Secondary and primary amine catalysts for iminium catalysis

New enantioenriched β-indolyl ketones as aromatase inhibitors: Unraveling heme-ligand interactions by MD simulation and MMPBSA analysis

A series of enantioenriched β-indolyl ketones as aromatase inhibitors (AI) is synthesized through the Michael-type Friedel-Crafts alkylation of indole. A highly efficient bifunctionalized amino catalyst is developed to access structurally diverse β-indolyl ketones in high yields (up to 91%) and excellent enantioselectivity (enantiomeric ratio up to 98:2). All the synthesized compounds demonstrated promising aromatase inhibitory potential, where ortho-substituted analogs (3c and 3e) were found most active with IC50 values of 0.68 and 0.90 µM, respectively. Both of these compounds exhibited significant cytotoxicity (IC50 = 0.34 and 0.37 µM) against the MCF-7 breast cancer cell line in the (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay. Molecular docking studies of the synthesized compounds demonstrate favorable binding interactions with the estrogens controlling CYP19A1 (3EQM) and metabolizing CYP3A4 (5VCC) enzymes. Molecular dynamic (MD) simulation analysis revealed the essentiality of heme-ligand interactions to build a stable protein-ligand complex. An average root mean square deviation of 0.35 nm observed during a 100-ns MD simulation and binding free energy in the range of -190 to -227 kJ/mol calculated by g_mmpbsa analysis authenticated the stability of the 3c-3EQM complex. ADMET and drug-likeness parameters supported the suitability of these indole derivatives as the drug lead to develop potent inhibitors for estrogen-dependent breast cancer.

Mechanism of a Dually Catalyzed Enantioselective Oxa-Pictet-Spengler Reaction and the Development of a Stereodivergent Variant

Enantioselective oxa-Pictet-Spengler reactions of tryptophol with aldehydes proceed under weakly acidic conditions utilizing a combination of two catalysts, an indoline HCl salt and a bisthiourea compound. Mechanistic investigations revealed the roles of both catalysts and confirmed the involvement of oxocarbenium ion intermediates, ruling out alternative scenarios. A stereochemical model was derived from density functional theory calculations, which provided the basis for the development of a highly enantioselective stereodivergent variant with racemic tryptophol derivatives.

Design of Bifunctional Amino Tf-Amide Organocatalysts and Application in Various Asymmetric Transformations

In this personal account, we describe our recent developments on the four types of amino Tf-amide catalysts in asymmetric transformations. Firstly, axially chiral biaryl-based secondary-amino Tf-amide catalyzed various stereoselective reactions via enamine intermediates. Secondly, pyrrolidine-based secondary-amino aliphatic Tf-amide catalyzed asymmetric direct Mannich reaction. Thirdly, chiral primary-amino aliphatic Tf-amide catalyzed asymmetric direct aldol reaction and conjugate addition. Finally, modified chiral amino aromatic Tf-amide catalyzed asymmetric transformations. These four different strategies are illustrated by using various organocatalyzed asymmetric transformations.

Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer

For molecules with a singlet ground state, the population of triplet states is mainly possible (a) by direct excitation and subsequent intersystem crossing or (b) by energy transfer from an appropriate sensitizer. The latter scenario enables a catalytic photochemical reaction in which the sensitizer adopts the role of a catalyst undergoing several cycles of photon absorption and subsequent energy transfer to the substrate. If the product molecule of a triplet-sensitized process is chiral, this process can proceed enantioselectively upon judicious choice of a chiral triplet sensitizer. An enantioselective reaction can also occur in a dual catalytic approach in which, apart from an achiral sensitizer, a second chiral catalyst activates the substrate toward sensitization. Although the idea of enantioselective photochemical reactions via triplet intermediates has been pursued for more than 50 years, notable selectivities exceeding 90% enantiomeric excess (ee) have only been realized in the past decade. This review attempts to provide a comprehensive survey on the various photochemical reactions which were rendered enantioselective by triplet sensitization.

Triplet Energy Transfer from Ruthenium Complexes to Chiral Eniminium Ions: Enantioselective Synthesis of Cyclobutanecarbaldehydes by [2+2] Photocycloaddition

Chiral eniminium salts, prepared from α,β-unsaturated aldehydes and a chiral proline derived secondary amine, underwent, upon irradiation with visible light, a ruthenium-catalyzed (2.5 mol %) intermolecular [2+2] photocycloaddition to olefins, which after hydrolysis led to chiral cyclobutanecarbaldehydes (17 examples, 49-74 % yield), with high diastereo- and enantioselectivities. Ru(bpz)3 (PF6 )2 was utilized as the ruthenium catalyst and laser flash photolysis studies show that the catalyst operates exclusively by triplet-energy transfer (sensitization). A catalytic system was devised with a chiral secondary amine co-catalyst. In the catalytic reactions, Ru(bpy)3 (PF6 )2 was employed, and laser flash photolysis experiments suggest it undergoes both electron and energy transfer. However, experimental evidence supports the hypothesis that energy transfer is the only productive quenching mechanism. Control experiments using Ir(ppy)3 showed no catalysis for the intermolecular [2+2] photocycloaddition of an eniminium ion.

Regioselective and Enantioselective Synthesis of β-Indolyl Cyclopentenamides by Chiral Anion Catalysis

The regioselective and enantioselective synthesis of β-indolyl cyclopentenamides, a versatile chiral building block, by asymmetric addition of indoles to α,β-unsaturated iminium intermediates has been achieved through chiral anion catalysis. Key to the success of this methodology is the generation of a chiral anion-paired ketone-type α,β-unsaturated iminium intermediate from α-hydroxy enamides. Preliminary mechanistic studies and DFT calculations are consistent with a mechanism involving multiple, concurrent pathways for isomerization of the initially formed azaallylcation into the key α,β-unsaturated iminium intermediate, all mediated by the phosphoric acid catalyst.

One-pot Fluorination and Organocatalytic Robinson Annulation for Asymmetric Synthesis of Mono- and Difluorinated Cyclohexenones

A one-pot fluorination and organocatalytic Robinson annulation sequence has been developed for asymmetric synthesis of 6-fluoroyclohex-2-en-1-ones and 4,6-difluorocyclohex-2-en-1-ones. The reactions promoted by cinchona alkaloid amine afforded products bearing two or three stereocenters in good to excellent yields with up to 99% ee and 20:1 dr.

Organocatalytic atroposelective synthesis of axially chiral styrenes

Axially chiral compounds are widespread in biologically active compounds and are useful chiral ligands or organocatalysts in asymmetric catalysis. It is well-known that styrenes are one of the most abundant and principal feedstocks and thus represent excellent prospective building blocks for chemical synthesis. Driven by the development of atroposelective synthesis of axially chiral styrene derivatives, we discovered herein the asymmetric organocatalytic approach via direct Michael addition reaction of substituted diones/ketone esters/malononitrile to alkynals. The axially chiral styrene compounds were produced with good chemical yields, enantioselectivities and almost complete E/Z-selectivities through a secondary amine-catalysed iminium activation strategy under mild conditions. Such structural motifs are important precursors for further transformations into biologically active compounds and synthetic useful intermediates and may have potential applications in asymmetric synthesis as olefin ligands or organocatalysts.

Many methods exist for the atroposelective synthesis of axially chiral biaryls, but axially chiral styrenes are much less explored. Here the authors report an organocatalytic procedure for the synthesis of configurationally stable, axially chiral styrenes with high enantio- and diastereoselectivities.

Synthesis of Enantiomeric Polyhydroxyalkylpyrrolidines from 1,3-Dipolar Cycloadducts. Evaluation as Inhibitors of a β-Galactofuranosidase

Enantiomeric 2,3,4-tris(hydroxyalkyl)-5-phenylpyrrolidines have been synthesized from the major cycloadducts obtained by the 1,3-dipolar cycloaddition of sugar enones with azomethine ylides derived from natural amino acids. Reduction of the ketone carbonyl group of the cycloadducts, which possess a basic structure of bicyclic 6-(menthyloxy)hexahydropyrano[4,3-c]pyrrol-7(6H)one, afforded a number of pyrrolidine-based bicyclic systems. A sequence of reactions, which involved hydrolysis of the menthyloxy substituent, reduction, N-protection, and degradative oxidation, afforded varied pyrrolidine structures having diverse configurations and patterns of substitution; in particular, polyhydroxylated derivatives have been obtained. The unprotected products were isolated as pyrrolidinium trifluoroacetates. Because of the furanose-like nature of the target trihydroxyalkyl pyrrolidines, these molecules have been evaluated as inhibitors of the β-galactofuranosidase from Penicillium fellutanum. The compounds showed practically no inhibitory activity for concentration of pyrrolidines in the range of 0.1-1.6 mM.

Asymmetric α,γ-Regioselective [3 + 3] Formal Cycloadditions of α,β-Unsaturated Aldehydes via Cascade Dienamine-Dienamine Catalysis

Asymmetric α,γ-regioselective [3 + 3] formal cycloadditions of α,β-unsaturated aldehydes and 2-nitroallylic acetates have been developed for the first time. These reactions proceeded through a domino Michael addition-Michael addition sequence via an unusual cascade dienamine-dienamine catalysis of a chiral secondary amine, and multifunctional cyclohexene derivatives were generally constructed in moderate yields with excellent stereoselectivity after simple treatment with K2CO3.

Theoretical study of the mechanism of highly diastereoselective formation of a strained 3-azabicyclo[3.2.0]heptane derivative

The mechanism of multicomponent reactions, especially stereoselective ones is of special importance in organic synthesis. The mechanism of diastereoselective formation of a highly strained 3-azabicyclo[3.2.0]heptane derivative through a catalyst-free multistep reaction is studied by DFT calculations in solution phase. The calculations on the two proposed mechanisms (pathways A and B) showed that the iminium ion, the relatively high energy species is not the approprite reaction intermediate. On the other hand, the lower activation energy of pathway B, as compared to that of pathway A, makes it as the preferred mechanism. The high stereoselectivity of the stepwise reaction may be attributed to the fact that the rate limiting and stereodifferentiating steps are the same. These findings may also account the formation of by-product in the experimental observations.

Improving catalyst activity in secondary amine catalysed transformations

The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increase the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed.

Direct approaches to nitriles via highly efficient nitrogenation strategy through C-H or C-C bond cleavage

Because of the importance of nitrogen-containing compounds in chemistry and biology, organic chemists have long focused on the development of novel methodologies for their synthesis. For example, nitrogen-containing compounds show up within functional materials, as top-selling drugs, and as bioactive molecules. To synthesize these compounds in a green and sustainable way, researchers have focused on the direct functionalization of hydrocarbons via C-H or C-C bond cleavage. Although researchers have made significant progress in the direct functionalization of simple hydrocarbons, direct C-N bond formation via C-H or C-C bond cleavage remains challenging, in part because of the unstable character of some N-nucleophiles under oxidative conditions. The nitriles are versatile building blocks and precursors in organic synthesis. Recently, chemists have achieved the direct C-H cyanation with toxic cyanide salts in the presence of stoichiometric metal oxidants. In this Account, we describe recent progress made by our group in nitrile synthesis. C-H or C-C bond cleavage is a key process in our strategy, and azides or DMF serve as the nitrogen source. In these reactions, we successfully realized direct nitrile synthesis using a variety of hydrocarbon groups as nitrile precursors, including methyl, alkenyl, and alkynyl groups. We could carry out C(sp(3))-H functionalization on benzylic, allylic, and propargylic C-H bonds to produce diverse valuable synthetic nitriles. Mild oxidation of C═C double-bonds and C≡C triple-bonds also produced nitriles. The incorporation of nitrogen within the carbon skeleton typically involved the participation of azide reagents. Although some mechanistic details remain unclear, studies of these nitrogenation reactions implicate the involvement of a cation or radical intermediate, and an oxidative rearrangement of azide intermediate produced the nitrile. We also explored environmentally friendly oxidants, such as molecular oxygen, to make our synthetic strategy more attractive. Our direct nitrile synthesis methodologies have potential applications in the synthesis of biologically active molecules and drug candidates.

Chiral primary-amine-catalyzed conjugate addition to α-substituted vinyl ketones/aldehydes: divergent stereocontrol modes on enamine protonation

Enantioselective protonation with a catalytic enamine intermediate represents a challenging, yet fundamentally important process for the synthesis of α-chiral carbonyls. We describe herein chiral primary-amine-catalyzed conjugate additions of indoles to both α-substituted acroleins and vinyl ketones. These reactions feature enamine protonation as the stereogenic step. A simple primary-tertiary vicinal diamine 1 with trifluoromethanesulfonic acid (TfOH) was found to enable both of the reactions of acroleins and vinyl ketones with good activity and high enantioselectivity. Detailed mechanistic studies reveal that these reactions are rate-limiting in iminium formation and they all involve a uniform H2 O/acid-bridged proton transfer in the stereogenic steps but divergent stereocontrol modes for the protonation stereoselectivity. For the reactions of α-branched acroleins, facial selections on H2 O-bridged protonation determine the enantioselectivity, which is enhanced by an OH⋅⋅⋅π interaction with indole as uncovered by DFT calculations. On the other hand, the stereoselectivity of the reactions with vinyl ketones is controlled according to the Curtin-Hammett principle in the CC bond-formation step, which precedes a highly stereospecific enamine protonation.

Readily accessible chiral at nitrogen cage structures

The reaction of glycine-N-methyl amide with paraformaldehyde in the presence of ytterbium triflate (1 mol%) leads to a novel cage structure 6 which is chiral at nitrogen. Single crystal X-ray analysis and DFT calculations suggest this cage structure is rigid and adopts a single low energy conformation. Use of single enantiomer α-amino amides results in two diastereomeric tertiary amines that differ in their absolute configuration at nitrogen. These diastereoisomers interconvert under acidic conditions but are configurationally stable under basic conditions and can be readily separated by either crystallisation or column chromatography. By reacting racemic chiral α-amino amides a third diastereomeric cage can also be isolated through this reaction protocol. Preparation of mixed cages by reacting two different α-amino amides is also possible allowing for greater structural diversity in the products to be attained. Preliminary mechanistic studies show that all three methylene units in the cage structure are labile and can be replaced under acidic reaction conditions.

Camphor-derived C1-symmetric chiral diamine organocatalysts for asymmetric Michael addition of nitroalkanes to enones

A series of stable C(1)-symmetric chiral diamines (2a–2l) were conveniently synthesized by condensing exo-(-)-bornylamine or (+)-(1S,2S,5R)-menthylamine with various commercially available Cbz-protected amino acids. Among them, 2a can efficiently promote the Michael addition of nitroalkanes to a broad scope of enones with high yields (up to 96%) and excellent enantioselectivities (up to 98%) under mild conditions.

A Molecular Dynamics (MD) and Quantum Mechanics/Molecular Mechanics (QM/MM) study on Ornithine Cyclodeaminase (OCD): a tale of two iminiums

Ornithine cyclodeaminase (OCD) is an NAD+-dependent deaminase that is found in bacterial species such as Pseudomonas putida. Importantly, it catalyzes the direct conversion of the amino acid L-ornithine to L-proline. Using molecular dynamics (MD) and a hybrid quantum mechanics/molecular mechanics (QM/MM) method in the ONIOM formalism, the catalytic mechanism of OCD has been examined. The rate limiting step is calculated to be the initial step in the overall mechanism: hydride transfer from the L-ornithine's C(α)-H group to the NAD+ cofactor with concomitant formation of a C(α)=NH(2)+ Schiff base with a barrier of 90.6 kJ mol-1. Importantly, no water is observed within the active site during the MD simulations suitably positioned to hydrolyze the C(α)=NH(2)+ intermediate to form the corresponding carbonyl. Instead, the reaction proceeds via a non-hydrolytic mechanism involving direct nucleophilic attack of the δ-amine at the C(α)-position. This is then followed by cleavage and loss of the α-NH(2) group to give the Δ1-pyrroline-2-carboxylate that is subsequently reduced to L-proline.

Transfer hydrogenation with Hantzsch esters and related organic hydride donors

In recent years, Hantzsch esters and their related organic hydride donors have been widely utilized in biomimetic approaches of asymmetric transfer hydrogenation (ATH) reactions. Various compounds containing C=C, C=N and C=O unsaturated functionalities could be reduced in the presence of organocatalysts or transition metal complexes, affording versatile chiral building blocks in high yields and excellent enantioselectivities under mild conditions. In this critical review, recent advances in this area are summarized and classified according to unsaturated functional groups being reduced and catalytic systems employed (91 references).

Organocatalyzed enantioselective aldol reaction of 1H-pyrrole-2,3-diones

The enantioselective cross aldol reaction of 1-benzyl 4,5-dioxo-2-aryl-4,5-dihydro-1H-pyrrole-3-carboxylates and ketones was studied with proline derivatives or cinchona alkaloid-derived primary amines as the catalysts for the first time. trans-4-Benzoyloxy-L-proline (15) was found to be the best catalyst for acyclic ketones. For cyclohexanone, the best results were achieved with 9-deoxy-9-epi-aminoquinine (18) as the catalyst in the presence of racemic 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (19) as the cocatalyst. Using these protocols, 3-alkyl-3-hydroxy-1H-pyrrol-2(3H)-one derivatives were obtained in excellent yields and good to high ee values (up to 94% ee).