Urea- and thiourea-substituted cinchona alkaloid derivatives as highly efficient bifunctional organocatalysts for the asymmetric addition of malonate to nitroalkenes: inversion of configuration at C9 dramatically improves catalyst performance

Role of Configuration at C6 in Catalytic Activity of l-Proline-Derived Bifunctional Organocatalysts

l-Proline-derived chiral bifunctional (thio)urea organocatalysts epi-PTU and epi-PU were newly synthesized, and their catalytic performances were compared with their C6 epimeric catalysts PTU and PU in various Michael reactions of nitrostyrene in terms of reactivities and stereoselectivities. The experimental results indicate that a proper relative stereochemistry at C2 and C6 in l-proline-derived bifunctional organocatalysts is important for successful catalysis and that catalysts (PTU and PU) with the 2S,6R configuration are much more efficient.

β2, 2 -Amino Acid N-Carboxyanhydrides Relying on Sequential Enantioselective C(4)-Functionalization of Pyrrolidin-2,3-diones and Regioselective Baeyer-Villiger Oxidation

A catalytic enantioselective entry to β^2, 2 -amino acids enabling their direct coupling with nucleophiles is described. The approach is based upon an effective bifunctional Brønsted base catalyzed construction of a quaternary carbon stereocenter at C₄ position of pyrrolidin-2,3-diones. Subsequent regioselective Baeyer-Villiger oxidation of the resultant adducts gives β^2, 2 -amino acid N-carboxyanhydrides as the reactive species, which can further react with nucleophiles. Following this strategy both, β^2, 2 -amino acid derivatives with different functionalities at the newly created stereocenter, and spirocyclic structures can be efficiently prepared.

Mechanism and Origins of Stereoselectivity in the Cinchona Thiourea- and Squaramide-Catalyzed Asymmetric Michael Addition of Nitroalkanes to Enones

We report density functional theory calculations that examine the mechanism and origins of stereoselectivity of Soós' landmark discovery from 2005 that cinchona thioureas catalyze the asymmetric Michael addition of nitroalkanes to enones. We show that the electrophile is activated by the catalyst's protonated amine and that the nucleophile binds to the thiourea moiety by hydrogen bonding. These results lead to the correction of published mechanistic work which did not consider this activation mode. We have also investigated the corresponding cinchona squaramide-catalyzed reaction and found that it proceeds by the same mechanism despite the differences in the geometry of the two catalysts' hydrogen-bond-donating groups, which demonstrates the generality of this mechanistic model.

Asymmetric amination of 2-substituted indolin-3-ones catalyzed by natural cinchona alkaloids

The natural quinine-catalyzed enantioselective amination of 2-substituted indolin-3-ones with azodicarboxylates has been developed.

A highly enantioselective Friedel–Crafts reaction of 3,5-dimethoxylphenol with nitroolefins mediated by a bifunctional quinine derived thiourea catalyst

A useful and highly enantioselective Friedel–Crafts reaction of 3,5-dimethoxylphenol with nitroolefins catalyzed by a bifunctional quinine derived thiourea catalyst was developed.

Practical synthesis of symmetrical thioureas and heterocyclic thiones in water

A simple and practical protocol for the synthesis of symmetric thioureas and heterocyclic thiones has been developed. It involves the reaction of amines with phenyl chlorothionoformate in water to afford the symmetrical thioureas or heterocyclic thiones in good to excellent yields. Various amines including aromatic, aliphatic and chiral amines survived the reaction conditions. Mild conditions, simple work-up and high yields as well as using water as solvent are the major advantages of this protocol.

Crystal structures of N-[(4-phenyl-thia-zol-2-yl)carbamo-thio-yl]benzamide and N-{[4-(4-bromo-phen-yl)thia-zol-2-yl]carbamo-thio-yl}benzamide from synchrotron X-ray diffraction

The title compounds, C17H13N3OS2, (I), and C17H12BrN3OS2, (II), are potential active pharmaceutical ingredients. Compound (I) comprises two almost planar fragments. The first is the central (carbamo-thio-yl)amide (r.m.s. deviation = 0.038 Å), and the second consists of the thia-zole and two phenyl rings (r.m.s. deviation = 0.053 Å). The dihedral angle between these planes is 15.17 (5)°. Unlike (I), compound (II) comprises three almost planar fragments. The first is the central N-(thia-zol-2-ylcarbamo-thio-yl)amide (r.m.s. deviation = 0.084 Å), and the two others comprise the bromo-phenyl and phenyl substituents, respectively. The dihedral angles between the central and two terminal planar fragments are 21.58 (7) and 17.90 (9)°, respectively. Both (I) and (II) feature an intra-molecular N-H⋯O hydrogen bond, which closes an S(6) ring. In the crystal of (I), mol-ecules form hydrogen-bonded layers parallel to (100) mediated by N-H⋯S and C-H⋯O hydrogen bonds. In the crystal of (II), mol-ecules form a three-dimensional framework mediated by N-H⋯Br and C-H⋯O hydrogen bonds, as well as secondary S⋯Br [3.3507 (11) Å] and S⋯S [3.4343 (14) Å] inter-actions.

Synthetic and Organocatalytic Studies of Quinidine Analogues with Ring-Size Modifications in the Quinuclidine Moiety

Six highly enantiopure analogues of [2.2.2] were synthesized with five- or seven-membered rings in the (original) quinuclidine skeleton. Five of these compounds were prepared through epoxide opening by a secondary cyclic amine, providing the nor- and homoquinuclidine moieties through five- and six-membered ring formation. This method failed in the case of seven-membered ring formation, so for that particular ring size a different synthetic route starting from 3-quinuclidone was applied. The six novel analogues were examined as organocatalysts in four asymmetric conjugate addition reactions and the results compared with those of known cinchona alkaloid catalysts. This study shows that modification of the quinuclidine ring can have a substantial influence on catalyst activity and enantioselectivity. To acquire more insight into the characteristics of the new catalysts, the pKaH values were determined by means of fluorescence spectroscopy. Furthermore, relative reaction rates of conjugate thiol additions reactions catalyzed by these quinidine analogues were measured through polarimetry.

Stereoselective Glycosylation of 2-Nitrogalactals Catalyzed by a Bifunctional Organocatalyst

The use of a bifunctional cinchona/thiourea organocatalyst for the direct and α-stereoselective glycosylation of 2-nitrogalactals is demonstrated for the first time. The conditions are mild, practical, and applicable to a wide range of glycoside acceptors with products being isolated in good to excellent yields. The method is exemplified in the synthesis of mucin type Core 6 and 7 glycopeptides.

Towards the total synthesis of keramaphidin B

The enantio- and diastereoselective Michael addition of a δ-valerolactone-derived pronucleophile to a substituted furanyl nitroolefin catalysed by a bifunctional cinchonine-derived thiourea has been used as the key stereocontrolling step in a new synthetic strategy to the heavily functionalised piperidine core of keramaphidin B.

Cupreines and cupreidines: an established class of bifunctional cinchona organocatalysts

Cinchona alkaloids with a free 6'-OH functionality are being increasingly used within asymmetric organocatalysis. This fascinating class of bifunctional catalyst offers a genuine alternative to the more commonly used thiourea systems and because of the different spacing between the functional groups, can control enantioselectivity where other organocatalysts have failed. In the main, this review covers the highlights from the last five years and attempts to show the diversity of reactions that these systems can control. It is hoped that chemists developing asymmetric methodologies will see the value in adding these easily accessible, but underused organocatalysts to their screens.

Non-covalent organocatalysis in asymmetric oxidative C(sp3)–H bond functionalization – broadening C–H bond coupling reactions

In this highlight, the design of cooperative non-covalent organocatalysis for the development of novel asymmetric oxidative C–H bond functionalization reactions is presented.

Catalytic enantioselective cascade Michael/cyclization reaction of 3-isothiocyanato oxindoles with exocyclic α,β-unsaturated ketones en route to 3,2′-pyrrolidinyl bispirooxindoles

Cascade Michael/cyclization reactions between 3-isothiocyanato oxindoles and exocyclic α,β-unsaturated ketones are shown to proceed in the presence of a tertiary amino-squaramide catalyst and furnish 3,2′-pyrrolidinyl bispirooxindoles with excellent enantioselectivities (up to 99 : 1 er).

Asymmetric cycloaddition reactions catalyzed by bifunctional thiourea and squaramide organocatalysts: recent advances

In this review, the recent developments in the field of enantioselective cycloaddition reactions using powerful bifunctional amine-thiourea and amine-squaramide organocatalysts have been described.

A highly enantioselective thiolation of sulfonyl indoles to access 3-sec-sulfur-substituted indoles in water

Highly enantioselective organocatalytic Michael addition of thiol to vinylogous imine intermediates generated in situ from sulfonylindoles in aqueous medium is presented.

Enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofurans using a Brønsted base/thiourea bifunctional catalyst

The enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofuran derivatives via intramolecular Michael addition has been developed using a bifunctional tertiary amine–thiourea catalyst.

Organocatalyzed enantioselective Michael addition/cyclization cascade reaction of 3-isothiocyanato oxindoles with arylidene malonates

Both enantiomers of functionalized 3,2′-pyrrolidinyl spirooxindoles were obtainedviaa Michael-cyclization cascade reaction using pseudoenantiomeric cinchona alkaloid derived thiourea based bifunctional catalysts.

The first catalytic asymmetric cycloadditions of imines with an enolisable anhydride

The first asymmetric cycloaddition reactions between imines and enolisable anhydrides is described.

Kinetic resolution of phosphoric diester by Cinchona alkaloid derivatives provided with a guanidinium unit

Cinchona alkaloid derivatives featuring a guanidinium group in diverse positions efficiently catalyze the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate (HPNP).

Organocatalytic enantioselective Mukaiyama–Mannich reaction of fluorinated enol silyl ethers and cyclic N-sulfonyl ketimines

The first catalytic asymmetric Mukaiyama–Mannich reaction of fluorinated silyl enol ethers and ketimines is developed, allowing highly enantioselective synthesis of benzosultam based β-fluorinated C^α-tetrasubstituted α-amino acid derivatives.

Advantages of Organic Halogen Bonding for Halide Recognition

The study of hydrogen bonding organocatalysis is rapidly expanding. Much research has been directed at making catalysts more active and selective, with less attention on fundamental design strategies. This study systematically increases steric hindrance at the active site of pH switchable urea organocatalysts. Incorporating strong intramolecular hydrogen bonds from protonated pyridines to oxygen stabilizes the active conformation of these ureas thus reducing the entropic penalty that results from substrate binding. The effect of increasing steric hindrance was studied by single crystal X-ray diffraction and by kinetics experiments of a benchmark reaction.