New auxiliaries for copper-catalyzed asymmetric Michael reactions: generation of quaternary stereocenters at room temperature

Copper(I)-Catalyzed Direct Oxidative Annulation of 1,3-Dicarbonyl Compounds with Maleimides: Access to Polysubstituted Dihydrofuran Derivatives

An efficient annulation method for the synthesis of polysubstituted dihydrofurans from 1,3-dicarbonyl compounds and maleimides is described. The reactions can afford furo[2,3-c]pyrrole derivatives with satisfactory yields. The developed strategy realizes the direct oxidative double C(sp³)-H functionalization in the presence of copper(I) salts and 2-(tert-butylperoxy)-2-methylpropane. Meanwhile, this protocol features a mild reaction condition and simple catalytic system. A reaction mechanism involving a single electron oxidation is also proposed.

Arynes and Cyclic Alkynes as Synthetic Building Blocks for Stereodefined Quaternary Centers

We report a facile method to synthesize stereodefined quaternary centers from reactions of arynes and related strained intermediates using β-ketoester-derived substrates. The conversion of β-ketoesters to chiral enamines is followed by reaction with in situ generated strained arynes or cyclic alkynes. Hydrolytic workup provides the arylated or alkenylated products in enantiomeric excesses as high as 96%. We also describe the one-pot conversion of a β-ketoester substrate to the corresponding enantioenriched α-arylated product. Computations show how chirality is transferred from the N-bound chiral auxiliary to the final products. These are the first theoretical studies of aryne trapping by chiral nucleophiles to set new stereocenters. Our approach provides a solution to the challenging problem of stereoselective β-ketoester arylation/alkenylation, with formation of a quaternary center.

Rational design of peptide derivatives for inhibition of MyD88-mediated toll-like receptor signaling in human peripheral blood mononuclear cells and epithelial cells exposed to Francisella tularensis

Small molecules were developed to attenuate proinflammatory cytokines resulting from activation of MyD88-mediated toll-like receptor (TLR) signaling by Francisella tularensis. Fifty-three tripeptide derivatives were synthesized to mimic a key BB-loop region involved in toll-like/interleukin-1 receptor recognition (TIR) domain interactions. Compounds were tested for inhibition of TNF-α, IFN-γ, IL-6, and IL-1β in human peripheral blood mononuclear cells (PBMCs) and primary human bronchial epithelial cells exposed to LPS extracts from F. tularensis. From 53 compounds synthesized and tested, ten compounds were identified as effective inhibitors of F. tularensisLPS-induced cytokines. Compound stability testing in the presence of human liver microsomes and human serum resulted in the identification of tripeptide derivative 7 that was a potent, stable, and drug-like small molecule. Target corroboration using a cell-based reporter assay and competition experiments with MyD88 TIR domain protein supported that the effect of 7 was through MyD88 TIR domain interactions. Compound 7 also attenuated proinflammatory cytokines in human peripheral blood mononuclear cells and bronchial epithelial cells challenged with a live vaccine strain of F. tularensis at a multiplicity of infection of 1:5. Small molecules that target TIR domain interactions in MyD88-dependent TLR signaling represent a promising strategy toward host-directed adjunctive therapeutics for inflammation associated with biothreat agent-induced sepsis.

Asymmetric enamine catalysis with β-ketoesters by chiral primary amine: divergent stereocontrol modes

α-Branched ketones remain a challenging type of substrates in aminocatalysis due to their congested structures as well as the associated difficulties in controlling chemo- and stereoselectivity. In this work, we have explored asymmetric aminocatalysis with α-substituted β-ketoesters. A simple chiral primary amine catalyst was identified to enable unprecedentedly effective catalysis of β-ketoesters in α-hydrazination and Robinson annulation reaction with good yields and high enantioselectivities. Stoichiometric experiments with preformed enamine ester intermediates revealed their enamine-catalytic nature as well as the critical roles of acidic additives in facilitating catalytic turnovers and in tuning the chemo- and stereoselectivity. With the identical catalytic system, the two reactions demonstrated opposite chiral inductions in terms of the absolute configurations of the newly formed stereogenic centers. Investigations into this intriguing issue by DFT have revealed divergent stereocontrol modes. For α-hydrazination, H-bonding-directed facial attack determines the stereoselectivity, whereas a steric model is applied to the Robinson annulation where dual activations of both β-ketoester and vinyl ketone/aldehyde are involved.

Enamides and enecarbamates as nucleophiles in stereoselective C-C and C-N bond-forming reactions

Because the backbone of most of organic compounds is a carbon chain, carbon-carbon bond-forming reactions are among the most important reactions in organic synthesis. Many of the carbon-carbon bond-forming reactions so far reported rely on nucleophilic attack of enolates or their derivatives, because those nucleophiles can be, in general, readily prepared from the corresponding carbonyl compounds. In this Account, we summarize the recent development of reactions using enamide and enecarbamate as a novel type of nucleophile. Despite their ready availability and their intrinsic attraction as a synthetic tool that enables us to introduce a protected nitrogen functional group, enamide and enecarbamate have rarely been used as a nucleophile, since their nucleophilicity is low compared with the corresponding metal enolates and enamines. A characteristic of enamides and enecarbamates is that those bearing a hydrogen atom on nitrogen are relatively stable at room temperature, while enamines bearing a hydrogen atom on nitrogen are likely to tautomerize into the corresponding imine form. Enamides and enecarbamates can be purified by silica gel chromatography and kept for a long time without decomposition. During the investigation of nucleophilic addition reactions using enamides and enecarbamates, it has been revealed that enamides and enecarbamates bearing a hydrogen atom on nitrogen react actually as a nucleophile with relatively reactive electrophiles, such as glyoxylate, N-acylimino ester, N-acylimino phosphonate, and azodicarboxylate, in the presence of an appropriate Lewis acid catalyst. Those bearing no hydrogen atom on nitrogen did not react at all. The products initially obtained from the nucleophilic addition of enamides and enecarbamates are the corresponding N-protected imines, which can be readily transformed to important functional groups, such as ketones by hydrolysis and N-protected amines by reduction or nucleophilic alkylation. In the nucleophilic addition reactions of enamides and enecarbamates to aldehydes, it was unveiled that the reaction proceeds stereospecifically, that is, (E)-enecarbamate gave anti product and (Z)-enecarbamate afforded syn product with high diastereoselectivity (>97/3). This fact can be rationalized by consideration of a concerted reaction pathway via a hydrogen-involved cyclic six-membered ring transition state. In the addition reactions to N-acylimino phosphonates, much higher turnover frequency was observed when enamides and enecarbamates were used as a nucleophile than was observed when silicon enolates were used. When silicon enolates were used, the intermediates bearing a strong affinity for the catalyst inhibited catalyst turnover, resulting in low enantioslectivity because of the dominance of the uncatalyzed racemic pathway. In the case of nucleophilic addition of enamides and enecarbamate, however, a fast intramolecular hydrogen transfer from the enecarbamate nitrogen may prevent the intermediate from trapping the catalyst for a long time, to afford the product with a high enantioselectivity. In conclusion, enamides and enecarbamates, although originally employed as just N-analogues to silicon enolates, have emerged as remarkably useful nucleophiles in a variety of Lewis acid-catalyzed reactions.

Coordination compounds based on 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid

Syntheses of 2,6-bis[((3S)-3-(methoxycarbonyl)-1,2,3,4-tetrahydroisoquinolin-2-yl)carbonyl]pyridine and its coordination compounds with Cu2+, Co2+, Co3+, or Fe3+ are described. By means of 1H- and 13C-NMR spectra it was proved that 2,6-bis[((3S)-3-(methoxycarbonyl)-1,2,3,4-tetrahydroisoquinolin-2-yl)carbonyl]pyridine as well as its coordination compound with Co3+ exist in the form of a mixture of three conformers, differing in the conformations at the two amide groups present. The prepared coordination compounds were tested in the enantioselective catalysis of the nitroaldol addition of nitromethane with 2-nitrobenzaldehyde or 4-nitrobenzaldehyde, and in the Michael addition of ethyl 2-oxocyclohexanecarboxylate to but-3-en-2-one.

Non-Biaryl Atropisomers in Organocatalysis

A new class of 6'-hydroxy cinchona alkaloids, with a non-biaryl atropisomeric functionalisation at position 5' of the quinoline core can be prepared by an easy amination procedure. These are the first derivatives for which the principle of atropisomerism is engrafted in the classical core of the cinchona alkaloids. The aminated cinchona alkaloids are effective organocatalysts for the Michael addition of beta-keto esters to acrolein and methyl vinyl ketone, in up to 93 % ee (ee=enantiomeric excess), as well as for the asymmetric Friedel-Crafts amination of a variety of 2-naphthols, permitting the preparation of the latter in up to 98 % ee. The aminated 8-amino-2-naphthol itself is the first chiral organocatalyst based on non-biaryl atropisomerism. The two enantiomers of this chiral primary amine can be used for the direct alpha-fluorination of alpha-branched aldehydes. The fluorinated compounds can thereby be accessed in up to 90 % ee.

Enantioselective construction of quaternary carbon centre catalysed by bifunctional organocatalyst

The bifunctional thiourea-tertiary amine derivatives of simple chiral diamines serve as highly enantioselective catalysts for the Michael addition of alpha-substituted cyanoacetates to vinyl sulfones, giving an efficient protocol for the construction of an all-carbon substituted quaternary stereocentre.

Generation of Quaternary Stereocenters by Asymmetric Michael Reactions: Enamine Regiochemistry as Configuration Switch

Regioselective enamine formation from cyclic beta-diketones 1 is obtained by the appropriate choice of activating agent: Brønsted acid catalyzed condensation gives endocyclic enamines 3 as the thermodynamically favored products. Activation with Lewis acid BF(3).OEt(2) affords betaines 8 as intermediate products, which can be reacted with L-valine diethylamide (2) to preferentially furnish exocyclic enamines 4 as kinetic products. Derivatives with quaternary stereocenters were accessible from both isomeric enamines by using asymmetric, copper(II)-catalyzed Michael reactions at ambient temperature. Both regioisomers afford the triketones 7 with the same constitution but bearing the opposite absolute configuration at the quaternary stereocenter. Thus, both enantiomers of the product are prepared by using the same chiral auxiliary derived from L-valine.

Ionic and Covalent Copper(II)-Based Catalysts for Michael Additions. The Mechanism

Cu(SbF6)2-AdamBox and copper(II) bis-(5-tert-butylsalicylaldehydate) catalyze the Michael addition in neutral media. Mechanistic studies, based on UV-vis, IR, and electrospray ionization mass spectrometry (ESI-MS), suggest that copper enolates of the beta-dicarbonyl formed in situ are the active nucleophilic species.

Lewis Acid Catalyzed Allylboration: Discovery, Optimization, and Application to the Formation of Stereogenic Quaternary Carbon Centers

A full account of the development of the first catalytic manifold for the additions of allylboronates to aldehydes is described. The thermal additions (both diastereospecific and enantioselective) of 2-carboxyester 3,3-disubstituted allylboronates 1 to both aromatic and aliphatic aldehydes give biologically and synthetically important exo-methylene butyrolactones 2 containing a beta-quaternary carbon center. Although the thermal reaction requires 14 d at room temperature to reach completion, the presence of certain metal salts allows for a 12-16 h reaction while preserving the diastereospecificity observed in the uncatalyzed process. Preliminary mechanistic studies on the origin of the catalytic effect are described as well as stereoselective transformations of lactones 2 into cyclic and acyclic stereotriads with potential usefulness as synthetic intermediates.

Transformation of an optically active decahydro-6-isoquinolone scaffold: perfect Felkin-Anh diastereoselectivity

Diastereomerically and enantiomerically pure decahydro-6-isoquinolone derivative 7 (>99% de, 97% ee) was obtained from the Michael addition product 3. Interestingly, aldehyde 7 reacted with a number of different Grignard reagents to give the secondary alcohols 9 in good yields as single diastereomers. This result can be explained by taking the Felkin-Anh model into account.

A novelL-neopentylglycine derivative as auxiliary for copper-catalyzed asymmetric Michael reactions

L-Neopentylglycine diethylamide (4a). was prepared from the new unnatural amino acid L-neopentylglycine (1). The utilization of amide 4a as a chiral auxiliary in the copper(II)-catalyzed asymmetric Michael reaction was investigated in comparison with L-valine diethylamide (4b). Cyclic beta-oxocarboxylates 7 react with 4a and 4b to give the respective enaminoesters 8, which were converted with methyl vinyl ketone (9) in the presence of 10 mol% Cu(OAc)(2). H2O at room temperature in acetone to yield the optically active Michael addition products (R)-10a, b with high selectivity independent of the starting enamine. In the case of the seven-membered beta-oxocarboxylate 7c, however, the valine-derived enamine 8f led to higher enantioselectivity for product 10c. Despite the bulkiness of the neopentyl group, the isopropyl group with an alpha-branch has a better stereoinducing effect.

Enantioselective Construction of Quaternary Stereocenters

The stereoselective formation of C-C bonds is of great importance for the synthesis of enantiomerically pure natural products and pharmaceuticals. A broad repertoire of chiral auxiliaries, reagents, and catalysts can be utilized for the reliable generation of tertiary stereocenters. In contrast, the synthesis of organic compounds with quaternary stereocenters is a much more demanding and challenging task. Every enantioselective synthetic method can demonstrate its value through the generation of a fully substituted carbon center. In this Minireview examples of newer stoichiometric and catalytic methods are summarized which have proved their suitability for the enantioselective construction of quaternary stereocenters.