l-Proline amides catalyze direct asymmetric aldol reactions of aldehydes with methylthioacetone and fluoroacetone

Direct aldol reactions of aldehydes with methylthio- and fluoroacetone catalyzed by proline amides have been investigated. l-Prolinamide 5e was found to be the best catalyst. Under the optimized reaction conditions, a series of aromatic and aliphatic aldehydes reacted smoothly with methylthioacetone, to generate 1-methylthio-4-hydroxyketones 3 in good yields and with high regio- and enantioselectivities. Excellent enantioselectivities of up to 98% ee were observed for aromatic aldehydes and even higher enantioselectivities of >99% ee were observed for aliphatic aldehydes. Asymmetric direct aldol reactions of fluoroacetone with aldehydes in the presence of 20 mol % of 5e preferentially occurred at the fluoromethyl group, yielding products with high enantioselectivities (up to 98% ee).     

Cu(I)-carbenoid- and Ag(I)-Lewis acid-catalyzed asymmetric intermolecular insertion of alpha-diazo compounds into N-H bonds

Chiral Cu(I)-bisoxazoline- and Cu(I)-PN-complexes were found to catalyze the intermolecular insertion of alpha-diazo compounds into N-H bonds. The insertion reactions proceed with enantioselectivities of up to 28% ee for the different alpha-diazo acetates into one of the N-H bonds of different amines. Analogous chiral Ag(I) complexes were found to give higher enantioselectivities of up to 48% ee, however, lower yields were obtained. There are indications, that the Ag(I)-mediated reactions follow a different reaction mechanism compared to the Cu(I)-catalyzed insertions. It is demonstrated that different alpha-amino acid derivatives can be obtained via this approach in good yields and with low to moderate enantioselectivities. However, the results obtained are the highest asymmetric inductions obtained for an intermolecular N-H insertion via chiral carbene complexes or chiral Lewis acid catalysis.     

Synthesis and application of novel imidazole and 1H-tetrazolic acid containing catalysts in enantioselective organocatalyzed Diels–Alder reactions

Herein we report studies on the organocatalytic Diels–Alder reaction using a variety of catalysts capable of activating α,β-unsaturated carbonyl compounds for reactions with dienes. The structurally attractive catalysts 4 and 14 were utilized in the enantioselective organocatalytic Diels–Alder reactions. Catalyst 4 provided the products in fair yields and more importantly in good enantioselectivities of up to 83% ee. Catalyst 14 was synthesized in high yield and was assessed in the enantioselective organocatalytic Diels–Alder reaction. Catalyst 14 proved to be a highly active and selective catalyst providing the products in high yield and high enantioselectivities up to 95% ee.     

Screening of a modular sugar-based phosphite-oxazoline ligand library in asymmetric Pd-catalyzed heck reactions

We have synthesised a library of phosphite-oxazoline ligands derived from readily available D-glucosamine. These ligands have been successfully screened in the palladium-catalysed Heck reaction of several substrates with high regio- (up to 99 %) and enantioselectivities (ee's up to 99 %) as well as with improved activities under standard thermal conditions. The results indicate that the catalytic performance is highly affected by the oxazoline and biarylphosphite substituents and the axial chirality of the biaryl moiety of the ligand. The Heck reactions were also performed under microwave irradiation conditions, allowing a considerably shorter reaction time (full conversion in minutes) maintaining the excellent regio- and enantioselectivities.     

Novel thiolated amino-alcohols as chiral ligands for copper-catalyzed asymmetric nitro-aldol reactions

Thiolated amino-alcohols have been synthesized and evaluated as a potential new class of chiral ligands for copper-catalyzed nitro-aldol reactions. The model nitro-aldol reaction took place smoothly at ambient temperature in the presence of catalytic amounts (5-15 mol %) of the ligands and copper(II) acetate to afford the nitro-aldol product in good to excellent yield without accompanying dehydration. Amino-alcohol ligands bearing N-(2-alkylthio)benzyl substituents provided only modest enantioselectivities (22-46% ee) while those carrying N-2-thienylmethyl substituents provided better enantioselectivities (up to 75% ee). A range of aromatic aldehydes were acceptable for the nitro-aldol reaction with nitromethane, giving moderate to good enantioselectivities (69-88% ee).     

Ferrocenylphosphines as new catalysts for Baylis-Hillman reactions

[reaction: see text] Readily available ferrocenyldialkylphosphines are effective air-stable catalysts for Baylis-Hillman reaction between aldehydes and acrylates, affording the corresponding adducts in high yields and short reaction times. A set of readily accessible planar chiral ferrocenyldialkylphosphines have been tested in asymmetric Baylis-Hillman reactions. The best enantioselectivities were obtained using Mandyphos as chiral catalyst (up to 65% ee).     

Small peptides catalyze highly enantioselective direct aldol reactions of aldehydes with hydroxyacetone: unprecedented regiocontrol in aqueous media

[reaction: see text] L-Proline-based small peptides have been developed as efficient catalysts for the asymmetric direct aldol reactions of hydroxyacetone with aldehydes. Chiral 1,4-diols 7, which are disfavored products in similar aldol reactions catalyzed by either aldolases or L-proline, were obtained in high yields and enantioselectivities of up to 96% ee with peptides 3 and 4 in aqueous media.     

Catalytic, asymmetric cyanohydrin synthesis in propylene carbonate

Propylene carbonate can be used as a green solvent for asymmetric cyanohydrin synthesis catalyzed by VO(salen)NCS. A range of 10 aromatic and aliphatic aldehydes gave high enantioselectivities (up to 93%) and conversions (up to 100%) in reactions carried out at or near room temperature with reaction times of 24 h or less.     

Mimicking enzymatic transaminations: attempts to understand and develop a catalytic asymmetric approach to chiral alpha-amino acids

Attempts are made to build a bridge between asymmetric catalysis and enzymatic reactions by mechanistic investigations and the development of a catalytic and enantioselective approach to amination of alpha-keto esters by primary amines catalyzed by chiral Lewis acids as a model for transamination enzymes. Different Lewis acids can catalyze the half-transamination of alpha-keto esters using primary amine nitrogen sources such as pyridoxamine and 4-picolylamine. The mechanistic studies of the Lewis-acid catalyzed half-transamination using deuterium-labelled compounds show the incorporation of deuterium atoms in several positions of the alpha-amino acid derivative, indicating that the enol of the alpha-keto ester plays an important role along the reaction path. The catalytic enantioselective reactions are dependent on the pKa-value of the solvent since enantioselectivities were only obtained in solvents with high pKa-values relative to methanol. However, stronger acidic conditions generally gave better yields, but poor enantioselectivities. A series of chiral Lewis acids were screened as catalysts for the enantioselective half-transamination reactions and moderate yields and enantioselectivities of up to 46% ee were obtained.     

Readily tunable and bifunctional L-prolinamide derivatives: design and application in the direct enantioselective Aldol reactions

[reaction: see text] Readily tunable and bifunctional L-prolinamides as novel organocatalysts have been developed, and their catalytic activities were evaluated in the direct asymmetric Aldol reactions of various aromatic aldehydes and cyclohexanone. High isolated yields (up to 94%), enantioselectivities (up to 99% ee), and anti-diastereoselectivities (up to 99:1) were obtained under the optimal conditions.     

Catalytic and highly enantioselective reactions of alpha-sulfonyl carbanions with chiral bis(oxazoline)s

The enantioselective reactions of lithiated benzyl trifluoromethyl sulfones with a substoichiometric amount of a bis(oxazoline) and various aldehydes is disclosed. The products were formed with excellent diastereo- and enantioselectivities. Fluorination of the sulfone with N-fluorobenzenesulfonimide and a stoichiometric amount of a bis(oxazoline) gave products with extremely high enantioselectivities (up to 99% ee; ee=enantiomeric excess). The enantioselective reaction was confirmed to proceed through a dynamic thermodynamic resolution pathway.     

Highly enantioselective aldol reactions catalyzed by reusable upper rim-functionalized calix[4]arene-based l-proline organocatalyst in aqueous conditions

A series of upper rim-functionalized calix[4]arene-based l-Proline derivatives have been synthesized and employed for the enantioselective aldol reactions between cyclic ketones and aromatic aldehydes in the presence of water. Good to excellent yields (up to 96%), enantioselectivities (up to 99% ee), as well as diastereoselectivities (up to 99:1 dr) were obtained under the optimal reaction conditions. Detailed experiments clearly showed that the hydrophobic calixarene platform not only contributed to the good reactivities and enantioselectivities, but also exhibited size-selective catalysis function. Moreover, the organocatalyst can be readily recovered and reused for several runs without significant loss in its enantioselectivity.     

Organocatalytic diastereo- and enantioselective Michael addition reactions of 5-aryl-1,3-dioxolan-4-ones

5-Aryl-1,3-dioxolan-4-one heterocycles derived from mandelic acid derivatives and hexafluoroacetone have been identified as new and effective pro-nucleophiles in highly diastereo- and enantioselective Michael addition reactions to nitro olefins catalyzed by bifunctional epi-9-amino-9-deoxy cinchona alkaloid derivatives. Diastereoselectivities up to 98% and enantioselectivities up to 89% for a range of nitro olefins and 5-aryl-1,3-dioxolan-4-ones under mild reaction conditions are reported.     

Highly enantioselective hetero-Diels-Alder reaction of Brassard diene with aromatic aldehydes

[reaction: see text] Hetero-Diels-Alder reactions of Brassard diene with aromatic aldehydes were carried out smoothly in the presence of titanium(IV) tridentate Schiff base complexes to give the corresponding chiral delta-lactones in high enantioselectivities (up to 99% ee) under mild conditions.     

Formal cycloaddition of disubstituted ketenes with 2-oxoaldehydes catalyzed by chiral N-heterocyclic carbenes

Chiral N-hetereocyclic carbenes were found to be efficient catalysts for the formal [2 + 2] cycloaddition reactions of alkyl(aryl)ketenes with 2-oxoaldehydes to afford beta-lactones with alpha-quaternary-beta-tertiary stereocenters in high yields with good diastereoselectivities and excellent enantioselectivities (up to 99% ee). Both alkyl(aryl)ketenes and diarylketene worked well in this reaction.     

Pyrrolidine-pyridine base catalysts for the enantioselective Michael addition of ketones to chalcones

A series of pyrrolidine-pyridine base organocatalysts have been developed and 3a found to be a highly effective catalyst for the asymmetric Michael addition reactions of ketones to chalcones. The reaction generated the corresponding products 1, 5-diketones in excellent diastereoselectivities (up to >99:1 dr) and enantioselectivities (up to 100% ee).     

Carbohydrate-derived alcohols as organocatalysts in enantioselective aldol reactions of isatins with ketones

The catalytic activity of carbohydrate-derived amino alcohols in the enantioselective aldol reaction of ketones with isatin and its derivatives has been examined for the first time. The carbohydrate-derived amino alcohols 5 were found to be efficient organocatalysts for asymmetric aldol reactions. A variety of isatins were used as substrates and the corresponding aldol products were obtained in high yields (up to 99%) and with moderate enantioselectivities (up to 75%).     

Nickel/Photoredox-Catalyzed Asymmetric Reductive Cross-Coupling of Racemic α-Chloro Esters with Aryl Iodides

A unique nickel/organic photoredox co-catalyzed asymmetric reductive cross-coupling between α-chloro esters and aryl iodides is developed. This cross-electrophile coupling reaction employs an organic reductant (Hantzsch ester), whereas most reductive cross-coupling reactions use stoichiometric metals. A diverse array of valuable α-aryl esters is formed under these conditions with high enantioselectivities (up to 94 %) and good yields (up to 88 %). α-Aryl esters represent an important family of nonsteroidal anti-inflammatory drugs. This novel synergistic strategy expands the scope of Ni-catalyzed reductive asymmetric cross-coupling reactions.     

Organophosphine bearing multiple hydrogen-bond donors for asymmetric Michael addition reaction of 1-oxoindane-2-carboxylic acid ester via dual-reagent catalysis

Multiple hydrogen bonds containing nucleophilic phosphines derived from dipeptide dual-reagents catalyzed asymmetric Michael addition reactions between indene esters and activated olefins in high yields and good to excellent enantioselectivities under mild reaction conditions. The success of current highly selective reactions should provide inspiration for expansion to other reactions and would open up new paradigms for the synthesis of indanone derivatives bearing chiral quaternary carbon centers.     

Aminophosphonates as organocatalysts in the direct asymmetric aldol reaction: towards syn selectivity in the presence of Lewis bases

Chiral alpha-aminophosphonates have been synthesized and their performance was evaluated as organocatalysts in the direct asymmetric aldol reaction. High enantioselectivities (up to 99% ee) were achieved for a range of substituted cyclohexanones and benzaldehydes. Several organic bases, such as DBU, DBN, and TMG, were used together with the alpha-aminophosphonates in the aldol reactions and were found to favor syn-selectivity.