Enantiopure azetidine-2-carboxamides as organocatalysts for direct asymmetric aldol reactions in aqueous and organic media

Synergistic Copper-Aminocatalysis for Direct Tertiary α-Alkylation of Ketones with Electron-Deficient Alkanes

In this study, a novel approach for the tertiary α-alkylation of ketones using alkanes with electron-deficient C─H bonds is presented, employing a synergistic catalytic system combining inexpensive copper salts with aminocatalysis. This methodology addresses the limitations of traditional alkylation methods, such as the need for strong metallic bases, regioselectivity issues, and the risk of over alkylation, by providing a high reactivity and chemoselectivity without the necessity for pre-functionalized substrates. The dual catalytic strategy enables the direct functionalization of C(sp3)─H bonds, demonstrating remarkable selectivity in the presence of conventional C(sp3)─H bonds that are adjacent to heteroatoms or π systems, which are typically susceptible to single-electron transfer processes. The findings contribute to the advancement of alkylation techniques, offering a practical and efficient route for the construction of C(sp3)─C(sp3) bonds, and paving the way for further developments in the synthesis of complex organic molecules.

New Strategy for the Synthesis of Some Valuable Chiral 1,3-Diols with High Enantiomeric Purity: New Organocatalyst, Asymmetric Aldol Reaction, and Reduction

Chiral 1,3-diols are highly valuable molecules used in industries such as pharmaceuticals, cosmetics, and agriculture. Therefore, in this study, a new strategy was developed to synthesize enantiomerically pure (>99% ee) 1,3-diols. New chiral 1,3-diols (5a-5q) with high enantiomeric purity were synthesized from aldol products chiral 1,3-keto alcohols (4a-4q), which are aldol products with different structures. Chiral 1,3-keto alcohols (4a-4q) were synthesized by a new asymmetric aldol method in the first step. This method was developed using a new proline-derived organocatalyst (3g) and Cu(OTf)2 as an additive in DMSO-H2O for the first time. Almost >99% ee was obtained using our developed aldol procedure. In the second step, original chiral diols (5a-5q) of high enantiomeric purity were obtained by asymmetric reduction of chiral keto alcohols with chiral oxazaborolidine reagents. In this way, a two-step asymmetric reaction was developed for chiral 1,3-diol enantiomers with high enantiomeric purity. The structures of all the original chiral compounds obtained were elucidated by infrared and nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis methods. Their enantiomeric excesses were determined by the chiral high-performance liquid chromatography method. Both keto alcohols and their corresponding chiral diols synthesized can be used as chiral starting materials and chiral source materials or intermediates in the synthesis of many biologically active molecules, or they can be used as chiral ligands in asymmetric synthesis, serving as organocatalysts.

Azaheterocyclic diphenylmethanol chiral solvating agents for the NMR chiral discrimination of alpha-substituted carboxylic acids

A series of small-membered heterocycle probes, so-called azaheterocycle-containing diphenylmethanol chiral solvating agents (CSAs), have been developed for NMR enantiodiscrimination. These chiral sensors were readily synthesized were inexpensive and efficiently used for the chiral analysis of alpha-substituted carboxylic acids. The sensing method was operationally simple and the processing was straightforward. Notably, we propose (S)-aziridinyl diphenylmethanol as a promising CSA, which has excellent chiral discriminating properties and offers multiple detectable possibilities pertaining to the 1H NMR signals of diagnostic split protons (including 25 examples, up to 0.194 ppm, 77.6 Hz). Its ability to detect the molecular recognition of fluorinated carboxylic acids were further investigated, with a good level of discrimination via the 19F NMR spectroscopic analysis. In addition, an accurate enantiomeric excess (ee) analysis of the p-methoxyl-mandelic acid with different optical compositions have been calculated based on the integration of well-separated proton signals.

Base-promoted diastereoselective α-alkylation of borane N-((S)-1'-phenylethyl)azetidine-2-carboxylic acid ester complexes

The base-promoted α-alkylation of N-((S)-1-phenylethyl)azetidine-2-carboxylic acid esters 1 was investigated. The use of diastereomerically pure borane complexes 3 as substrates, which are easily prepared from 1, dramatically improved the yields and diastereoselectivities of α-alkylated products 2. For example, the treatment of tert-butyl ester (1S,2S,1'S)-3a with 2.4 equivalents of lithium bis(trimethysilyl)amide (LiHMDS) at 0 °C followed by 2.6 equivalents of benzyl bromide afforded α-benzylated (2S,1'S)-2aa in 90% yield as almost a single diastereomer. Our method enables the production of optically active α-substituted azetidine-2-carboxylic acid esters starting from commercially available (S)-1-phenylethylamine, which is one of the least expensive chiral compounds.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Synthesis of 3-Indolylglycine Derivatives via Dinuclear Zinc Catalytic Asymmetric Friedel-Crafts Alkylation Reaction

A direct asymmetric Friedel-Crafts (F-C) alkylation reaction between a wide range of indoles and ethyl 2-(4-methoxyphenylimino)acetate catalyzed by Trost's dinuclear complex is reported. A series of 3-indolylglycine derivatives were synthesized in enantioselectivity of up to >99% enantiomeric excess (ee) using 10 mol% catalyst loading under mild conditions. This atom economic reaction could be run on a gram scale without impacting its enantioselectivity. The absolute stereochemistry of catalytic products was determined by correlation with a known configuration compound. A possible mechanism was proposed for the asymmetric induction.

Highly modular dipeptide-like organocatalysts for direct asymmetric aldol reactions in brine

The dipeptide-like organocatalysts have been developed for asymmetric aldol reactions in brine to achieve high yields and enantioselectivities with 1 mol% catalyst-loading.