1H-indole-3,5′-oxolane]-2,2′-dione (1) have been achieved.

Copper/guanidine-catalyzed asymmetric alkynylation of isatin-derived ketimines

Asymmetric alkynylation of isatin-derived ketimines was achieved using an easily available chiral guanidine ligand in combination with CuI under mild reaction conditions.

Highly Enantioselective Synthesis of Propargyl Amides through Rh-Catalyzed Asymmetric Hydroalkynylation of Enamides: Scope, Mechanism, and Origin of Selectivity

Chiral propargyl amides are particularly useful structural units in organic synthesis. The enantioselective synthesis of propargyl amide is highly desirable. Conventional approach involves the use of a stoichiometric amount of metal reagent or chiral auxiliary. In comparison, direct alkynylation with terminal alkyne is attractive because it avoids the use of stoichiometric organometallic reagent. The asymmetric coupling of aldehyde, amine, and alkyne (A³-coupling) provides an efficient method for the synthesis of N-alkyl and N-aryl-substituted propargyl amines, but this strategy is not amenable for the direct enantioselective synthesis of propargyl amide. We have developed a new strategy and report here a Rh-catalyzed asymmetric hydroalkynylation of enamides. Alkynylations occur regioselectively at the α position of an enamide to produce chiral propargyl amides. High yield and enantioselectivity were observed. Previous alkynylation methods to prepare chiral propargyl amine involve the nucleophilic addition to an electron-deficient imine. In contrast, our current approach proceeds through regioselective hydroalkynylation of an electron-rich alkene. Kinetic studies indicated that migratory insertion of the enamide to the rhodium hydride is turnover limiting. Computational studies revealed the origin of regio- and enantioselectivities. This novel strategy provides an efficient method to access chiral propargyl amides directly from terminal alkynes.

Copper-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines

A Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines was developed, providing the corresponding chiral α-tertiary amines with up to 98% ee.

Highly efficient synthesis of chiral quaternary 3-aminooxindoles promoted by zinc(ii) chloride via Et2Zn-catalysed addition of Grignard reagents to isaltin-derived N-tert-butanesulfinyl ketimines

An approach to chiral quaternary 3-aminooxindoles via Et₂Zn catalyzed diastereoselective addition of various Grignard reagents to broad isaltin-derived N-tert-butanesulfinyl ketimines promoted by zinc(ii) chloride.

Enantioselective alkynylation of N-sulfonyl α-ketiminoesters via a Friedel–Crafts alkylation strategy

Enantioselective Ni(ClO₄)₂-catalyzed alkynylation of N-sulfonyl-α-ketiminoesters is developed via a Friedel–Crafts-type pathway, yielding propargylic amides bearing quaternary stereocenters in excellent enantioselectivities.

Direct access to N-unprotected tetrasubstituted propargylamines via direct catalytic alkynylation of N-unprotected trifluoromethyl ketimines

Direct catalytic alkynylation of N-unprotected trifluoromethyl ketimines allowed an unprecedented direct access to N-unprotected α-tetrasubstituted primary amines without additional deprotection steps.

Enantioselective addition of diphenyl phosphonate to ketimines derived from isatins catalyzed by binaphthyl-modified organocatalysts

Chiral binaphthyl-modified squaramide-catalyzed enantioselective addition of diphenyl phosphonate to ketimines derived from isatins has been achieved. This method affords practical and efficient access to chiral 3-amino-3-phosphonyl-substituted oxindole derivatives in high yields with excellent enantioselectivities (up to 99% ee).

Mechanistic Studies and Expansion of the Substrate Scope of Direct Enantioselective Alkynylation of α-Ketiminoesters Catalyzed by Adaptable (Phebox)Rhodium(III) Complexes

Mechanistic studies and expansion of the substrate scope of direct enantioselective alkynylation of α-ketiminoesters catalyzed by adaptable (phebox)rhodium(III) complexes are described. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytic cycle, and the generation of more active (acetato-κ(2)O,O')(alkynyl)(phebox)rhodium(III) complexes from the starting (diacetato)rhodium(III) complexes limited the overall reactivity of the reaction. These findings, as well as facile exchange of the alkynyl ligand on the (alkynyl)rhodium(III) complexes led us to use (acetato-κ(2)O,O')(trimethylsilylethynyl)(phebox)rhodium(III) complexes as a general precatalyst for various (alkynyl)rhodium(III) complexes. Use of the (trimethylsilylethynyl)rhodium(III) complexes as precatalysts enhanced the catalytic performance of the reactions with an α-ketiminoester derived from ethyl trifluoropyruvate at a catalyst loading as low as 0.5 mol % and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.