Highly Enantioselective Organocatalytic Trifluoromethyl Carbinol Synthesis—A Caveat on Reaction Times and Product Isolation

Chiral Anthranyl Trifluoromethyl Alcohols: Structures, Oxidative Dearomatization and Chiroptical Properties

Chiral trifluoromethyl alcohol groups were introduced at the hindered ortho positions of 9,10-diphenylanthracenes to investigate their effects on the physical properties and reactivity towards oxidative dearomatization. In such compact structures, the position in different quadrants and the preferred orientation of the -CH(OH)CF₃ groups were determined by the relative and absolute configurations of each stereoisomer, respectively. As a consequence, the stereochemistry governs the organization of the H-bonded molecules in single crystals (homochiral dimers vs ribbon), whereas in chlorinated solvents, they all behave as discrete compounds. Concerning their reactivity, the stereospecific dearomative oxidation of these molecules leads to 9,10-bis-spiro-isobenzofuran-anthracenes, when using organic single-electron transfer oxidants. The chiroptical properties of the alcohols and the corresponding dearomatized products were compared and showed an important modulation of the intensity.

Catalytic Diastereo- and Enantioselective Synthesis of Tertiary Trifluoromethyl Carbinols through a Vinylogous Aldol Reaction of Alkylidenepyrazolones with Trifluoromethyl Ketones

A diastereo- and enantioselective organocatalytic aldol reaction between alkylidenepyrazolones and trifluoromethyl ketones leading to chiral tertiary alcohols bearing a trifluoromethyl group is presented. The methodology is based on the use of a bifunctional organocatalyst in order to activate the γ-hydrogen atoms of the alkylidenepyrazolone nucleophile and the carbonyl group of the trifluoromethylarylketone providing highly functionalized trifluoromethyl alcohols with moderate yields, excellent diastereoselectivity, and moderate to good enantioselectivity. Experiments monitoring the conversion by ¹H NMR and the enantiomeric excess by HPLC with the reaction time showed that full conversion of the starting materials is not achieved and that the enantiomeric excess decreases upon extended times, probably due to the reversibility of the reaction.

Varying the Directionality of Protein Catalysts for Aldol and Retro-Aldol Reactions

Natural aldolase enzymes and created retro-aldolase protein catalysts often catalyze both aldol and retro-aldol reactions depending on the concentrations of the reactants and the products. Here, we report that the directionality of protein catalysts can be altered by replacing one amino acid. The protein catalyst derived from a scaffold of a previously reported retro-aldolase catalyst, catalyzed aldol reactions more efficiently than the previously reported retro-aldolase catalyst. The retro-aldolase catalyst efficiently catalyzed the retro-aldol reaction but was less efficient in catalyzing the aldol reaction. The results indicate that protein catalysts with varying levels of directionality in usually reversibly catalyzed aldol and retro-aldol reactions can be generated from the same protein scaffold.

Highly Diastereoselective Construction of Carbon- Heteroatom Quaternary Stereogenic Centers in the Synthesis of Analogs of Bioactive Compounds: From Monofluorinated Epoxyalkylphosphonates to α-Fluoro-, β-, or γ-Amino Alcohol Derivatives of Alkylphosphonates

The synthesis of the stable surrogates of an important amino acid (R)-4-amino-3-hydroxybutyric acid (GABOB) such as substituted hydroxy aminophosphonic acids bearing a quaternary stereogenic center is presented. Highly diastereoselective formations of fluorinated spiroepoxy alkylphosphonate or related tertiary carbon-containing oxiranes from β-keto phosphonates possessing methyl, phenyl, or cyclohexenyl substituents, are reported. Stereoselective acid-promoted epoxide opening by bromide or azide followed by reduction/protection afforded tertiary bromides or N-Boc derivatives of β-amino-γ-hydroxy alkylphosphonates in most cases, while the reactions of oxiranes with different amines yielded their β-hydroxy-γ-amino regioisomers. Surprisingly, during the synthesis of amino phosphonic acids, we observe that the acid-induced rearrangement proceeded in a high diastereospecific manner, leading finally to substituted β-hydroxy-γ-aminoalkylphosphonic acids.

Fluorinated Ketones as Trapping Reagents for Visible-Light-Induced Singlet Nucleophilic Carbenes

Singlet nucleophilic carbenes (SNCs) containing only one heteroatom donor remain underutilized in chemical synthesis. We recently discovered that visible-light-induced SNC intermediates can be trapped by fluorinated ketones via 1,2-carbonyl addition to afford benzoin-type products. This discovery represents a rare example of nucleophilic carbenes reacting with ketones and delivers an efficient, user-friendly, and scalable process for accessing fluorinated tertiary alcohol derivatives driven only by light circumventing the use of exogenous catalysts or additives.

Syntheses and Applications of Singh’s Catalyst

Singh’s catalyst has emerged as one of the most promising and valuable catalysts in the field of asymmetric synthesis. Since its discovery, it has proven to be one of the best organocatalysts for asymmetric direct aldol reactions, and is equally efficient in aqueous and organic media. In this Short Review, we summarize reactions utilizing Singh’s catalyst under various conditions.

1 Introduction

2 Synthesis of Singh’s Catalyst

3 Applications in Asymmetric Synthesis

4 Conclusion

NHC-catalyzed enantioselective synthesis of β-trifluoromethyl-β-hydroxyamides

The N-heterocyclic carbene (NHC)-catalyzed formal [2 + 2] cycloaddition between α-aroyloxyaldehydes and trifluoroacetophenones, followed by ring opening with an amine or a reducing agent is described. The resulting β-trifluoromethyl-β-hydroxyamide and alcohol products are produced with reasonable diastereocontrol (typically ≈70:30 dr) and excellent enantioselectivity, and they can be isolated in moderate to good yield as a single diastereoisomer.

Direct Catalytic Asymmetric Synthesis of Oxindole-Derived δ-Hydroxy-β-ketoesters by Aldol Reactions

Direct asymmetric synthesis of δ-hydroxy-β-ketoesters was accomplished via regio- and enantioselective aldol reactions of β-ketoesters with isatins catalyzed by cinchona alkaloid thiourea derivatives. The C-C bond formation of the reactions occurred only at the γ-position of the β-ketoesters. Reaction progress monitoring and product stability analyses under the conditions that included the catalyst indicated that the γ-position reaction products were formed kinetically. Various δ-hydroxy-β-ketoesters bearing 3-alkyl-3-hydroxyoxindole cores relevant to the development of bioactive molecules were synthesized.

Asymmetric vinylogous aldol addition of alkylidene oxindoles on trifluoromethyl-α,β-unsaturated ketones

A novel vinylogous aldol addition of alkylidene oxindole with 1-trifluoromethyl-3-alkylidene-propan-2-ones is presented. The reaction, catalyzed by a bifunctional tertiary amine, provides an efficient application of the vinylogous reactivity of oxindoles for the preparation of enantioenriched trifluoromethylated allylic alcohols.

Palladium-Catalyzed Negishi Coupling of α-CF3 Oxiranyl Zincate: Access to Chiral CF3-Substituted Benzylic Tertiary Alcohols

We report a Pd-catalyzed stereospecific α-arylation of optically pure 2,3-epoxy-1,1,1-trifluoropropane (TFPO). This method allows for the direct and reliable preparation of optically pure 2-CF₃-2-(hetero)aryloxiranes, which are precursors to many CF₃-substituted tertiary alcohols. The use of continuous-flow methods has allowed the deprotonation of TFPO and subsequent zincation at higher temperature compared to that under traditional batch conditions.

Screening, Molecular Cloning, and Biochemical Characterization of an Alcohol Dehydrogenase from Pichia pastoris Useful for the Kinetic Resolution of a Racemic β-Hydroxy-β-trifluoromethyl Ketone

The stereoselective synthesis of chiral 1,3-diols with the aid of biocatalysts is an attractive tool in organic chemistry. Besides the reduction of diketones, an alternative approach consists of the stereoselective reduction of β-hydroxy ketones (aldols). Thus, we screened for an alcohol dehydrogenase (ADH) that would selectively reduce a β-hydroxy-β-trifluoromethyl ketone. One potential starting material for this process is readily available by aldol addition of acetone to 2,2,2-trifluoroacetophenone. Over 200 strains were screened, and only a few yeast strains showed stereoselective reduction activities. The enzyme responsible for the reduction of the β-hydroxy-β-trifluoromethyl ketone was identified after purification and subsequent MALDI-TOF mass spectrometric analysis. As a result, a new NADP(+) -dependent ADH from Pichia pastoris (PPADH) was identified and confirmed to be capable of stereospecific and diastereoselective reduction of the β-hydroxy-β-trifluoromethyl ketone to its corresponding 1,3-diol. The gene encoding PPADH was cloned and heterologously expressed in Escherichia coli BL21(DE3). To determine the influence of an N- or C-terminal His-tag fusion, three different recombinant plasmids were constructed. Interestingly, the variant with the N-terminal His-tag showed the highest activity; consequently, this variant was purified and characterized. Kinetic parameters and the dependency of activity on pH and temperature were determined. PPADH shows a substrate preference for the reduction of linear and branched aliphatic aldehydes. Surprisingly, the enzyme shows no comparable activity towards ketones other than the β-hydroxy-β-trifluoromethyl ketone.

Asymmetric chemoenzymatic synthesis of 1,3-diols and 2,4-disubstituted aryloxetanes by using whole cell biocatalysts

A simple and efficient chemoenzymatic approach for the asymmetric synthesis of 1,3-diols and 2,4-disubstituted aryloxetanes has been established.

An efficient approach for the construction of trifluoromethylated all-carbon quaternary stereocenters: enantioselective Ni(ii)-catalyzed Michael addition of 2-acetyl azaarene to β,β-disubstituted nitroalkenes

Michael addition of 2-acetyl azaarenes with β,β-disubstituted nitroalkenes afforded chiral compounds in good yields and with excellent enantioselectivities.

A ligand-free strategy for the copper-catalysed direct alkynylation of trifluoromethyl ketones

A superbase-induced copper-catalyzed direct alkynylation method for trifluoromethyl ketones was developed without any additional ligand under mild conditions.