Organocatalytic asymmetric remote aziridination of 2,4-dienals

A Synthesis of α-Alkyl Cycloenones by Pd-Catalyzed Suzuki-Miyaura Coupling with Cyclic Morita-Baylis-Hillman Adducts

We herein disclose a Pd-catalyzed Suzuki-Miyaura coupling of cyclic Morita-Baylis-Hillman adducts with organoboronic acids under mild conditions, which allows for a rapid access to diverse α-alkyl substituted cycloenones. The advantage of this method resides in the employment of functionalized allyl alcohols as the unprecedented electrophilic partners in the absence of external activators.

Rhodium(II)-Catalyzed Enantioselective Intermolecular Aziridination of Alkenes

C₄-Symmetrical dirhodium(II) tetracarboxylates are highly efficient catalysts for the asymmetric intermolecular aziridination of substituted alkenes with sulfamates. The reaction proceeds with high levels of efficiency and chemoselectivity to afford aziridines with excellent yields of up to 95% and enantiomeric excesses of up to 99%. The scope of the alkene aziridination includes mono-, di-, and trisubstituted olefins as well as the late-stage functionalization of complex substrates. The reaction can be performed on a gram-scale with a catalyst loading of 0.1 mol %. Our DFT study led us to propose a two-spin-state mechanism, involving a triplet Rh-nitrene species as key intermediate to drive the stereocontrolled approach and activation of the substrate.

Monitoring Reaction Intermediates to Predict Enantioselectivity Using Mass Spectrometry**

Enantioselective reactions are at the core of chemical synthesis. Their development mostly relies on prior knowledge, laborious product analysis and post‐rationalization by theoretical methods. Here, we introduce a simple and fast method to determine enantioselectivities based on mass spectrometry. The method is based on ion mobility separation of diastereomeric intermediates, formed from a chiral catalyst and prochiral reactants, and delayed reactant labeling experiments to link the mass spectra with the reaction kinetics in solution. The data provide rate constants along the reaction paths for the individual diastereomeric intermediates, revealing the origins of enantioselectivity. Using the derived kinetics, the enantioselectivity of the overall reaction can be predicted. Hence, this method can offer a rapid discovery and optimization of enantioselective reactions in the future. We illustrate the method for the addition of cyclopentadiene (CP) to an α,β‐unsaturated aldehyde catalyzed by a diarylprolinol silyl ether.

Cascade synthetic strategies opening access to medicinal-relevant aliphatic 3- and 4-membered N-heterocyclic scaffolds

Cascade reactions are often 'employed' by nature to construct structurally diverse nitrogen-containing heterocycles in a highly stereoselective fashion, i.e., secondary metabolites important for pharmacy. Nitrogen-containing heterocycles of three- and four-membered rings, as standalone and bicyclic compounds, inhibit different enzymes and are pharmacophores of approved drugs or drug candidates considered in many therapies, e.g. anticancer, antibacterial or antiviral. Domino transformations are in most cases in line with modern green chemistry concepts due to atom economy, one-pot procedures often without use the protective groups, time-saving and at markedly lower costs than multistep transformations. The tandem approaches can help to obtain novel N-heterocyclic scaffolds, functionalized according to structural requirements of the target in cells, taking into account the nature of functional group and stereochemistry. On the other hand cascade strategies allow to modify small N-heterocyclic rings in a systematic way, which is beneficial for structure-activity relationship (SAR) analyses. This review is focused on the biological relevance of the N-heterocyclic scaffolds with smaller 3- and 4-membered rings among approved drugs and leading structures of drug candidates. The cascade synthetic strategies offering N-heterocyclic scaffolds, at relatively good yields and high stereoselectivity, are discussed here. The review covers mainly years from 2015 to 2021.

Recent development in asymmetric organocatalytic domino reactions involving 1,6-addition as a key step

This article highlights the significant development in stereoselective domino reactions involving 1,6-addition as a crucial step.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Asymmetric organocatalytic double 1,6-addition: rapid access to chiral chromans with molecular complexity

We have developed a highly δ-selective organocatalytic asymmetric double 1,6-addition protocol by tethering ortho-hydroxyphenyl substituted p-QMs onto the linear unbiased 2,4-dienals towards the formation of synthetically enriched chiral chromans.

Organocatalytic Olefin Aziridination via Iminium-Catalyzed Nitrene Transfer: Scope, Limitations, and Mechanistic Insight

Olefin aziridination via organocatalytic nitrene transfer offers potential complementarity to metal-catalyzed methods; however there is a lack of reports of such reactions in the literature. Herein is reported a method that employs an iminium salt to catalyze the aziridination of styrenes by [ N-( p-toluenesulfonyl)imino]phenyliodinane (PhINTs). These reactions are hypothesized to proceed via a diaziridinium salt as the active oxidant. In addition to outlining the scope and limitations of the method, evidence for a polar, stepwise mechanism is presented, which provides new insight into the nature of iminium catalysis of nitrene transfer.

Synthetic approach to skeletally diverse nitrogen heterocycles from dicyano-2-methylenebut-3-enoates

Divergent synthesis of three types of azaheterocycles, including vinylaziridines, pyrroline and 2-aminopyridines, has been achieved through [2 + 1], formal-[4 + 1] and [5 + 1] annulations.

Efficient Access to Chiral Trisubstituted Aziridines via Catalytic Enantioselective Aza-Darzens Reactions

Herein, we report a Zn-ProPhenol catalyzed aza-Darzens reaction using chlorinated aromatic ketones as nucleophilic partners for the efficient and enantioselective construction of complex trisubstituted aziridines. The α-chloro-β-aminoketone intermediates featuring a chlorinated tetrasubstituted stereocenter can be isolated in high yields and selectivities for further derivatization. Alternatively, they can be directly transformed to the corresponding aziridines in a one-pot fashion. Of note, the reaction can be run on gram-scale with low catalyst loading without impacting its efficiency. Moreover, this methodology was extended to α-bromoketones which are scarcely used in enantioselective catalysis because of their sensitivity and lack of accessibility.

Old tricks, new dogs: organocatalytic dienamine activation of α,β-unsaturated aldehydes

Chiral secondary amines are some of the most commonly used kinds of catalysts. They have become a reliable tool for the α- and β-activation of carbonyl compounds, via HOMO, SOMO or LUMO activation pathways. Recently, chemists have turned their attention to the development of novel organocatalytic strategies for remote functionalisation, targeting stereocentres even more distant from the catalyst-activation site, through dienamine, trienamine, and vinylogous iminium ion pathways (γ-, ε- and δ-positions, respectively). Here we outline and discuss the state-of-the-art in dienamine activation, classifying examples according to the different reactive activation pathways followed by the formed dienamine intermediate (1,3-, 1,5-, 2,5- and 4,5-functionalisation) and the reaction type developed, as determined by the structure and the nature of electrophiles and nucleophiles.

Directed electrostatic activation in enantioselective organocatalytic cyclopropanation reactions: a computational study

Cyclopropane rings are versatile building blocks in organic chemistry. Their synthesis, by the reaction of sulfur ylides with α,β-unsaturated carbonyl compounds, has recently aroused renewed interest after the discovery of efficient catalysis by using (S)-indoline-2-carboxylic acid. In order to rationalize the behavior of this catalyst, MacMillan proposed a directed electrostatic activation (DEA) mechanism, in which the negative carboxylate group interacts with the positive thionium moiety, thus reducing the activation energy and increasing the reaction rate. More recently, Mayr refuted some of MacMillan conclusions, but accepted the DEA mechanism as a justification for the experimental high reaction rates. In contrast, our results indicate that the selectivity obtained in the process seems to result from several strong hydrogen bond interactions between the two reacting species, while no strong evidence for a DEA mechanism was found. We also concluded that the hydrogen bonds don't improve the reaction rate by lowering the activation energy of the rate-determining step, but can do it by promoting efficient reaction trajectories due to long-range complexation of the reagents. Finally, our results confirm that the cyclopropanation reaction occurs by a two-step mechanism, and that the overall enantioselectivity depends on the relative energies of the two steps, averaged by the relative populations of the iminium intermediates that are initially formed in the reaction.

Enantioselective Vinylogous Organocascade Reactions

Cascade reactions are powerful tools for rapidly assembling complex molecular architectures from readily available starting materials in a single synthetic operation. Their marriage with asymmetric organocatalysis has led to the development of novel techniques, which are now recognized as reliable strategies for the one-pot enantioselective synthesis of stereochemically dense molecules. In recent years, even more complex synthetic challenges have been addressed by applying the principle of vinylogy to the realm of organocascade catalysis. The key to the success of vinylogous organocascade reactions is the unique ability of the chiral organocatalyst to transfer reactivity to a distal position without losing control on the stereo-determining events. This approach has greatly expanded the synthetic horizons of the field by providing the possibility of forging multiple stereocenters in remote positions from the catalyst's point of action with high selectivity, while simultaneously constructing multiple new bonds. This article critically describes the developments achieved in the field of enantioselective vinylogous organocascade reactions, charting the ideas, the conceptual advances, and the milestone reactions that have been essential for reaching highly practical levels of synthetic efficiency.

Base-catalyzed controllable reaction of 3-ylideneoxindoles with O-Boc hydroxycarbamates for the synthesis of amidoacrylates and spiroaziridine oxindoles

A base-catalyzed divergent reaction of 3-ylideneoxindoles and O-Boc hydroxycarbamates has been developed to provide various amidoacrylates and spiroaziridine oxindoles with high yields.

Direct Catalytic Asymmetric Doubly Vinylogous Michael Addition of α,β-Unsaturated γ-Butyrolactams to Dienones

An asymmetric doubly vinylogous Michael addition (DVMA) of α,β-unsaturated γ-butyrolactams to sterically congested β-substituted cyclic dienones with high site-, diastereo-, and enantioselectivity has been achieved. An unprecedented DVMA/vinylogous Michael addition/isomerization cascade reaction affords chiral fused tricyclic γ-lactams with four newly formed stereocenters.

An unprecedented base-promoted domino reaction of methyleneindolinones and N-tosyloxycarbamates for the construction of bispirooxindoles and spiroaziridine oxindoles

An efficient and unprecedented domino reaction of methyleneindolinones andN-tosyloxycarbamates was realized to construct bispirooxindoles and spirooxindole aziridines.

Site and stereoselectivity in sulfa-Michael addition to equivocally activated conjugated dienes

Two terminally-linked Michael acceptors can react in either a 1,4- or 1,6-fashion with thiols depending on the choice of activation method.