Chiral C2-Symmetric CuII Complexes as Catalysts for Enantioselective Hetero-Diels–Alder Reactions

Air-stable and recyclable , the Cu^II–(bis)oxazoline complex 1 efficiently catalyzes diastereo- and enantioselective hetero-Diels–Alder reactions between β,γ-unsaturated α-keto acid derivatives 2 and vinyl ethers for the synthesis of substituted dihydropyrans 3 [Eq. (1)]. Results of the crystal structure analysis of 1 in combination with calculations on model compounds indicate the formation of a reactive intermediate through replacement of the two H₂O ligands with a chelating substrate. X=OEt, N(OMe)Me; R=alkyl, aryl, alkoxy, thiobenzyl.     

Synthesis of Novel Chiral Diol Ligands for the Enantioselective Reactions

The chiral diols ((,(,((,((-Tetraaryl-1,3-dioxolane-4,5-dimethanols) have been used for the formation of cyclic titanates and similar derivatives of Mg, Al and Zr. These complexes of oxophilic metal centers were employed in catalytic and stiochiometric enantioselective reactions such as enantioselective addition reactions of carbon-centered nucleophiles to aldehydes1, [2+2] cycloadditions2, and Diels-Alder reactions3. So far, the two hydrogen atoms of all this type of diols are in trans form…     

手性催化剂在不对称羰基ene反应中的应用

手性高烯丙基醇类化合物是一类非常重要的有机合成中间体,被广泛应用于药物分子和天然产物的合成中,不对称羰基ene反应是构建此类化合物最有效的方法之一,近年来已经取得了巨大进展。目前,在不对称羰基ene反应中的手性催化剂主要有如Mg、Ca、Sc、Ti、Co、Ni、Cu、Rh、Pd、Pt等金属与手性配体络合形成的配合物,以及一些手性有机小分子,大部分都取得了较好的催化活性与对映选择性。本文就此评述了各类手性催化剂在不对称羰基ene反应中的应用,不对称诱导反应的机理,以及催化剂分子结构及反应条件对催化活性和对映选择性的影响。     

Highly Enantioselective [3+2] Coupling of Indoles with Quinone Monoimines Promoted by a Chiral Phosphoric Acid

Highly enantioselective [3+2] coupling of 3‐substituted indoles with quinone monoimines promoted by a chiral phosphoric acid has been reported. A large variety of benzofuroindolines were prepared in moderate to good yields (up to 98 %) with generally excellent enantioselectivities (up to 99 % ee).     

Chiral Organocatalysts in Enantioselective CO2 Utilization Reactions

The development of efficient CO₂ utilization reactions has gained a significant amount of attention in recent years. Although transformations of CO₂ to produce basic chemicals have been extensively investigated, the development of catalytic enantioselective CO₂ utilization reactions for the preparation of fine chemicals remains limited at this stage. Several excellent methods for catalytic enantioselective CO₂ utilization using chiral metal complex catalysts have been reported. Many researchers have also focused on developing organocatalyzed approaches to enantioselective CO₂ utilization, and several excellent examples have appeared in recent years. Herein, recent advances in catalytic enantioselective CO₂ utilization reactions using chiral organocatalysts are reviewed to provide a forecast for organocatalyzed enantioselective CO₂ utilization research.     

Chiral Guanidine Catalyzed Enantioselective Reactions

Pinches like a crab : The utilization of guanidines as catalysts has been growing at a steady pace. They possess high pK_a values and activate through dual hydrogen‐bonding modes of activation. This Focus Review highlights the development of chiral guanidine catalysts in asymmetric organic transformations.

     

C2-Symmetric Bicyclic Diols as Chiral Ligands in the titanate-catalyzed enantioselective addition of alkylzinc reagents to aldehydes

Enantiomerically pure C₂-symmetric 1,4-diols embodying bicyclic C-frameworks were synthesized by means of asymmetric carbo-Diels-Alder reactions as key steps (Scheme 1). They were investigated as chiral ligands in the enantioselective addition of ZnEt₂ to aromatic aldehydes. In the presence of 20–40 mol-% of the titanates formed from these diols and [Ti(i-PrO)₄] at ?78°, the respective 1-arylpropanols were obtained with enantiomer ratios up to 93:7 (Scheme 2, Table).     

Enantioselective Diels-Alder Reactions with G-Quadruplex DNA-Based Catalysts

DNA in command: An enantioselective Diels-Alder reaction can be achieved using human telomeric G-quadruplex DNA-based catalysts. The absolute configuration of the product can be reversed when the conformation of G-quadruplex DNA is switched from antiparallel to parallel, and both the reaction rate and the enantioselectivity of the Diels-Alder reaction were found to be dependent on the DNA sequence.     

Enantioselective Aldol Reactions Catalyzed by Chiral Phosphine Oxides

The development of enantioselective aldol reactions catalyzed by chiral phosphine oxides is described. The aldol reactions presented herein do not require the prior preparation of the masked enol ethers from carbonyl compounds as aldol donors. The reactions proceed through a trichlorosilyl enol ether intermediate, formed in situ from carbonyl compounds, which then acts as the aldol donor. Phosphine oxides activate the trichlorosilyl enol ethers to afford the aldol adducts with high stereoselectivities. This procedure was used to realize a directed cross‐aldol reaction between ketones and two types of double aldol reactions (a reaction at one/two α position(s) of a carbonyl group) with high diastereo‐ and enantioselectivities.     

Highly Enantioselective Aldol Reactions between Acetaldehyde and Activated Acyclic Ketones Catalyzed by Chiral Primary Amines

Highly enantioselective cross‐aldol reactions between acetaldehyde and activated acyclic ketones are reported for the first time. Various acyclic ketones, such as saturated and unsaturated keto esters, reacted with acetaldehyde in the presence of a chiral primary amine and a Brønsted acid to afford optically enriched tertiary alcohols in good yields and with excellent enantioselectivities. Trifluoromethyl ketones were tolerable under the reaction conditions, thereby affording the trifluoromethyl carbinol in good‐to‐excellent yields and enantioselectivities. Structural modification of the chiral amines from the same chiral source switched the stereoselectivity of the products. The utility of aldol chemistry was demonstrated in the brief synthesis of functionally enriched δ‐lactones. Theoretical calculations on the transition‐state structure indicated that the protonated tertiary amine could effectively activate the carbonyl group of a keto ester to promote the addition process through hydrogen‐bonding interaction and, simultaneously, provide an appropriate attacking pattern for the approach of the keto ester to the enamine, which is formed from acetaldehyde and the chiral catalyst, on a particular face, resulting in high enantioselectivity.     

Synthesis of Novel Chiral C2-Symmetric Bisoxazoline Ligands Containing 2,5-Di( m -substituted)phenyl-1,3,4-oxadiazole

Abstract

Seven novel chiral C₂-symmetric substituted bisoxazoline ligands containing 2,5-di(m-substituted)phenyl-1,3,4-oxadiazole have been synthesized from 2,5-di-(m-carboxylphenyl)-1,3,4-oxadiazole and aminoalcohol by NaOH or Et₃N cyclization method via halogenated amide intermediate.     

Synthesis of C 2-Symmetric Chiral Amino Alcohols: Their Usage as Organocatalysts for Enantioselective Opening of Epoxide Ring

A series of β-amino alcohols derivatives were synthesized from (R)-2-amino-1-butanol and (S)-1,2-propanediol, and they have been used as organocatalaysts in the racemic ring opening of epoxide in good yields with high enantiomeric excess (up to 97%).     

Enantioselective Synthesis of Biarylphenylglycines and Their Use as Chiral Ligands in the Lewis Acid Catalyzed Diels-Alder Reaction

The enantioselective syntheses of naphthyl-substituted-N-tosylphenylglycines are described. These unnatural amino acids were tested as ligands for the chiral oxazaborolidine catalyzed Diels-Alder reaction between cyclopentadiene and 2-bromoacrolein.     

The Enantioselective Addition of Diethylzinc to Aldehydes Catalyzed by 2-Methylquinoline Derived Chiral Ligands

One of the most important and fundamental synthetic procedures for the establishment of a carbon-carbon bond stereoselectively is the enantioselective addition of organometallic reagents to aldehydes affording chiral secondary alcohols1. In this process, a frequently used method is to perform the reaction in the presence of a chiral ligand such as amino alcohol2. Here, we first report that two new chiral ligands3, which were readily available by the reaction of chiral ketone with lithiated 2-m…     

Asymmetric Baylis–Hillman Reactions Promoted by Chiral Imidazolines

The coupling of electrophiles with activated alkenes by using tertiary amines or phosphines is generally known as the Baylis–Hillman reaction. It is a useful and atom‐economical carbon–carbon bond‐forming reaction that generates multifunctionalized products. This reaction is notoriously slow; yields are often low and substrate‐dependent. The asymmetric reaction is still limited especially for unactivated olefins such as acrylates. Imidazolines have been developed as ligands in metal‐catalyzed reactions and have also been used as privileged structures in diversity‐oriented synthesis. A series of novel chiral imidazolines were prepared and used to develop asymmetric Baylis–Hillman reactions. These imidazolines promote the reactions of various aromatic aldehydes with unactivated acrylates. Enantiomeric excesses of up to 60 % and high yields were obtained by using stoichiometric amounts of the promoter. Furthermore, the imidazolines are also suitable promoters for the reactions between aromatic aldehydes and alkyl vinyl ketones. Enantiomeric excesses of up to 78 % and high yields were obtained with 50 mol % of an imidazoline with a chiral methylnaphthyl group. These chiral imidazolines are easily prepared from commercially available amino alcohols and can be easily recovered for reuse without loss of product enantioselectivity.