Multicatalytic Beckmann rearrangement of 2-hydroxylarylketone oxime: Switchable synthesis of benzo[d]oxazoles and N-(2-hydroxylaryl)amides

A switchable synthesis route is developed for benzo[d]oxazole derivatives and (2-hydroxylaryl)benzamide from 2-hydroxylbenzeneketoxime using organomolecules (BOP-Cl, and CNC) and Lewis acid cocatalyzed Beckmann rearrangement (BR) reaction. Further, this reaction is switched using different organocatalysts.     

Synthesis of polyhydroquinoline derivatives through unsymmetric Hantzsch reaction using organocatalysts

A novel and green approach for efficient and rapid synthesis of polyhydroquinoline derivatives via unsymmetric Hantzsch reaction using organocatalysts at room temperature was reported. The process is a simple, environmentally friendly, rapid, and high yielding reaction for the synthesis of polyhydroquinoline derivatives. The catalytic efficiency of various small organocatalysts such as l-proline, trans-4-hydroxy-l-proline, l-thiaproline, dl-phenylglycine, and (−)-cinchonidine was studied under aqueous, organic, and solvent free conditions.     

Direct Asymmetric Mannich‐Type Reaction of α‐Isocyanoacetates with Ketimines using Cinchona Alkaloid/Copper(II) Catalysts

The enantioselective direct Mannich‐type reaction of ketimines with α‐isocyanoacetates has been developed. Excellent yields and enantioselectivity were observed for the reaction of various ketimines and α‐isocyanoacetates using cinchona alkaloid/Cu(OTf)₂ and a base. Both enantiomers of the products could be obtained by using pseudoenantiomeric chiral catalysts. This process offers an efficient route for the synthesis of α,β‐diamino acids.     

Practical Synthesis of both Enantiomeric Amino Acid,Mannich, and Aldol Derivatives by Asymmetric Organocatalysis

In this account, we describe our recent developments in organocatalyzed asymmetric methodologies for the practical synthesis of both enantiomers. The synthesis of both enantiomeric products by using both enantiomeric catalysts is the most traditional way. In addition, use of two pseudoenantiomeric catalysts provides a more practical approach for the synthesis of both enantiomeric products. Even more efficient is the use of a single enantiomeric catalyst in the presence or absence of certain additives for obtaining both enantiomeric products. These three different strategies are illustrated by using certain organocatalyzed asymmetric approaches.     

Recent topics in the desymmetrization of meso-diols

The desymmetrization of meso compounds is one of the most effective strategies for asymmetric synthesis. This digest focuses on recent progress in the desymmetrization of meso-diols and their derivatives. The topics discussed here include methods for the enzymatic acylation of meso-diols and the hydrolysis of meso-diesters, acylation, related reactions of meso-diols with organocatalysts and metal catalysts, the oxidation of meso-diols by enzymes, organocatalysts, and metal catalysts, and the desymmetrization of meso-dicarbamates with metal catalysts. The desymmetrization of meso-diols using tandem reactions is also discussed.     

anti-Selective direct asymmetric Mannich reactions catalyzed by chiral pyrrolidine-based amino sulfonamides

The novel pyrrolidine-based amino sulfonamides (R,R)-2, (S)-3, and (S)-4 were designed and synthesized as organocatalysts and successfully applied for the anti-selective direct asymmetric Mannich reaction.     

Recent advances in organocatalytic asymmetric synthesis of polysubstituted pyrrolidines

Chiral substituted pyrrolidines are important N-heterocyclic structural motifs, existing in many natural products, drug candidates, ligands and organocatalysts. We summarise herein the recent (between January 2010 and July 2013) developments on synthesising the chiral polysubstituted pyrrolidines through asymmetric organocatalysis. The organocatalytic strategies for constructing the pyrrolidine scaffolds can be divided into one-step and sequential approaches, respectively. The straightforward one-step approach is mainly the [3+2] cycloaddition based on the iminium activation, chiral Brønsted acid catalysis, bifunctional organocatalysis and SOMO activation. In the sequential approach (multi-step or one-pot reactions), the primary construction of chiral linear precursors is followed by the sequential cyclisation. Other important strategies, such as the organocatalytic bromoaminocyclisation were also described. These organocatalytic strategies have enriched the synthetic chemistry of chiral pyrrolidines, especially towards the target-, diversity- and application-oriented synthesis. New organocatalytic approaches are thus expected for the facile construction of polysubstituted pyrrolidines with well-controlled stereochemistry and for the practical synthesis of pyrrolidine-related natural alkaloids, drug candidates and functional proline derivatives.     

Facile direct synthesis of unsymmetrical ureas from N-Alloc-, N-Cbz-, and N-Boc-protected amines using DABAL-Me3

A practical synthetic method for the direct synthesis of unsymmetrically substituted ureas from N-Alloc-, N-Cbz-, and N-Boc-protected amines is described. In this study, efficient direct conversion of the Alloc-, Cbz-, and Boc-carbamate compounds to ureas was achieved in the presence of DABAL-Me₃, an air stable and easily handled reagent. Using this reaction method, both protected aromatic and aliphatic amines were successfully transformed into various trisubstituted and tetrasubstituted ureas with high yields without side product. Our findings offer promising guidelines for direct preparation of useful ureas from N-Alloc-, N-Cbz-, and N-Boc-carbamates.     

Enantioselective Synthesis of 3-Heterosubstituted-2-amino-1-ols by Sequential Metal-Free Diene Aziridination/Kinetic Resolution

A general protocol for the enantioselective synthesis of 3-heterosubstituted-2-amino-1-ols was developed based on metal- free intramolecular regio- and stereoselective diene aziridination and regioselective opening. Kinetic resolution of the resulting (1′-NR¹R² and 1′-SR)-4-oxazolidinones was performed using ABCs organocatalysts, expanding the application of this methodology.     

Chiral pyridine N-oxides derived from monoterpenes as organocatalysts for stereoselective reactions with allyltrichlorosilane and tetrachlorosilane

The synthesis of enantiomerically pure C₂-symmetric dipyridylmethane ligands and related N,N′-dioxides is reported. A procedure for the synthesis of a few new enantiomerically pure C₂-symmetric pyridine N-oxides and the preparation of four pyridine N-oxides with oxygen and nitrogen atoms as further coordinating elements in the heterocycle framework is described. All compounds were prepared from naturally occurring monoterpenes. These new compounds were assessed as organocatalysts in two different reactions, namely the allylation of aldehydes with allyltrichlorosilane that afforded homoallylic alcohols in good yields and up to 85% ee and the stilbene oxide opening by the addition of tetrachlorosilane that gave chlorohydrin in quantitative yield and up to 70% ee.     

A novel and efficient one-pot synthesis of furo[3′,4′:5,6]pyrido[2,3-c]pyrazole derivatives using organocatalysts

A novel approach to one-pot synthesis of dihydrofuro[3′,4′:5,6]pyrido[2,3-c]pyrazole and indeno[2′,1′:5,6]pyrido[2,3-c]pyrazole derivatives have been investigated using organocatalysts that are recyclable. This new protocol has the advantages of environmental friendliness, higher yield, low cost, as well as convenient operation. The catalytic efficiency of various small organocatalysts such as l-proline, trans-4-hydroxy-l-proline, l-thiaproline, cinchonidine, (+)-cinchonine, and dl-2-phenylglycine has also been studied.     

Phosphinoyl-aziridines as a new class of chiral catalysts for enantioselective Michael addition

A series of new optically pure phosphine oxides containing chiral aziridine subunit were synthesized in good yields and applied as organocatalysts in asymmetric Michael reaction of various aliphatic aldehydes with β-nitrostyrene. The corresponding organocatalysts were synthesized starting from optically pure aziridines in a few simple efficient steps. The appropriate Michael adducts were obtained in most cases in very high chemical yield (up to 97%), excellent enantioselectivity (up to 98% of ee) and diastereoselectivity (up to 95:5 of dr).     

Linking Conformational Flexibility and Kinetics: Catalytic 1,4‐Type Friedel–Crafts Reactions of Phenols Utilizing 1,3‐Diamine‐Tethered Guanidine/Bisthiourea Organocatalysts

Herein, we present details of our conformationally flexible, 1,3‐diamine‐tethered guanidine/bisthiourea organocatalysts for chemo‐, regio‐, and enantioselective 1,4‐type Friedel–Crafts reactions of phenols. These organocatalysts show a unique stereo‐discrimination governed by the differential activation entropy (ΔΔS^≠), rather than by the differential activation enthalpy (ΔΔH^≠). Extensive kinetic analyses using Eyring plots for a series of guanidine/bisthiourea organocatalysts revealed the key structural motif in the catalysts associated with a large magnitude of differential activation entropy (ΔΔS^≠). A plausible guanidine–thiourea cooperative mechanism for the enantioselective Friedel–Crafts reaction is proposed.     

Reactivity of 3H-1,2,4-dithiazole-3-thiones and 3H-1,2-dithiole-3-thiones as sulfurizing agents for oligonucleotide synthesis

The reactivity of 5-amino-3H-1,2,4-dithiazole-3-thiones substituted at their amino group and 5-amino-3H-1,2-dithiole-3-thiones substituted at their amino group and C4 toward compounds containing P(III) atoms has been studied. N,N-Disubstituted-N′-(3-thioxo-3H-1,2,4-dithiazol-5-yl)methanimidamides were selected as novel efficient sulfur transfer reagents suitable for DNA and RNA synthesis.     

Synthesis of chiral isoindolinones via asymmetric propargylation/lactamization cascade

A Zn-mediated propargylation/lactamization cascade reaction with chiral 2-formylbenzoate derived N-tert-butanesulfinyl imines was realized, which provided a practical and efficient method for the synthesis of chiral isoindolinones. High diastereoselectivities (up to 97:3 dr) and good reaction yields were observed for most examined cases.     

Enantioselective organocatalytic synthesis of medicinally privileged 2-amino-4H-chromene-3-carbonitriles via a cascade process

An efficient organocatalytic cascade approach toward medicinally privileged 2-amino-4H-chromene-3-carbonitriles is reported. The enantioselective synthesis of these compounds is achieved in high yields and good to high ee values (up to 96% yield and 89% ee) using bifunctional amine-thiourea organocatalysts developed in our laboratory.     

Direct Enantioselective Vinylogous Mannich Reaction of Ketimines with γ‐Butenolide by Using Cinchona Alkaloid Amide/Zinc(II) Catalysts

A direct enantioselective vinylogous Mannich reaction of ketimines with γ‐butenolide has been developed. Good yields and enantioselectivities were observed for the reaction of various ketimines by using a cinchona alkaloid amide/Zn(OTf)₂ catalyst and Et₃N. Both enantiomers of the products could be obtained by using pseudoenantiomeric chiral catalysts.     

Highly diastereo- and enantioselective direct aldol reactions promoted by water-compatible organocatalysts bearing a pyrrolidinyl-camphor structural scaffold

Efficient synthetic routes have been developed for the synthesis of a series of pyrrolidinyl-camphor containing organocatalysts (1-10). Structural modifications were made by varying the stereo- and electronic properties of the camphor scaffold and the aromatic substituents. These readily tunable and amphiphilic organocatalysts were evaluated for the direct asymmetric aldol reaction of various aromatic aldehydes and cyclohexanone either in organic solvents or in the presence of water. The aldol reaction proceeded smoothly with excellent chemical yields (up to 99%), enantioselectivities (up to 99% ee), and anti-diastereoselectivities (up to 99:1) with a catalytical amount of the bifunctional organocatalysts (20 mol %) under optimal reaction conditions. Mechanistic transition models are proposed and the stereochemical bias of the asymmetric aldol reaction is presented.     

Room temperature synthesis of glycerol carbonate catalyzed by N-heterocyclic carbenes

N-Heterocyclic carbenes were used as efficient organocatalysts for the synthesis of glycerol carbonate from glycerol and dimethyl carbonate. The reaction takes place in a liquid mixture of reactants without solvent at room temperature. It provides glycerol carbonate in high yield using 2.6–4 mol % of catalyst in a reaction period of 20–30 min.     

Threonine-surfactant organocatalysts for the highly diastereo- and enantioselective direct anti-Mannich reactions of hydroxyacetone

In this work, several new l-threonine derivatives as organocatalysts were synthesized in one step for the first time by the reaction of threonine with acyl chlorides at room temperature in trifluoroacetic acid on a large-scale without protecting groups involved or chromatographic techniques, and those threonine-surfactant organocatalysts mediated the direct asymmetric anti-Mannich reactions of hydroxyacetone and anilines with aldehydes to synthesize anti-1,2-amino alcohols in good yields (75-93%) and highly enantioselectivities (up to 99% ee).