Loss Of Optical Purity Research Articles

Chiral cyclopalladated complex promoted asymmetric synthesis of diester-substituted P,N-ligands via stepwise hydrophosphination and hydroamination reactions

A series of enantiomerically pure 1,2-diester substituted P,N-ligands incorporating two chiral carbons in the backbone were generated in high yields and high stereoselectivity from acetylenedicarboxylate via initial hydrophosphination using diphenylphosphine followed by hydroamination with various primary and secondary amines. The reactions were activated and stereochemically controlled by the organopalladium complex containing ortho-palladated (S)-(1-(dimethylamino)ethyl)naphthalene under mild conditions. The absolute stereochemistry and the coordination chemistry of P,N-products were determined by the single crystal X-ray diffraction analysis. All the chiral P,N-ligands could be liberated from the palladium template without loss of optical purity. Subsequent recomplexation to selected chiral palladium centers confirmed the optical purity of the new functionalized chiral P,N-ligands.

ChemInform Abstract: Stereoselective Synthesis of Substituted 1,3‐Oxazolidines via Pd‐Catalyzed Carboamination Reactions of O‐Vinyl‐1,2‐amino Alcohols.

The stereoselective synthesis of 2,4- and 2,5-disubstituted 1,3-oxazolidines is accomplished via Pd-catalyzed carboamination of O-vinyl-1,2-amino alcohol derivatives. The transformations generate cis-disubstituted products with good to excellent diastereoselectivity, and enantiomerically enriched substrates are converted without loss of optical purity. In addition to yielding synthetically useful products that are difficult to generate with existing methods, these transformations illustrate that electron-rich enol ethers are viable substrates for alkene carboamination processes.

Catalytic Cycloetherification Using Bifunctional Thiourea Catalysts

The Matsubara group presents a catalytic asymmetric cycloetherification for the synthesis of enantioenriched 2-substituted tetra­hydrofuran (THF) derivatives via an intramolecular oxy-Michael addition using bifunctional thiourea catalyst 1. 2-Substituted THFs were obtained in high enantioselectivities and excellent isolated yields in the presence of 3 mol% of quinine-derived bifunctional catalyst 1. The authors show that the catalyst loading could be lowered to 1 mol% without loss of optical purity. Also, product 2 was transformed to a valuable synthetic intermediate 3 by means of Baeyer-Villiger oxidation and reduction.

Catalytic Asymmetric Synthesis of Isoxazoline-N-oxides through Conjugate Addition–Cyclization under Phase-Transfer Conditions

Abstract Catalytic asymmetric synthesis of various isoxazoline-N-oxides having a tetrasubstituted carbon has been accomplished by asymmetric phase-transfer conjugate addition of bromomalonate to nitroolefins and subsequent ring-closing O-alkylation. The obtained isoxazoline-N-oxide was readily converted to the corresponding oxime, isoxazoline, and lactam without loss of optical purity.

Synthesis of 4-Quinolones via Cyclocondensation of Substituted ortho-Amidoacetophenones: A Refit to the Camps Cyclization by Applying Trimethylsilyl Trifluoromethanesulfonate/Triethylamine

A modification of the classical Camps cyclization is described. A series of substituted 4-quinolone derivatives is prepared via trimethylsilyl trifluoromethanesulfonate/triethylamine induced cyclocondensation of substituted ortho-amidoacetophenones. The process shows a broad substrate scope and allows selective preparation of 2-aryl- and 2-alkyl-substituted 4-quino­lones. Enantiopure starting materials react without loss of optical purity using the modified conditions. Subsequent transformations of the products involving preparation of a 4-quinolyl nonaflate and O-selective methylation are also described.

Stereoselective Synthesis of Substituted 1,3-Oxazolidines via Pd-Catalyzed Carboamination Reactions of O-Vinyl-1,2-Amino Alcohols

The stereoselective synthesis of 2,4- and 2,5-disubstituted 1,3-oxazolidines is accomplished via Pd-catalyzed carboamination of O-vinyl-1,2-amino alcohol derivatives. The transformations generate cis-disubstituted products with good to excellent diastereoselectivity, and enantiomerically enriched substrates are converted without loss of optical purity. In addition to yielding synthetically useful products that are difficult to generate with existing methods, these transformations illustrate that electron-rich enol ethers are viable substrates for alkene carboamination processes.

Silver-Catalyzed Carbon Dioxide Incorporation and Rearrangement on Propargylic Derivatives

Abstract A silver/DBU catalyst system was developed for the effective synthesis of cyclic carbonate and oxazolidinone from the reaction of CO2 with propargylic alcohols and propargylic amines, respectively, in high yields under mild conditions. It was found that the [3,3]-sigmatropic Meyer–Schuster-type rearrangement of the propargylic alcohol was mediated by CO2 in DMF to afford the corresponding α,β-unsaturated carbonyl compounds in high yields. The silver salt combined with the chiral Schiff base ligand could be applied to enantioselective chemical CO2 incorporation into various bispropargylic alcohols to produce the corresponding cyclic carbonate in high yields with high enantioselectivity. The absolute configuration was determined by VCD spectroscopy as well as by X-ray analysis. These products were found to be active for the aminolysis reaction to afford the corresponding carbamate derivatives in high yields without any loss of optical purity.

Enantioselective gem-Chlorofluorination of Active Methylene Compounds Using a Chiral Spiro Oxazoline Ligand

Highly enantioselective gem-chlorofluorination of active methylene compounds was carried out by using a copper(II) complex of a chiral spiro pyridyl monooxazoline ligand. This reaction yielded α-chloro-α-fluoro-β-keto esters and α-chloro-α-fluoro-β-keto phosphonates with up to 92% ee. The resulting dihalo β-keto ester was converted into various α-fluoro-α-heteroatom-substituted carbonyl compounds via nucleophilic substitution without loss of optical purity. A fully protected β-amino acid with a gem-chlorofluoromethylene function was also synthesized.

Novel Synthesis of Optically Active Bishomotyrosine Derivatives Using the Friedel-Crafts Reaction in Triflic Acid

We report here a novel synthesis of optically active bishomotyrosine. The bishomotyrosine skeleton was constructed by using a Friedel-Crafts reaction between phenol and optically active N-Tfa-Glu(Cl)-OMe in triflic acid under the mild condition. Reduction and subsequent deprotection then afforded bishomotyrosine derivatives without any loss of optical purity.

ChemInform Abstract: Efficient Synthesis of Racemic and Chiral Alkenyl Sulfoxides by Palladium‐Catalyzed Suzuki Coupling.

Alkenyl sulfoxide derivatives are obtained in high yields through a palladium-catalyzed Suzuki/Miyaura cross-coupling reaction of racemic and chiral 1-halo sulfoxides with aryl and alkenyl boronic acids. Chiral substrates react with no loss of optical purity and high optical yields. The reaction takes place with different palladium catalysts, such as Pd(PPh 3 ) 4 or Pd(OAc) 2 /DABCO. Although nitrogen ligands like DABCO lead to an active palladium catalyst, they are less effective than the phosphine ones.

Highly Efficient Kinetic Resolution of Racemic Carboxylic Acids

Shiina and co-workers report a high­y efficient kinetic resolution of racemic α-aryl-alkanoic acids using acyl transfer Birman catalyst 1 (V. B. Birman, X. Li Org. Lett. 2006, 8, 1351) to afford optically enriched esters utilizing achiral ­alcohols (s-factor: up to 484). The obtained bis-(α-naphthyl)-methyl esters could be easily transformed into the corresponding chiral acids, such as ibuprofen and naproxen, without loss of optical purity.

ChemInform Abstract: Preparation of Chiral Aziridines from Chiral Oxiranes with Retention of Configuration.

Chiral oxiranes are converted into chiral aziridines viaβ-hydroxyalkly aryl sulfides and then β-tosylaminoalkyl aryl sulfides without loss of optical purity and with overall retention of configuration.

Efficient synthesis of racemic and chiral alkenyl sulfoxides by palladium-catalyzed Suzuki coupling

Alkenyl sulfoxide derivatives are obtained in high yields through a palladium-catalyzed Suzuki/Miyaura cross-coupling reaction of racemic and chiral 1-halo sulfoxides with aryl and alkenyl boronic acids. Chiral substrates react with no loss of optical purity and high optical yields. The reaction takes place with different palladium catalysts, such as Pd(PPh 3) 4 or Pd(OAc) 2/DABCO. Although nitrogen ligands like DABCO lead to an active palladium catalyst, they are less effective than the phosphine ones.

ChemInform Abstract: Intramolecular Alkylation of an α-Sulfinyl Vinylic Carbanion without Loss of Optical Purity: A Novel Access to Chiral Cyclic Vinylic Sulfoxides.

Novel cyclisation via intramolecular α-alkylation of vinylic sulfoxides was studied and cyclic vinylic sulfoxides of various ring sizes (5–7) were synthesised from both (E)- and (Z)-isomers without loss of optical purity.

ChemInform Abstract: Reaction of Various Oxiranes and Carbon Dioxide. Synthesis and Aminolysis of Five-Membered Cyclic Carbonates.

Various oxiranes reacted with carbon dioxide at 100 °C using lithium bromide as a catalyst under atmospheric pressure to afford the corresponding five-membered cyclic carbonates quantitatively. The rate of the reaction increased as the bulkiness of substituents on the oxirane ring was reduced or an electron-withdrawing group was introduced on the oxirane ring. The stereochemistry of the reaction of oxirane and carbon dioxide was retention without loss of optical purity. When substituted phenylethylene carbonates were reacted with benzylamine, the selectivity to afford secondary alcohol increased as the electron-withdrawing ability of the para-substituent increased.

Asymmetric Synthesis of Chiral Silacarboxylic Acids and Their Ester Derivatives

Sila analogues: The first asymmetric synthesis of silacarboxylic acids with a stereogenic center at the silicon atom has been achieved from chiral nonracemic silanols, without loss of optical purity. Silacarboxylic acids can be converted into their corresponding esters using a Mitsunobu-type reaction.

Routes for the Synthesis of (2S)-2-Methyltetrahydropyran-4-one from Simple Optically Pure Building Blocks

Routes to (2S)-2-methyltetrahydropyran-4-one of high optical purity starting from readily available chiral pool precursors and suitable for large-scale manufacture are described. In one approach, the key step is cyclisation of (S)-5-hydroxyhex-1-en-3-one, derived either from an alkyl (S)-3-hydroxybutyrate or (S)-propylene oxide. Formation of the tetrahydropyran ring directly via an intramolecular oxy-Michael reaction under acid-catalysed conditions resulted in loss of optical purity, whereas proceeding through the intermediate (2S)-2-methyl-2,3-dihydropyran-4-one, via an oxidative Pd-catalysed ring closure, followed by hydrogenation of the alkenyl bond, preserved the optical purity. An alternative approach to (2S)-2-methyl-2,3-dihydropyran-4-one is also reported, again starting from an alkyl (S)-3-hydroxybutyrate by elaboration to a carbonyl-protected (6S)-6-methyl-5,6-dihydropyran-2,4-dione derivative, followed by partial reduction and dehydration. Alternatively, the carbonyl group can be reduced out completely in one step to furnish (2S)-2-methyltetrahydropyran-4-one directly after deprotection.

Diastereoselective alkynylation of chiral phosphinoylimines: preparation of optically active propargylamines

Chiral phosphinoylimines were prepared from methylphenylphosphonamides and tested as new chiral and activated imines. The addition of aluminum acetylides was proved to be highly diastereoselective while lithium or magnesium acetylides gave poor results. The cleavage of the chiral auxiliary was done under mild conditions and allows the recovery of the starting phosphonamide without loss of optical purity.

Efficient Chemoenzymatic Synthesis of Chiral Pincer Ligands

Chiral, nonracemic pincer ligands based on the 6-phenyl-2-aminomethylpyridine and 2-aminomethylbenzo[h]quinoline scaffolds were obtained by a chemoenzymatic approach starting from 2-pyridyl and 2-benzoquinolyl ethanone. In the enantiodifferentiating step, secondary alcohols of opposite absolute configuration were obtained by a baker's yeast reduction of the ketones and by lipase-mediated dynamic kinetic resolution of the racemic alcohols. Their transformation into homochiral 1-methyl-1-heteroarylethanamines occurred without loss of optical purity, giving access to pincer ligands used in enantioselective catalysis.

1,2‐Diarylethanols by Alternative Regioselective Reductive Ring‐Opening of 2,3‐Diaryloxiranes

AbstractNon‐symmetrical trans‐2,3‐diaryloxiranes have been regioselectively opened by catalytic hydrogenation over Pd/C, NaBH4/Pd and [Cp2TiCl]/H2O. Although in the catalytic hydrogenation reactions the epoxides were mainly opened at the β‐carbon with respect to the substituted aryl ring in all cases, with the [Cp2TiCl]/H2O system the regioselectivity was affected by the electronic properties of the aryl residues, the epoxides being opened on the carbon bearing the most electron‐releasing or the least electron‐withdrawing group. With the NaBH4/Pd system different regioisomers were obtained depending on the substituents. Starting from enantioenriched epoxides, no loss of optical purity was observed in the alcohols formed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)