Chiral 1,1'-binaphthyl-2,2'-diammonium salt catalysts for the enantioselective Diels-Alder reaction with alpha-acyloxyacroleins

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

Novel asymmetric aminocatalytic cycloadditions are described between formyl cycloheptatrienes and 6,6-dimethylfulvene that lead to [4 + 2], [6 + 2], and [4 + 6] cycloadducts. The unprecedented reaction course is dependent on the position of the formyl functionality in the cycloheptatriene core, and each formyl cycloheptatriene isomer displays a distinct reactivity pattern. The formyl cycloheptatriene isomers are activated by a chiral primary diamine catalyst, and the activation mode is dependent on the position of the formyl functionality relative to the cycloheptatriene core. The [4 + 2] and [6 + 2] cycloadducts are formed via rare iminocatalytic inverse electron-demand cycloadditions, while the [4 + 6] cycloadduct is formed by a normal electron-demand cycloaddition. The reactivity displayed by the different formyl cycloheptatrienes was investigated by DFT calculations. These computational studies account for the different reaction paths for the three isomeric formyl cycloheptatrienes. The aminocatalytic [4 + 2], [6 + 2], and [4 + 6] cycloadditions proceed by stepwise processes, and the interplay between conjugation, substrate distortion, and dispersive interactions between the fulvene and aminocatalyst mainly defines the outcome of each cycloaddition.

Stereoselective Construction of Fluorinated Quaternary Stereogenic Centers via an Organocatalytic Asymmetric exo-Selective Diels-Alder Reaction in the Presence of Water

A catalytic, asymmetric Diels-Alder reaction of α-fluoro α,β-unsaturated aldehydes and cyclopentadiene was developed using diarylprolinol silyl ether as an organocatalyst. The reaction proceeds in toluene with trifluoroacetic acid as an additive (condition A). Perchloric acid salt of diarylprolinol silyl ether also promotes the reaction using water as a reaction medium (condition B). In both cases, excellent exo-selectivity and enantioselectivity were obtained with generation of a fluorinated quaternary chiral center.

A miniaturized analytical method based on molecularly imprinted absorbents for selective extraction of (S)-1,1'-binaphthyl-2,2'-diamine and combinatorial screening of polymer precursors by computational simulation

Chiral resolution of binaphthylamine is often a toilful conundrum in the field of analytical chemistry and biomedicine. The work puts forward a selective, sensitive, and miniaturized analytical method based on molecularly imprinted polymers (MIPs) as adsorbent for miniaturized tip solid-phase extraction (MTSPE) in the separation of binaphthylamine enantiomer. This method combines the advantages of MIPs (high selectivity), MTSPE (low consumption), and high-performance liquid chromatography (HPLC, high sensitivity). A simple synthesis methodology of MIP (P2) was conducted through bulk polymerization with (S)-(-)-1,1'-binaphthyl-2,2'-diamine (S-DABN) as template together with methacrylic acid monomer, and ethylene glycol dimethacrylate as cross-linker in proper porogen, realizing a selective recognition and efficient enrichment for S-DABN. The method exhibited appreciable linearity (0.06-1.00 mg ml^-1 ), low quantification limit (0.056 mg ml^-1 ), good absolute recoveries (45.70%-69.29%), and high precision (relative standard deviations ≤ 3.54%), along with low consumption (0.50 ml sample solution and 25.0 mg adsorbent). Based on the density functional theory, computational simulation was used to make a preliminary prediction for rational design of MIPs and gave a reasonable elaboration involving the potential mechanism of templates interacting with functional monomers. The adsorption kinetics and thermodynamics were investigated to evaluate the recombination process of substrates. In addition, the selectivity of MIPs for S-DABN was obtained by MIP-MTSPE coupled with HPLC, which supports the feasibility of this convenient design process. The proposed method was employed for selective extraction of S-DABN and exhibited promising potential in the application of chiral analysis.

α-Functionally Substituted α,β-Unsaturated Aldehydes as Fine Chemicals Reagents: Synthesis and Application

α-Functionalized α,β-unsaturated aldehydes is an important class of compounds, which are widely used in fine organic synthesis, biology, medicine and pharmacology, chemical industry, and agriculture. Some of the 2-substituted 2-alkenals are found to be the key metabolites in plant and animal cells. Therefore, the development of efficient methods for their synthesis attracts the attention of organic chemists. This review focusses on the recent advances in the synthesis of 2-functionally substituted 2-alkenals. The approaches to the preparation of α-alkyl α,β-unsaturated aldehydes are not included in this review.

A multi-substrate screening approach for the identification of a broadly applicable Diels-Alder catalyst

When developing a synthetic methodology, chemists generally optimize a single substrate and then explore the substrate scope of their method. This approach has led to innumerable and widely-used chemical reactions. However, it frequently provides methods that only work on model substrate-like compounds. Perhaps worse, reaction conditions that would enable the conversion of other substrates may be missed. We now show that a different approach, originally proposed by Kagan, in which a collection of structurally distinct substrates are evaluated in a single reaction vessel, can not only provide information on the substrate scope at a much earlier stage in methodology development, but even lead to a broadly applicable synthetic methodology. Using this multi-substrate screening approach, we have identified an efficient and stereoselective imidodiphosphorimidate organocatalyst for scalable Diels-Alder reactions of cyclopentadiene with different classes of α,β-unsaturated aldehydes.

First aromatic amine organocatalysed activation of α,β-unsaturated ketones

The first example of a chiral aromatic amine used to activate α,β-unsaturated ketones in aminocatalysis.

Aromatic Aminocatalysis

Aromatic aminocatalysis refers to transformations that employ aromatic amines, such as anilines or aminopyridines, as catalysts. Owing to the conjugation of the amine moiety with the aromatic ring, aromatic amines demonstrate distinctive features in aminocatalysis compared with their aliphatic counterparts. For example, aromatic aminocatalysis typically proceeds with slower turnover, but is more active and conformationally rigid as a result of the stabilized aromatic imine or iminium species. In fact, the advent of aromatic aminocatalysis can be traced back to before the renaissance of organocatalysis in the early 2000s. So far, aromatic aminocatalysis has been widely applied in bioconjugation reactions through transamination; in asymmetric organocatalysis through imine/enamine tautomerization; and in cooperative catalysis with transition metals through C-H/C-C activation and functionalization. This Focus Review summarizes the advent of and major advances in the use of aromatic aminocatalysis in bioconjugation reactions and organic synthesis.

Asymmetric [4+2] annulations to construct norcamphor scaffolds with 2-cyclopentenone via double amine–thiol catalysis

A double catalytic system combining chiral amine and 2-mercaptobenzoic acid is applied for α′,β-regioselective [4+2] annulations of 2-cyclopentenone with diverse activated alkenes.

A Simple Tetraminocalix[4]arene as a Highly Efficient Catalyst under "On-Water" Conditions through Hydrophobic Amplification of Weak Hydrogen Bonds

The simple tetraminocalix[4]arene 1, which contains weak H-bond-donor NH₂ groups, is reported to be a highly efficient organocatalyst for the Vinylogous Mukaiyama Aldol Reaction (VMAR) of 2-(trimethylsilyloxy)furan 5 with α-ketoesters 6 a-l under "on-water" conditions owing to the hydrophobic amplification of weak H-bond interactions. The catalytic efficiency of calixarene catalyst 1 was shown to be closely related to its recognition abilities towards the reactants 5 and 6 through a multipoint recognition model. The proposed model provided good explanations for the differences on the reaction rate acceleration and on the stereoselectivity observed with different substrates.

Asymmetric Diels-Alder Reaction of α-Substituted and β,β-Disubstituted α,β-Enals via Diarylprolinol Silyl Ether for the Construction of All-Carbon Quaternary Stereocenters

The asymmetric Diels-Alder reaction of α-substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl-substituted diarylprolinol silyl ether to afford the exo-isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels-Alder reaction of a β,β-disubstituted α,β-unsaturated aldehyde, good exo-selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all-carbon quaternary stereocenters are constructed enantioselectively.

Catalytic discrimination between formyl groups in regio- and stereoselective intramolecular cross-aldol reactions

Catalytic discrimination between inequivalent formyl groups was achieved using an aniline-type acid-base catalyst for the regio-, diastereo-, and enantioselective intramolecular cross-aldol reactions of enolizable dials. Although l-proline gave a mixture of the regio- and stereoisomeric products in the presence of an N-containing 1,6-dial, the aniline-type catalyst afforded anti-3,4-disubstituted pyrrolidine in high regio-, and stereoselectivity beyond the background reaction, which led to the regioisomeric 2,3-disubstituted products. The mild reactivity of the aniline-type amine facilitated catalytic discrimination between the inequivalent formyl groups. Kinetic isotope effect studies and reductive amination experiments suggested that the regioselectivity was controlled under the enamine-forming steps.

An enantioselective Diels-Alder reaction of 1,2-dihydropyridines with α-acyloxyacroleins catalyzed by a chiral primary ammonium salt

The first enantioselective Diels-Alder reaction of 1,2-dihydropyridines with α-acyloxyacroleins catalyzed by a chiral primary ammonium salt has been developed and it offers more efficient routes to key synthetic intermediates of alkaloids, for which the direct preparations were unavailable before. The asymmetric induction can be understood through the optimized geometry of an iminium salt aqua complex derived from the catalyst and the dienophile.

New insights into the asymmetric Diels–Alder reaction: the endo- and S-selective retro-Diels–Alder reaction

The endo- and S-selective retro-Diels–Alder reactions have been proposed and verified in an imidazolethione-catalyzed asymmetric Diels–Alder reaction.

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.

Molecular assembly of an achiral phosphine and a chiral primary amine: a highly efficient supramolecular catalyst for the enantioselective Michael reaction of aldehydes with maleimides

A combined catalyst system of a cinchonidine-derived primary amine and triphenylphosphine (CD-NH2 /PPh3 ) exhibited high catalytic performance in the Michael reaction of aldehydes with maleimides, thereby affording the corresponding functionalized aldehydes in excellent yields (up to 99 %) and enantioselectivities (>99 % ee). More interestingly, the significance of the phosphine in enhancing the enantioselectivities in the chiral-primary-amine-catalyzed Michael reaction was revealed. Furthermore, we explored the origin of the reaction mechanism in the Michael addition promoted by the dual organocatalytic system. On the basis of experimental results and spectroscopic analysis, such as UV/Vis, fluorescence emission (FL), NMR, and circular dichroism (CD) spectroscopy, as well as ESI-MS, we found that the molecular assembly of phosphine and primary amine played a crucial role in this enantioselective reaction, in which a possible supramolecular complex was formed as an effective chiral catalyst through noncovalent molecular interactions of a cinchona alkaloid-derived primary amine with triphenylphosphine.

Enantioselective Diels-Alder reaction of α-(acylthio)acroleins: a new entry to sulfur-containing chiral quaternary carbons

A catalytic and enantioselective Diels-Alder reaction of α-(carbamoylthio)acroleins induced by an organoammonium salt of chiral triamine is described. α-(Carbamoylthio)acroleins are designed and synthesized as new sulfur-containing dienophiles for the first time. The Diels-Alder reaction affords chiral tertiary thiol precursors with up to 91% ee.

Synthesis of axially chiral amino acid and amino alcohols via additive-ligand-free Pd-catalyzed domino coupling reaction and subsequent transformations of the product amidoaza[5]helicene

Novel optically active axially chiral amino acid and amino alcohols have been synthesized efficiently via lactam ring-opening, with the aid of an optically active alcohol, amidoaza[5]helicene 5, which has been readily prepared by an additive-ligand-free Pd catalyzed domino coupling reaction in a single step. The stereostructures of these chiral molecules have also been clarified.

Two Remarkable Epimerizations Imperative for the Success of an Asymmetric Total Synthesis of (+)-Aigialospirol

Two remarkable epimerization processes were uncovered during our pursuit of an enantioselective synthesis of (+)-aigialospirol featuring a cyclic acetal tethered ring-closing metathesis. Through modeling, we were able to turn these two unexpected epimerizations to our advantage via modeling to ensure a successful and concise total synthesis, thereby firmly establishing cyclic acetal tethered RCM as a powerful strategy in natural product synthesis. Most importantly, calculations allowed us to fully understand the nature and the mechanistic course of these two epimerizations that were imperative to the total synthesis efforts.