Catalytic Enantioselective Reaction Research Articles

Construction of Chiral β-Trifluoromethyl Alcohols Enabled by Catalytic Enantioselective Aldol-Type Reaction of CF3CHN2.

A zinc-/quinine-mediated enantioselective Aldol-type reaction of trifluorodiazoethane (CF3CHN2) with various aldehydes is described. This study demonstrated the feasibility of utilizing CF3CHN2 as an effective hard nucleophile in catalytic asymmetric transformations. Furthermore, the synthetic utility of this protocol is exemplified by the construction of a diverse set of chiral β-trifluoromethylated alcohols, including a valuable HDAC inhibitor precursor.

A 1,3,2‐Diazaphospholene‐Catalyzed Reductive Claisen Rearrangement

Abstract1,3,2‐Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O‐bound phospholene enolates and provide a proof‐of‐principle for catalytic enantioselective reactions.

A 1,3,2-Diazaphospholene-Catalyzed Reductive Claisen Rearrangement.

1,3,2-Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O-bound phospholene enolates and provide a proof-of-principle for catalytic enantioselective reactions.

Catalytic Enantioselective Aldol‐Type Reaction Using α‐Fluorinated Enolates

AbstractOptically active α‐fluoroalkyl substituted alcohols are widely present in drugs and pharmaceutically active compounds. Catalytic enantioselective aldol‐type reaction using fluorinated enolates constitutes a powerful method for the synthesis of α‐fluorinated β‐hydroxy carbonyl compounds, which are versatile building blocks to access α‐fluorinated alcohols bearing different functionalities through an array of transformations based on carbonyl group. This Focus Review summarizes recent achievements of the catalytic enantioselective aldol reactions using fluorinated enolates derived from five different types of precursors, discusses in detail the limitation of each method, and outlines synthetic opportunities still present.

Benzophenone Schiff bases of glycine derivatives: Versatile starting materials for the synthesis of amino acids and their derivatives

This review focuses on the introduction and early development, in solution, of phase-transfer catalyzed (PTC) reactions to afford racemic or enantioenriched natural and unnatural amino acids. To form monosubstituted amino acids alkylation reactions are performed on the benzophenone Schiff base of glycine. For α,α-disubstituted amino acids the activated intermediate is an aldimine derivative of the monosubstituted amino acid. Enantioenriched products are produced by organocatalysis using derivatives of Cinchona alkaloids as the phase-transfer catalyst. Selectivity for monoalkylatation and lack of product racemization depend on the acidities of the glycine imines, and dialkylated products are formed from aldimine esters of monoalkyl amino acids. The racemic and catalytic enantioselective reactions of a cationic glycine equivalent with organoboranes, organometallics and malonate anion are discussed as are other reactions of these versatile Schiff bases derivatives.

Asymmetric α-Amination Reaction of Alkenoate Cyclic Esters Catalyzed by Chiral Tin Alkoxides

A catalytic enantioselective α-amination reaction of alkenoate cyclic esters with dialkyl azodicarboxylates was achieved by using a 3,3′-di(1-naphthyl)-substituted (R)-BINOL–dibromostannane complex as a chiral precatalyst in the presence of a sodium alkoxide and an alcohol. Optically active α-hydrazino ketones were obtained in moderate to high yields and with up to 91% ee in the presence of the chiral tin alkoxide generated in situ.

Enantioselective Catalytic Transformations of Barbituric Acid Derivatives

Since the beginning of the 20th century, numerous research efforts made elegant use of barbituric acid derivatives as building blocks for the elaboration of more complex and useful molecules in the field of pharmaceutical chemistry and material sciences. However, the construction of chiral scaffolds by the catalytic enantioselective transformation of barbituric acid and derivatives has only emerged recently. The specific properties of these rather planar scaffolds, which also encompass either a high Brønsted acidity concerning the native barbituric acid or the marked electrophilic character of alkylidene barbituric acids, required specific developments to achieve efficient asymmetric processes. This review covers the enantioselective catalytic reactions developed for barbituric acid platforms using an organocatalytic and metal-based enantioselective sequences. These achievements currently allow several unique addition and annulation reactions towards the construction of high valued chiral heterocycles from barbituric acid derivatives along with innovative enantioselective developments.

Noncovalent Catalysis for Enantioselective Direct Aldol Reaction of 3‐Acetylcoumarins to Pyrazole‐4,5‐diones

AbstractThe first catalytic enantioselective direct aldol reaction of 3‐acetylcoumarins to pyrazole‐4,5‐diones is reported and shown to proceed through noncovalent substrate activation by a quinine‐derived bifunctional tertiary amino‐amide as the catalyst. The densely functionalized products, consisting of an oxygen‐containing quaternary stereocenter and bearing two biologically relevant heterocycles, are generally obtained in high yields with moderate to excellent enantioselectivities.

Desymmetrization of meso-bisphosphates using copper catalysis and alkylzirconocene nucleophiles

The desymmetrization of meso-compounds is a useful synthetic method, as illustrated by numerous applications of this strategy in natural product synthesis. Cu-catalyzed allylic desymmetrizations enable the enantioselective formation of carbon-carbon bonds, but these transformations are limited in substrate scope and by the use of highly reactive premade organometallic reagents at cryogenic temperatures. Here we show that diverse meso-bisphosphates in combination with alkylzirconium nucleophiles undergo highly regio-, diastereo- and enantio-selective Cu-catalyzed desymmetrization reactions. In addition, C2-symmetric chiral bisphosphates undergo stereospecific reactions and a racemic substrate undergoes a Cu-catalyzed kinetic resolution. The reaction tolerates functional groups incompatible with many common organometallic reagents and provides access to a broad range of functionalized carbo- and hetero-cyclic structures. The products bear up to three contiguous stereogenic centers, including quaternary centers and spirocyclic ring systems. We anticipate that the method will be a useful complement to existing catalytic enantioselective reactions.

Catalytic enantioselective iodolactonization reactions.

The halolactonization reaction is a useful chemical transformation for the construction of lactones from γ- or δ-substituted alkenoic carboxylic acids or carboxylic esters. Traditionally, the stereoselectivity of these reactions has been controlled by the substrates or the reagents. The substrate-controlled method has been extensively studied and applied in the synthesis of many natural products. However, catalytic, enantioselective iodolactonizations of γ- or δ-substituted alkenoic carboxylic acids have only recently been developed. This review article highlights the advances that have emerged over the last decade.

Dinuclear zinc cooperative catalytic three-component reactions for highly enantioselective synthesis of 3,3'-dihydrofuran spirooxindoles.

A new dinuclear zinc cooperative catalytic enantioselective three-component reaction via a domino Knoevenagel /Michael/cyclization sequence has been developed. A series of chiral 3,3'-dihydrofuran spirooxindoles are obtained in excellent yields (up to 99%) and enantioselectivities (up to 99% ee). In addition, the reaction is carried out smoothly on a gram scale under mild conditions.

Catalytic enantioselective Michael addition reactions between in situ detrifluoroacetylatively generated 3-fluorooxindole-derived enolates and 1-(1-(phenylsulfonyl)vinylsulfonyl)benzene

This research presents the first example of catalytic enantioselective Michael addition reactions between 1-(1-(phenylsulfonyl)vinylsulfonyl)benzene and detrifluoroacetylatively in situ generated 3-fluorooxindole-derived enolates. The optimal catalyst includes Cu(OTf)2 and (1S,2S)-1,2-diphenylethane-1,2-diamine featuring NHR functionalities. The reactions are conducted in THF at -20 °C using DIPEA as a base. The additions show reasonably high degree of structural generality and, therefore, synthetic value for preparation of a new family of compounds possessing pharmacophoric 3-fluorooxindole moiety, solofeaturing quaternary C-F stereogenic center.

Recent advances in catalytic Wittig-type reactions based on P(III)/P(V) redox cycling

Abstract Numerous organic transformations are based on the use of stoichiometric amounts of phosphorus reagents. The formation of phosphane oxides from phosphanes is usually the thermodynamic driving force for these reactions. The stoichiometric amounts of phosphane oxide which are formed as by-products often significantly hamper the product purification. Organophosphorus catalysis based on P(III)/P(V) redox cycling aims to address these problems. Herein we present our recent advances in developing catalytic Wittig-type reactions. More specifically, we reported our results on catalytic Wittig reactions based on readily available Bu3P=O as pre-catalyst as well as the first microwave-assisted version of this reaction and the first enantioselective catalytic Wittig reaction utilizing chiral phosphane catalysts. Further developments led to the implementation of catalytic base-free Wittig reactions yielding highly functionalized alkylidene and arylidene succinates.

Catalytic Enantioselective Aldol Reactions of Unprotected Carboxylic Acids under Phosphine Oxide Catalysis

AbstractThe first catalytic enantioselective aldol reaction of various unprotected carboxylic acids is described. In the presence of a chiral bis(phosphine oxide) as a Lewis base catalyst, carboxylic acids were activated with silicon tetrachloride to form the corresponding bis(trichlorosilyl)enediolates in situ, which subsequently underwent an aldol reaction with an aldehyde or a ketone to produce β‐hydroxycarboxylic acids in high enantioselectivities of up to 92 % ee.

Catalytic Enantioselective Aldol Reactions of Unprotected Carboxylic Acids under Phosphine Oxide Catalysis.

The first catalytic enantioselective aldol reaction of various unprotected carboxylic acids is described. In the presence of a chiral bis(phosphine oxide) as a Lewis base catalyst, carboxylic acids were activated with silicon tetrachloride to form the corresponding bis(trichlorosilyl)enediolates in situ, which subsequently underwent an aldol reaction with an aldehyde or a ketone to produce β-hydroxycarboxylic acids in high enantioselectivities of up to 92 % ee.

Catalytic Enantioselective Decarboxylative Mannich-Type Reaction of N-Unprotected Isatin-Derived Ketimines.

The first catalytic enantioselective decarboxylative Mannich-type reaction of N-unprotected ketimines is reported, directly providing N-unprotected 3-tetrasubstituted 3-aminooxindoles in high yield and ee without protection/deprotection steps. The utility of this reaction is demonstrated in the short step synthesis of (+)-AG-041R.

The Important Role of the Byproduct Triphenylphosphine Oxide in the Magnesium(II)-Catalyzed Enantioselective Reaction of Hemiacetals and Phosphorus Ylides.

By employing a simple in situ generated magnesium catalyst, a direct asymmetric reaction between hemiacetals and phosphorus ylides was achieved through a tandem Wittig-oxa-Michael reaction sequence. Enantioenriched chromans, isochromans, and tetrahydropyrans were obtained in good chemical yields, and (-)-erythrococcamide B was synthesized in enantioenriched form. The byproduct triphenylphosphine oxide was identified as a necessary additive for this process.

The Important Role of the Byproduct Triphenylphosphine Oxide in the Magnesium(II)‐Catalyzed Enantioselective Reaction of Hemiacetals and Phosphorus Ylides

AbstractBy employing a simple in situ generated magnesium catalyst, a direct asymmetric reaction between hemiacetals and phosphorus ylides was achieved through a tandem Wittig–oxa‐Michael reaction sequence. Enantioenriched chromans, isochromans, and tetrahydropyrans were obtained in good chemical yields, and (−)‐erythrococcamide B was synthesized in enantioenriched form. The byproduct triphenylphosphine oxide was identified as a necessary additive for this process.

Recent Advances in the Catalytic Enantioselective Reactions of para-Quinone Methides.

In recent years, the catalytic enantioselective reactions of p-QMs have drawn much attention, because of their distinctive reactivities. As such, a variety of chiral catalytic systems have been developed. This Focus Review describes the reactions of p-QMs according to the chiral catalyst system used, with the aim of provide the first complete picture of the exciting developments in this field. Special emphasis is placed on recent work regarding their reaction mechanisms.

Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis‐Dioxidomolybdenum(VI) Complexes

Two enantiomerically pure chiral dioxidomolybdenum(VI) complexes (1 and 2) of the type [MoO2L] (L = dianionic, tetradentate ONNO‐ligand) were synthesized and investigated in enantioselective oxidation reactions. The solid‐state structures of complex 1 and 2, determined via single‐crystal X‐ray diffraction analysis revealed two fundamentally different coordination geometries: a C1‐symmetric cis‐β isomer (Λ‐1), and a C2‐symmetric cis‐α isomer (Δ‐2). In both cases, only one of the two possible helical enantiomers (Λ‐ or Δ‐helix) was formed. The complexes were examined as precatalysts in the epoxidation of the challenging prochiral substrate trans‐stilbene, using either tert‐butyl hydroperoxide (TBHP) or cumyl hydroperoxide (CHP) as oxidants. The asymmetric cis‐β complex 1 was found to be significantly more active in the epoxidation than its cis‐α counterpart 2, asymmetric induction was, however, negligible for both complexes. The complexes were also tested in catalytic enantioselective sulfoxidation reactions where chiral induction could be achieved, albeit small. The observed putative molybdenum oxido‐peroxido intermediate 1‐O2 could be identified as an important precomplex before formation of the active catalyst in sulfoxidation.