Highly Enantioselective Organocatalyticsyn- andanti-Aldol Reactions in Aqueous Medium

Enantioselective vinylogous Mukaiyama aldol reaction of α-ketoesters under bifunctional organocatalysis

A highly enantioselective vinylogous Mukaiyama aldol reaction to ketoesters catalysed by a hydrogen-bond-donor-based bifunctional organocatalyst is presented. The addition of silyloxy dienol ether gives rise to multifunctional chiral tertiary alcohols bearing a versatile α,β-unsaturated aldehyde with excellent enantiocontrol.

Tripeptide-Catalyzed Asymmetric Aldol Reaction Between α-ketoesters and Acetone Under Acidic Cocatalyst-Free Conditions

Here, we report the tripeptide-catalyzed asymmetric aldol reaction between α-ketoesters and acetone under acidic cocatalysts-free conditions. H-Pro-Tle-Gly-OH 3g-catalyzed reactions between α-ketoesters and acetone resulted in up to 95% yield and 88% ee. Analysis of the transition state using density functional theory (DFT) calculations revealed that the tert-butyl group in 3g played an important role in enantioselectivity.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Water in asymmetric organocatalytic systems: a global perspective

Asymmetric organocatalysis often operates under near ambient conditions, which means it is air and moisture compatible. However, in many examples water is indeed necessary for achieving excellent catalytic results. Ranging from the addition of small amounts of water to a reaction, to complex catalytic systems in the presence of water as the only reaction medium, this review offers an illustrative classification of the uses of water in asymmetric organocatalysis.

Synthesis of functionalized isoxazole–oxindole hybrids via on water, catalyst free vinylogous Henry and 1,6-Michael addition reactions

Various isoxazole–oxindole hybrids were synthesized via vinylogous Henry reaction of 3,5-dimethyl-4-nitroisoxazole and isatin under catalyst free conditions in water. The products obtained were functionalized using 1,6-Michael addition reaction.

Amine-catalyzed direct aldol reactions of hydroxy- and dihydroxyacetone: biomimetic synthesis of carbohydrates

This article presents comprehensive studies on the application of primary, secondary, and tertiary amines as efficient organocatalysts for the de novo synthesis of ketoses and deoxyketoses. Mimicking the actions of aldolase enzymes, the synthesis of selected carbohydrates was accomplished in aqueous media by using proline- and serine-based organocatalysts. The presented methodology also provides direct access to unnatural L-carbohydrates from the (S)-glyceraldehyde precursor. Determination of the absolute configuration of all obtained sugars was feasible using a methodology consisting of concerted ECD and VCD spectroscopy.

Primary-tertiary diamine-catalyzed Michael addition of ketones to isatylidenemalononitrile derivatives

Simple primary-tertiary diamines easily derived from natural primary amino acids were used to catalyze the Michael addition of ketones with isatylidenemalononitrile derivatives. Diamine 1a in combination with D-CSA as an additive provided Michael adducts in high yield (up to 94%) and excellent enantioselectivity (up to 99%). The catalyst 1a was successfully used to catalyze the three-component version of the reaction by a domino Knoevenagel–Michael sequence. The Michael adduct 4a was transformed into spirooxindole 6 by a reduction with sodium borohydride in a highly enantioselective manner.

Reactions of pyruvates: organocatalytic synthesis of functionalized dihydropyrans in one pot and further transformations to functionalized carbocycles and heterocycles

Concise cascade reactions of pyruvates with aldehydes that generate functionalized dihydropyran derivatives in one pot have been developed. The products, dihydropyrans, were further concisely transformed to various functionalized molecules.

Chiral picolylamines for Michael and aldol reactions: probing substrate boundaries

Here we report on inroads concerning increased substrate breadth via the picolylamine organocatalyst template, a vicinal chiral diamine based on a pyridine-primary amine motif. The addition of cyclohexanone to β-nitrostyrene has many catalyst solutions, but cyclopentanone and isobutyraldehyde additions continue to be challenging. PicAm-3 (10 mol%) readily allows the Michael addition of cyclopentanone or isobutyraldehyde (5.0 equiv.) to β-nitrostyrene derivatives. By contrast, PicAm-1 (7.0 mol%) is optimal for catalyzing the aldol reaction of cyclohexanone or cycloheptanone (3.3 equiv.) with aromatic aldehydes. Eighteen products are reported and for each reaction type new products are reported (4b-d, 9c). Very good yields and stereoselectivities are generally noted. The reactions, which require an acid additive, proceed via a transient chiral enamine and a mechanistic case is put forth for a bifunctional catalysis model.

Selective access to both diastereoisomers in an enantioselective intramolecular Michael reaction by using a single chiral organocatalyst and application in the formal total synthesis of (-)-epibatidine

Two in one: Both diastereoisomers of 4-nitro-3-substituted cyclohexanones are accessed selectively by an intramolecular Michael reaction using a single chiral aminocatalyst (see scheme). Mechanistic studies show that the reaction is selective for the cis-diastereoisomer and that the trans-diastereoisomer arises over time. DFT calculations suggest that the cis-selectivity is due to a favorable electrostatic interaction between the iminium ion and the nucleophile.

Hydrophobic substituent effects on proline catalysis of aldol reactions in water

Derivatives of 4-hydroxyproline with a series of hydrophobic groups in well-defined orientations have been tested as catalysts for the aldol reactions. All of the modified proline catalysts carry out the intermolecular aldol reaction in water and provide high diastereoselectivity and enantioselectivity. Modified prolines with aromatic groups syn to the carboxylic acid are better catalysts than those with small hydrophobic groups (1a is 43.5 times faster than 1f). Quantum mechanical calculations provide transition structures, TS-1a(water) and TS-1f(water), that support the hypothesis that a stabilizing hydrophobic interaction occurs with 1a.

Asymmetric syn-selective direct aldol reaction of protected hydroxyacetone catalyzed by primary amino acid derived bifunctional organocatalyst in the presence of water

A new series of water compatible primary-tertiary diamine catalysts derived from natural primary amino acids bearing a hydrophobic side chain have been synthesized. These new primary-tertiary diamine-Brønsted acid conjugates bifunctional organocatalysts efficiently catalyzes the asymmetric direct syn selective cross-aldol reaction of different protected hydroxyacetone with various aldehydes in high yield (94%) and high enantioselectivity (up to 97% ee of syn) and dr of 91 : 9 (syn/anti) under mild reaction conditions.

Highly diastereo- and enantioselective direct Barbas-List aldol reactions promoted by novel benzamidoethyl and benzamidopropyl prolinamides in water

Four novel benzamido-functionalized prolinamides have been prepared and tested as organocatalysts for enantioselective aldol reaction of aldehydes and cyclic ketones in water. In particular, prolinamide derived from achiral ethylene diamine was the best catalyst leading to anti aldols in excellent diastereomeric (up to 98/2) and enantiomeric (up to 99/1) ratios, thereby showing that lateral amide functionalities might be a key issue for facilitating "in water" chemistry. These catalysts are cheaper and easier to prepare than those previously described.

Direct asymmetric aldol reaction with recyclable fluorous organocatalyst

Direct asymmetric aldol reactions of aldehydes with ketones in the presence of a catalytic amount of fluorous sulfonamide 4 and trifluoroacetic acid result in the corresponding aldol products in high yields with up to 96% ee. The fluorous organocatalyst 4 can be readily recovered from the reaction mixture by fluorous solid-phase extraction and could be reused without a significant loss of the catalytic activity and enantioselectivity.

Organocatalytic reactions in water

Organocatalysts have emerged as a third major way of catalyzing a wide variety of reactions, besides metal catalysts and biocatalysts. They have gained tremendous importance because of their green chemistry perspective. The criteria for green chemistry would be largely fulfilled if the major component of the reaction mixture, i.e. the solvent, is water which is a suitable solvent in various biosynthetic reactions. In this feature article we have described reactions promoted by organocatalysts in a large excess of water, without any organic solvent or excess of any reactant. We have also explained the structural features required for organocatalysts to work well in aqueous media.