Organocatalytic highly enantioselective α-selenenylation of aldehydes

Chiral Lewis Base Catalyzed Enantioselective Selenocyclization of 1,1-Disubstituted Alkenes: Asymmetric Synthesis of Selenium-Containing 4H-3,1-Benzoxazines

An enantioselective selenocyclization of 1,1-disubstituted alkenes was achieved for the first time, which is enabled by a novel combination of a chiral BINAM-derived sulfide and an achiral Lewis acid. Various selenium-containing 4H-3,1-benzoxazines, which are widely present in a range of medicinally relevant molecules, were readily obtained in moderate to good yields and good to excellent enantioselectivities. A series of tetrasubstituted carbon stereocenters were facilely constructed.

Enantioselective organocatalytic syntheses of α-selenated α- and β-amino acid derivatives

Selenium-containing amino acids are valuable targets but methods for the stereoselective α-selenation of simple amino acid precursors are rare. We herein report the enantioselective electrophilic α-selenation of azlactones (masked α-amino acid derivatives) and isoxazolidin-5-ones (masked β-amino acids) using Cinchona alkaloids as easily accessible organocatalysts. A variety of differently substituted derivatives was accessed with reasonable levels of enantioselectivities and further studies concerning the stability and suitability of these compounds for further manipulations have been carried out as well.

Use of Standard Addition to Quantify In Situ FTIR Reaction Data

FTIR spectroscopy is a common in situ reaction monitoring technique used in modern academic and industrial environments. The FTIR signals collected during the course of a reaction are proportional to the concentration of the reaction components but not intrinsically quantitative. To make FTIR data quantitative, precalibration or offline analyses of reaction samples are required, which diminishes the unique benefits of in situ reaction monitoring techniques. Herein, we report the use of standard addition as a convenient method to obtain quantitative FTIR data.

Synthesis of enantioenriched α-heteroatom functionalised aldehydes by chiral organocatalysis and their synthetic applications

Asymmetric organocatalysis is a versatile method for the enantioselective α-functionalisation of aldehydes. The synthetic scope for chiral α-heteroatom substituted aldehydes is examined including their applications in synthesis.

Recent Advances on C-Se Bond-forming Reactions at Low and Room Temperature

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Over the last decades, many methods have been reported for the synthesis of selenium- heterocyclic scaffolds because of their interesting reactivities and applications in the medicinal as well as in the material chemistry. This review describes the recent numerous useful methodologies on C-Se bond formation reactions which were basically carried out at low and room temperature.

Enantioselective addition of selenosulfonates to α,β-unsaturated ketones

An organo-catalyzed enantioselective addition of selenosulfonates to α,β-unsaturated ketones was developed for the first time. With a chiral squaramide as an efficient catalyst, the desired α-selenylated ketones were obtained in a good yields with high enantioselectivity up to 89% ee, and good results could be obtained on a gram scale. The products could also be efficiently transformed into useful building blocks with a propenylic stereocenter; the strategy presented in this study may find further applications in organic synthesis.

Direct Construction of 2,3-Dihydroxy-2,3-diaryltetrahydrofurans via N-Heterocyclic Carbene/Base-Mediated Domino Reactions of Aromatic Aldehydes and Vinyl Selenone

A one-pot, stereoselective construction of 2,3-dihydroxy-2,3-diaryltetrahydrofurans has been achieved via N-heterocyclic carbene (NHC)/base-mediated domino reactions of aldehydes and vinyl selenone. The products containing two contiguous quaternary hydroxyl functionalities among the three stereocenters are obtained advantageously as either acetals or ketals through the formation of five new chemical bonds in a single operation. This report constitutes an altogether different reactivity of vinyl selenone in comparison with the corresponding sulfones and phosphonates under NHC/base-mediated reactions.

Silver-catalyzed carbon–selenium cross-coupling using N-(phenylseleno)phthalimide: an alternate approach to the synthesis of organoselenides

Silver(I) catalyzed phenylselenylation of terminal alkynes and organoboronic acids has been demonstrated using N-(phenylseleno)phthalimide as an electrophilic SePh donor. A wide variety of terminal alkynes and organoboronic acids are selenylated efficiently to produce the corresponding alkynyl and diaryl selenides, respectively, in good yields. Silver(I) acts as a Lewis acid in this process.

Solvent-Dependent Facile Synthesis of Diaryl Selenides and Biphenols Employing Selenium Dioxide

Biphenols are important structure motifs for ligand systems in organic catalysis and are therefore included in the category of so‐called “privileged ligands”. We have developed a new synthetic pathway to construct these structures by the use of selenium dioxide, a stable, powerful, and commercially available oxidizer. Our new, and easy to perform protocol gives rise to biphenols and diaryl selenides depending on the solvent employed. Oxidative treatment of phenols in acetic acid yields the corresponding biphenols, whereas conversion in pyridine results in the preferred formation of diaryl selenides. As a consequence, we were able to isolate a broad scope of novel diaryl selenides, which could act as pincer‐like ligands with further applications in organic synthesis or as ligands in transition metal catalysis.

Explaining Anomalies in Enamine Catalysis: "Downstream Species" as a New Paradigm for Stereocontrol

Enantioselective organocatalysis by diarylprolinol ethers is remarkably selective and efficient for a wide range of transformations involving a number of different activation modes, including HOMO activation via enamines. A simple steric model based on facial discrimination in the attack of an enamine on an electrophile has been invoked to rationalize high enantioselectivity. In a number of reactions, however, experimental observations have persistently left us with mechanistic puzzles that fail to fit neatly into this simple picture. Further studies involving both kinetic profiling of reaction networks and NMR spectroscopic characterization of the structures of intermediate species helped us to address these puzzles. This work led to the proposal of a new paradigm for stereocontrol in asymmetric aminocatalysis, demonstrating that the ultimate stereochemical outcome may not, in fact, be determined solely in the stereogenic bond-forming step between enamine and electrophile. The identification of stable species occurring downstream from the addition of an electrophile to an enamine, and the discovery of kinetic features that are diagnostic of the presence of such species, allows development of a new mechanistic framework that reveals a hierarchical selection. Both kinetic and thermodynamic processes associated with downstream intermediates can exert an influence on the ultimate enantioselectivity. Interestingly, the role of these species may be either to enhance or to erode selectivity established at the enamine-electrophile step. The reversibility of steps preceding and subsequent to the stereogenic bond-forming step is an important factor, as are reaction parameters that may stabilize or destabilize intermediates, including the nature of the electrophile counterion and the solvent. These concepts hold implications for the future design and optimization of asymmetric catalytic processes, because such design does not necessarily feature the same parameters at the second hierarchical level as it does at the first. Examples presented to highlight these issues include the conjugate addition of aldehydes to nitroalkenes and the α-chlorination and α-selenylation of aldehdyes using diarylprolinol ether catalysts commonly assumed to follow a steric model for enantioselectivity. While the new paradigm for stereocontrol involving downstream intermediates is developed here for enamine catalysis, the same concepts may hold for other organocatalytic modes of activation.

Solvent effects in catalysis: implementation for modelling of kinetics

A mathematical framework is developed for analysis of solvent dependent reaction rates and selectivity in the case of complex catalytic reactions by incorporating solvent permittivity into the rate expressions.

[2,3]-Sigmatropic rearrangement of allylic selenimides: strategy for the synthesis of peptides, peptidomimetics, and N-aryl vinyl glycines

The scope of the NCS-mediated amination/[2,3]-sigmatropic rearrangement of enantioenriched allylic selenides has been expanded to provide access to three new product classes. The use of N-protected amino acid amides provides a novel strategy for accessing peptide chains containing unnatural vinyl glycine amino acid residues. Also reported is the use of amino acid esters, allowing the diastereoselective synthesis of N,N-dicarboxymethylamines, a motif found in a number of pharmaceuticals. Furthermore, use of a range of N-aromatic and N-heteroaromatic amines allows the formation of enantioenriched N-arylamino acids, a motif found in a number of synthetically and biologically interesting compounds.

Asymmetric, organocatalytic, three-step synthesis of alpha-hydroxy-(E)-beta,gamma-unsaturated esters

An efficient and enantiocontrolled three-step synthesis of alpha-hydroxy-(E)-beta,gamma-unsaturated esters is reported. Enantioenriched alpha-selenyl aldehydes, prepared in one step by asymmetric, organocatalytic alpha-selenylation of aldehydes, were directly subjected to a Wittig reaction followed by allylic selenide to selenoxide oxidation and final spontaneous [2,3]-sigmatropic rearrangement to yield the target compounds in 43-65% overall yield and in 94-97% ee.

Asymmetric, organocatalytic, three-step synthesis of gamma-hydroxy-(E)-alpha,beta-unsaturated sulfones and esters

Efficient and enantiocontrolled synthesis of gamma-hydroxy-alpha,beta-unsaturated sulfones and esters are reported through the reaction of enantioenriched alpha-selenyl aldehydes with EWG-stabilized carbanions and then a one-pot selenide oxidation, in situ epoxide formation, and final in situ epoxide opening.

Alpha,alpha-diarylprolinol ethers: new tools for functionalization of carbonyl compounds

Alpha,alpha-diaryl(dialkyl)prolinol ethers constitute a potent organocatalyst family which has been shown to be very general for a broad range of reactions involving enamine and iminium ion activation or a combination of both. The reactions are characterized by an efficient steric control approach and can lead to a variety of alpha-, beta-, gamma-, and alpha,beta-functionalized carbonyl compounds with excellent stereocontrol. As a full expression of their catalytic activity, these compounds are also excellent promoters of elegant cascade processes and valuable catalysts in water-compatible systems.

Asymmetric aminocatalysis--gold rush in organic chemistry

Catalysis with chiral secondary amines (asymmetric aminocatalysis) has become a well-established and powerful synthetic tool for the chemo- and enantioselective functionalization of carbonyl compounds. In the last eight years alone, this field has grown at such an extraordinary pace that it is now recognized as an independent area of synthetic chemistry, where the goal is the preparation of any chiral molecule in an efficient, rapid, and stereoselective manner. This has been made possible by the impressive level of scientific competition and high quality research generated in this area. This Review describes this "Asymmetric Aminocatalysis Gold Rush" and charts the milestones in its development. As in all areas of science, progress depends on human effort.