Enantioselective Synthesis of anti-β-Hydroxy-α-amido Esters via Transfer Hydrogenation

Enantioselective Transfer Hydrogenation of α-Methoxyimino-β-keto Esters

α-Methoxyimino-β-keto esters are reported to undergo highly enantioselective catalytic transfer hydrogenation using the Noyori-Ikariya complex RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of formic acid-triethylamine and dimethylformamide at 25 °C. The experimental study performed on over 25 substrates combined with computational analysis revealed that a Z-configured methoxyimino group positioned alpha to a ketone carbonyl leads to higher reactivity and mostly excellent enantioselectivity within this substrate class. Density functional theory calculations of competing transition states were used in rationalizing the origins of enantioselectivity and the possible role of the methoxyimino group in the reaction outcome.

Ru(II)-Catalyzed Asymmetric Transfer Hydrogenation of α-Alkyl-β-Ketoaldehydes via Dynamic Kinetic Resolution

The (R,R)-Teth-TsDPEN-Ru(II) complex promoted the one-pot double C=O reduction of α-alkyl-β-ketoaldehydes through asymmetric transfer hydrogenation/dynamic kinetic resolution (ATH-DKR) under mild conditions. In this process, ten anti-2-benzyl-1-phenylpropane-1,3-diols (85:15 to 92:8 dr) were obtained in good yields (41-87%) and excellent enantioselectivities (>99% ee for all compounds). Notably, the preferential reduction of the aldehyde moiety led to the in situ formation of 2-benzyl-3-hydroxy-1-phenylpropan-1-one intermediates. These intermediates played a crucial role in enhancing both reactivity and stereoselectivity through hydrogen bonding.

Regio- and Enantioselective Synthesis of 1,2-Diamines by Formal Hydroamination of Enamines: Scope, Mechanism, and Asymmetric Synthesis of Orthogonally Protected Bis-Piperazines as a Privileged Scaffold

We have previously reported the first formal hydroamination of enamines for the synthesis of chiral 1,2-diamines. Here, we describe: (i) the discovery, optimization, and substrate expansion of this reaction; (ii) a novel and straightforward protocol for the "click-type" synthesis of enamines in quantitative yield utilizing sodium sulfate in a dual role as an ancillary and dehydrating agent without the need for workup or purification; (iii) the application of this methodology to the first enantioselective synthesis of orthogonally protected 1,1'-(1-(4-fluorophenyl)ethane-1,2-diyl) piperazines, a scaffold for rapid lead optimization in drug discovery; (iv) a computational investigation into the mechanism and rationalization of the enantioselectivities of the reaction.

Tetramate Derivatives by Chemoselective Dieckmann Ring Closure of threo-Phenylserines and Their Antibacterial Activity

A general route, which provides direct access to substituted bicyclic tetramates, making use of Dieckmann cyclization of oxazolidines derived from threo-arylserines, is reported; the latter were found to be available by an efficient aldol-like reaction of glycine with some substituted benzaldehydes under alkaline conditions. The tetramates were found to release chelated metal cations acquired during chromatographic purification by mild acid wash. Some compounds in the library showed good antibacterial activity against Gram-positive bacteria. Cheminformatic analysis demonstrates that the most active compounds were Ro5-compliant and occupy a narrow region of chemical space, distinct from that occupied by other known antibiotics, with the most potent compounds having 399 < M_w < 530 Da; 3.5 < cLogP < 6.6; 594 < MSA <818 Ų; 9.6 < rel. PSA <13.3%. MIC values were shifted to higher concentrations when tested in the presence of HSA or blood, but was not completely abolished, consistent with a plasma protein binding (PPB) effect.

Synthesis of MeBmt and related derivatives via syn-selective ATH-DKR

The unusual α-amino, β-hydroxy acid MeBmt is a key structural feature of cyclosporin A, an important naturally occurring immunosuppressant and antiviral agent. We present a convergent synthesis of MeBmt which relies on new aspects of dynamic kinetic resolution (DKR) to establish simultaneously the chirality at C(2) and C(3). We also show that this route is applicable to the synthesis of other derivatives.

Asymmetric Transfer Hydrogenation of rac-α-(Purin-9-yl)cyclopentones via Dynamic Kinetic Resolution for the Construction of Carbocyclic Nucleosides

An asymmetric transfer hydrogenation via dynamic kinetic resolution of a broad range of rac- α-(purin-9-yl)cyclopentones was first developed. A series of cis-β-(purin-9-yl)cyclopentanols were obtained with up to 97% yield, >20/1 dr, and >99% ee. This also provides an efficient synthetic route to a variety of chiral carbocyclic nucleosides.

Enantioselective synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters: application to the total synthesis of (+)-alexine

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed. This method gives moderate to excellent yields with high chemo-, diastereo- and enantioselectivities for a broad range of substrates. In order to highlight the versatility of the methodology it was applied in an efficient asymmetric synthesis of the polyhydroxylated pyrrolizidine alkaloid (+)-alexine.

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed.

Access to Anti or Syn 2-Amino-1,3-diol Scaffolds from a Common Decarboxylative Aldol Adduct

A straightforward synthetic pathway allowing the access to anti or syn 2-amino-1,3-diol scaffolds is presented. The strategy relies on a diastereoselective organocatalyzed decarboxylative aldol reaction of a N-Boc-hemimalonate that is easily formed from commercial N-Boc-diethyl malonate. Although this method has been optimized previously with the N-Bz-hemimalonate analogue, this key step was reinvestigated with the N-Boc derivative to improve the required reaction time, the yield, and the diastereoselectivity. The new conditions enhance this transformation, and quantitative yields and anti/syn ratios up to 96:4 can be obtained. The anti aldol product was easily isolated in pure form and then taken forward as the key precursor in the preparation of both a set of ten N-/O-alkylated anti 2-amino-1,3-diol derivatives and the syn congeners.

Rhodium-mediated asymmetric transfer hydrogenation: a diastereo- and enantioselective synthesis of syn-α-amido β-hydroxy esters

The use of a Rh- instead of a Ru-complex in the asymmetric transfer hydrogenation of α-benzoylamino β-keto esters allowed a reversal of diastereoselectivity and an efficient access to a variety of syn α-amido β-hydroxy esters.

Stereoselective and Catalytic Synthesis of anti-β-Alkoxy-α-azido Carboxylic Derivatives

Direct addition of a chiral N-azidoacetyl thiazolidinethione to a variety of dialkyl acetals catalyzed by a commercially available and structurally simple nickel(II) complex gives access in good yields and a highly stereocontrolled manner to anti-β-alkoxy-α-azido carboxylic derivatives which, in turn, can be easily converted into a wide array of enantiomerically pure compounds.

A new cost-effective Ru-chloramphenicol base derivative catalyst for the asymmetric transfer hydrogenation/dynamic kinetic resolution of N-Boc α-amino-β-ketoesters and its application to the synthesis of the chiral core of vancomycin

Herein we describe the application of Ru-chloramphenicol base complexes catalyzed highly diastereo- and enantioselective transfer hydrogenation of N-Boc α-amino-β-ketoesters for the asymmetric synthesis of anti-N-Boc-β-hydroxy-α-amino esters.

Efficient Synthesis of Differentiated syn-1,2-Diol Derivatives by Asymmetric Transfer Hydrogenation-Dynamic Kinetic Resolution of α-Alkoxy-Substituted β-Ketoesters

Asymmetric transfer hydrogenation was applied to a wide range of racemic aryl α-alkoxy-β-ketoesters in the presence of well-defined, commercially available, chiral catalyst Ru(II) -(N-p-toluenesulfonyl-1,2-diphenylethylenediamine) and a 5:2 mixture of formic acid and triethylamine as the hydrogen source. Under these conditions, dynamic kinetic resolution was efficiently promoted to provide the corresponding syn α-alkoxy-β-hydroxyesters derived from substituted aromatic and heteroaromatic aldehydes with a high level of diastereoselectivity (diastereomeric ratio (d.r.)>99:1) and an almost perfect enantioselectivity (enantiomeric excess (ee)>99 %). Additionally, after extensive screening of the reaction conditions, the use of Ru(II) - and Rh(III) -tethered precatalysts extended this process to more-challenging substrates that bore alkenyl-, alkynyl-, and alkyl substituents to provide the corresponding syn α-alkoxy-β-hydroxyesters with excellent enantiocontrol (up to 99 % ee) and good to perfect diastereocontrol (d.r.>99:1). Lastly, the synthetic utility of the present protocol was demonstrated by application to the asymmetric synthesis of chiral ester ethyl (2S)-2-ethoxy-3-(4-hydroxyphenyl)-propanoate, which is an important pharmacophore in a number of peroxisome proliferator-activated receptor α/γ dual agonist advanced drug candidates used for the treatment of type-II diabetes.

Enantioconvergent synthesis of functionalized γ-butyrolactones via (3 + 2)-annulation

A dynamic kinetic resolution of β-halo α-keto esters in an asymmetric homoenolate reaction is described. A chiral N-hetereocyclic carbene catalyzes the a(3) → d(3)-umpolung addition of α,β-enals to racemic α-keto esters, forming γ-butyrolactones with three contiguous stereocenters. The addition occurs with high regio-, diastereo-, and enantiocontrol. This methodology constitutes an intermolecular DKR process to set three stereocenters during the key bond forming event.

Asymmetric transfer hydrogenation of α-amino β-keto ester hydrochlorides through dynamic kinetic resolution

The development of Ru-catalyzed asymmetric transfer hydrogenation of α-amino β-keto ester hydrochlorides is described.

Stereocontrolled synthesis of syn-β-Hydroxy-α-amino acids by direct aldolization of pseudoephenamine glycinamide

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55-98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

Stereoselective synthesis of 4-substituted-cyclic sulfamidate-5-carboxylates by asymmetric transfer hydrogenation accompanied by dynamic kinetic resolution and applications to concise stereoselective syntheses of (−)-epi-cytoxazone and the taxotere side-chain

DKR driven, asymmetric transfer hydrogenations of cyclic sulfamidate imine-5-carboxylates were developed.

Asymmetric synthesis of anti-β-amino-α-hydroxy esters via dynamic kinetic resolution of β-amino-α-keto esters

A method for the asymmetric synthesis of enantioenriched anti-α-hydroxy-β-amino acid derivatives by enantioconvergent reduction of the corresponding racemic α-keto esters is presented. The requisite α-keto esters are prepared via Mannich addition of ethyl diazoacetate to imines followed by oxidation of the diazo group with Oxone. Implementation of a recently developed dynamic kinetic resolution of β-substituted-α-keto esters via Ru(II)-catalyzed asymmetric transfer hydrogenation provides the title motif in routinely high diastereo- and enantioselectivity.

Asymmetric synthesis of diverse glycolic acid scaffolds via dynamic kinetic resolution of α-keto esters

The dynamic kinetic resolution of α-keto esters via asymmetric transfer hydrogenation has been developed as a technique for the highly stereoselective construction of structurally diverse β-substituted-α-hydroxy carboxylic acid derivatives. Through the development of a privileged m-terphenylsulfonamide for (arene)RuCl(monosulfonamide) complexes with a high affinity for selective α-keto ester reduction, excellent levels of chemo-, diastereo-, and enantiocontrol can be realized in the reduction of β-aryl- and β-chloro-α-keto esters.

Asymmetric transfer hydrogenation coupled with dynamic kinetic resolution in water: synthesis of anti-β-hydroxy-α-amino acid derivatives

The use of asymmetric transfer hydrogenation combined with dynamic kinetic resolution for the synthesis of β-hydroxy-α-(tert-butoxycarbonyl)amino esters in water is described. This procedure provides the desired amino alcohols in good yields, diastereoselectivities, and enantioselectivities. A surfactant is employed to achieve good yields due to the hydrophobic nature of both the catalyst and substrate. The reaction setup is operationally simple, and nondegassed water can be used as the solvent.

N-Heterocyclic carbene-catalyzed direct cross-aza-benzoin reaction: Efficient synthesis of α-amino-β-keto esters

An efficient catalytic synthesis of α-amino-β-keto esters has been newly developed. Cross-coupling of various aldehydes with α-imino ester, catalyzed by N-heterocyclic carbene, leads chemoselectively to α-amino-β-keto esters in moderate to good yields with high atom efficiency. The reaction mechanism is discussed, and it is proposed that the α-amino-β-keto esters are formed under thermodynamic control.

Dynamic kinetic resolution of α-keto esters via asymmetric transfer hydrogenation

The dynamic kinetic resolution of β-aryl α-keto esters has been accomplished using a newly designed (arene)RuCl(monosulfonamide) transfer hydrogenation catalyst. This dynamic process generates three contiguous stereocenters with remarkable diastereoselectivity through a reduction/lactonization sequence. The resulting enantioenriched, densely functionalized γ-butyrolactones are of high synthetic utility, as highlighted by several secondary derivatizations.