Direct organocatalytic asymmetric reductive Mannich-type reactions

A direct asymmetric reductive Manich-type reaction that allows for the formation of three contiguous stereocenters with high chemo-, diastereo-, and enantioselectivity is presented.     

The catalytic asymmetric Mannich-type reactions in aqueous media

The catalytic, diastereo- and enantioselective Mannich-type reaction of a hydrazono ester with silyl enol ethers in aqueous media has been successfully achieved with ZnF(2), a chiral diamine ligand, and trifluoromethanesulfonic acid.     

Direct organocatalytic aldol reactions in buffered aqueous media

Organocatalytic cross-aldol reactions catalyzed by cyclic secondary amines in aqueous media provide a direct route to a variety of aldols including carbohydrate derivatives and may warrant consideration as a prebiotic route to sugars.     

Direct catalytic asymmetric anti-selective Mannich-type reactions

The simple chiral pyrrolidine catalyzed asymmetric anti-selective Mannich-type reaction is presented; the reaction gives the corresponding amino acid derivatives with 10:1- >19:1 dr and 97-99% ee.     

Direct asymmetric three-component organocatalytic anti-selective Mannich reactions in a purely aqueous system

The direct three-component Mannich reactions of O-benzyl hydroxyacetone with p-anisidine and aromatic or aliphatic aldehydes in the presence of an L-threonine-derived catalyst afforded anti-1,2-amino alcohols in good-to-excellent yields and with enantioselectivities of up to 97%. This study is the first demonstration that direct three-component Mannich reactions can be promoted by a primary amino acid in water.     

Advances on asymmetric organocatalytic 1,4-conjugate addition reactions in aqueous and semi-aqueous media

Asymmetric Michael addition reactions or 1,4-conjugate addition reactions are considered to be the fundamental C–C bond-forming reaction for the construction of chiral β-nitro, β-carbonyl and several other important structural motifs in organic synthesis. After the development of many chiral metal complexes as catalysts, a tremendous growth in the design and applications of variety of new chiral organocatalysts for asymmetric 1,4-conjugate addition reactions has been witnessed over the last two decades. Initially, asymmetric organocatalysis has been performed in organic solvents, but gradually performing the same catalysis in aqueous and semi-aqueous media became a necessity due to environmental aspects as well as to achieve better reactivity and selectivity. The structural and functional diversity of the chiral organocatalysts derived from natural and synthetic sources utilized differently the water either as the sole solvent, co-solvent or additive for optimising their best performances. In the present review, we discuss a detailed and comprehensive report on the advancement in the field of asymmetric organocatalytic 1,4-conjugate addition reactions in aqueous and semi-aqueous media.     

Bioinspired organocatalytic asymmetric reactions

Several small organic molecule catalysts are reminiscent of natural enzymes in their mode of action and substrate interaction/activation. This striking similarity has been a great source of inspiration for the development of new organocatalytic asymmetric processes. A few representative examples, mostly dealing with catalysts interacting through multiple hydrogen-bonds (synthetic oxyanion holes), are highlighted in this perspective.     

Catalytic asymmetric aldol reactions in aqueous media

Nature has perfected the stereospecific aldol reaction by using aldolase enzymes. While virtually all the biochemical aldol reactions use unmodified donor and acceptor carbonyls and take place under catalytic control in an aqueous environment, the chemical domain of the aldol addition has mostly relied on prior transformation of carbonyl substrates, and the whole process traditionally is carried out in anhydrous solvents. The area of aqua-asymmetric aldol reactions has received much attention recently in light of the perception both of its green chemistry advantages and its analogy to eon-perfected enzyme catalysis. Both chiral metal complexes and small chiral organic molecules have been recently reported to catalyze aldol reactions with relatively high chemical and stereochemical efficiency. This tutorial review describes recent developments in this area.     

A one-pot organocatalytic asymmetric entry to tetrahydrothioxanthenones

A simple catalytic asymmetric synthesis of tetrahydrothioxanthenones is presented. The organocatalytic enantioselective domino reactions between 2-mercaptobenzaldehyde and α,β-unsaturated cyclic ketones proceed with excellent chemoselectivity to give the corresponding tetrahydrothioxanthenones in high yields and moderate to good enantioselectivities.     

Catalytic asymmetric carbon-carbon bond-forming reactions in aqueous media

Use of water as a reaction solvent or co-solvent has received much attention in synthetic organic chemistry. Recently, successful examples of catalytic asymmetric carbon-carbon bond formation in aqueous media have been developed. Most of these examples show characteristic features that are realized only in the presence of water. The role of water in these reactions is also discussed here.     

Direct organocatalytic asymmetric alpha-chlorination of aldehydes

The direct organocatalytic enantioselective alpha-chlorination of aldehydes has been developed. The reaction proceeds for a series of different aldehydes with NCS as the chlorine source using easily available catalysts such as l-proline amide and (2R,5R)-diphenylpyrrolidine. The alpha-chloro aldehydes are obtained in up to 99% yield and up to 95% ee. The synthetic utility of the enantioselective alpha-chlorination of aldehydes is demonstrated by transformation of the alpha-chloro aldehydes to the corresponding alpha-chloro alcohols (>90% yield) by standard reduction and further transformation to both a terminal epoxide and amino alcohol, both obtained without loss of optical purity. Oxidation of the alpha-chloro aldehydes followed by esterification gave optically active alpha-chloro esters without loss of optical purity. It is demonstrated that these optically active alpha-chloro esters can be converted into nonproteinogenic amino acids in overall high yields, maintaining the enantiomeric excess obtained in the catalytic enantioselective alpha-chlorination step.     

Direct catalytic asymmetric cross-aldol reactions in ionic liquid media

Enantioselective proline-catalyzed direct asymmetric cross-aldol reactions with aldehydes were performed in ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate media, which simplified product isolation and catalyst recycling, affording 3-hydroxy aldehydes in high yield with excellent stereoselectivity. In addition, the enhanced reactivity of the asymmetric cross-aldol reactions in ionic liquid allowed the catalyst loading to be lowered significantly.     

Direct asymmetric aldol reaction in aqueous media using polymer-supported peptide

PEG-PS resin-supported tripeptide/zinc chloride catalyst system has been developed for use in the direct asymmetric aldol reaction of acetone with aldehydes in aqueous media. The peptide catalyst could be separated from the reaction mixture by filtration, and was reusable at least five times without significant change in its activity and selectivity.     

Formal asymmetric enone aminohydroxylation: organocatalytic one-pot synthesis of 4,5-disubstituted oxazolidinones

A formal asymmetric organocatalytic aminohydroxylation reaction of enones has been achieved via an aziridination-double S(N)2 sequence. As a part of the reaction design, the generated amino alcohol products are isolated as the corresponding oxazolidinones in good yields and excellent stereoselectivities.     

Direct conversion of aldehydes to amides,tetrazoles, and triazines in aqueous media by one-pot tandem reactions

A variety of aldehydes reacted with iodine in ammonia water at room temperature to give the nitrile intermediates, which were trapped by addition of hydrogen peroxide, sodium azide, or dicyandiamide to produce their corresponding amides, tetrazoles, and 1,3,5-triazines in modest to high yields. The one-pot tandem reactions were conducted in water media, and the products were obtained simply by extraction or filtration.     

Enantioselective organocatalytic tandem Michael-Aldol reactions: one-pot synthesis of chiral thiochromenes

A highly enantioselective (S) diphenylpyrrolinol silyl ether promoted tandem Michael-aldol reaction of alpha,beta-unsaturated aldehydes with 2-mercaptobenzaldehydes has been developed. The method affords one-pot access to chiral and synthetically useful thiochromenes in high yields and high enantioselectivities from readily available compounds.     

Acyclic β-amino acid catalyzed asymmetric anti-selective Mannich-type reactions

The ability of a primary amine containing acyclic β³-amino acids to catalyze direct asymmetric anti-selective Mannich-type reactions is presented. The reactions are generally highly diastereo- and enantioselective to give the corresponding Mannich products with up to >19:1 dr (anti/syn) and 88–99% ee.     

Remarkably stable chiral zirconium complexes for asymmetric Mannich-type reactions

Isolable, air-stable, storable, and highly selective chiral zirconium catalysts for asymmetric Mannich-type reactions have been developed. The reactions of imines with silicon enolates proceeded smoothly using 1-10 mol % of the powdered zirconium catalyst to afford the corresponding adducts in high yields with high stereoselectivities. The catalyst could be recovered and reused without significant loss of activity. On the other hand, zirconium single crystals for X-ray analysis were obtained, and the crystals also showed high performance in the asymmetric Mannich-type reactions. Although NMR analyses of these zirconium catalysts showed different structures in dichloromethane, the formation of the same key intermediate from the different catalysts was indicated.