Catalytic asymmetric bromination and chlorination of beta-ketoesters

The first general catalytic asymmetric bromination and chlorination of beta-ketoesters has been developed. The reactions proceed for both acyclic and cyclic beta-ketoesters catalyzed by chiral bisoxazolinecopper(II) complexes giving the corresponding optically active alpha-bromo- and alpha-chloro-beta-ketoesters in high yields and moderate to good enantioselectivities. For the optically active chlorinated products the isolated yields are in the range of 88-99 % and the enantiomeric excesses up to 77 % ee, while the optically active brominated adducts are formed in 70-99 % isolated yield and up to 82 % ee. Based on the absolute configuration of the optically active products, the face selectivity for the catalytic enantioselective halogenation is discussed based on a bidentate coordination of the beta-ketoester to the chiral catalyst and a X-ray structure of chiral alpha,gamma-diketoesterenolatebisoxazolinecopper(II) complex.     

Titanium(IV)-catalyzed enantioselective sulfenylation of beta-ketoesters

A [Ti(TADDOLato)] complex (1) catalyzes the enantioselective sulfenylation of beta-ketoesters using phenylsulfenyl chloride, giving ees of up to 88% and yields of up to 95%. The reaction does not require the presence of a base. [reaction: see text]     

Organocatalytic asymmetric 1,6-additions of beta-ketoesters and glycine imine

The first organocatalytic enantioselective 1,6-addition of beta-ketoesters and benzophenone imine to electron-poor delta-unsubstituted dienes using cinchona alkaloids under phase-transfer conditions is demonstrated. The scope of the reaction for the beta-ketoesters is outlined for reactions with different delta-unsubstituted dienes having ketones, esters, and sulfones as electron-withdrawing substituents giving the corresponding optically active products in good yields and enantioselectivities in the range of 90-99% ee. The 1,6-addition also proceeds with a number of cyclic beta-ketoesters having different ring sizes, ring systems and substituents in high yields and enantioselectivities. The potential of this new organocatalytic 1,6-addition for beta-ketoesters is demonstrated by a two-step synthesis of the bicyclo[3.2.1]octan-8-one structure, a bicyclic bridged skeleton occurring in a variety of natural compounds. Benzophenone imines also undergo the organocatalytic asymmetric 1,6-addition to the activated dienes in high yields and with enantioselectivities from 92% to 98% ee, except in one case. The synthetic utility of this asymmetric reaction is demonstrated by the two-step transformation of the allylated alpha-amino acid derivative to highly attractive optically active pyrrolidines.     

Organocatalytic asymmetric direct alpha-alkynylation of cyclic beta-ketoesters

The first organocatalytic enantioselective direct alpha-alkynylation of beta-ketoesters and 3-acyl oxindoles is described. It is demonstrated that activated beta-halo-alkynes undergo nucleophilic acetylenic substitution catalyzed by chiral phase-transfer compounds to afford the alkynylated products in high yields and excellent enantioselectivities. The potential of the reaction is first demonstrated for various alkynylating reagents having chloride and bromide as the leaving groups and substituents such as allyl and alkyl esters, amides, ketones, and sulfones. These reactions proceed with 74-99% yield and 88-97% ee. Then the scope in nucleophile is demonstrated for a large number of cyclic beta-ketoesters with various ring-sizes and for oxindoles as well. The corresponding optically active products are formed in high yields and with enantioselectivities up to 98% ee. The procedure allows for the stereocontrolled attachment of an ethynyl unit in the alpha-position to the carbonyl compound by facile removal of the activating group, and this has been demonstrated for a number of the optically active allyl esters. Furthermore, the synthesis of optically active 1,4-enynes is also shown. The isolation and characterization by X-ray analysis of the catalyst with p-nitrophenolate as the counterion allowed us to propose a model of the catalyst-substrate intermediate which might account for the observed enantioselectivity of the organocatalytic enantioselective alpha-alkynylation reaction. Furthermore, it is suggested that this intermediate is also the reactive species for a number of other electrophiles adding to beta-ketoesters giving enantioselectivities in the range of 90-98% ee.     

Thermomorphic system with non-fluorous phase-tagged Ru(BINAP) catalyst: facile liquid/solid catalyst separation and application in asymmetric hydrogenation

The thermomorphic BINAP derivative 1 tagged with long alkyl chains was prepared from (S)-5,5'-diamino BINAP and applied to Ru-catalyzed asymmetric hydrogenation of beta-ketoesters under homogeneous conditions in 3:1 (v/v) ethanol/1,4-dioxane at 60 degrees C with high enantioselectivity (up to 98% ee). Results indicated that the Ru(1) catalyst was easily recovered by simple cooling and precipitation and could be used for at least four cycles without any loss of enantioselectivity.     

Highly enantioselective approach to geminal bisphosphonates by organocatalyzed Michael-type addition of beta-ketoesters

A valuable organocatalyzed protocol has been developed for the asymmetric synthesis of bisphosphonate derivatives, a class of pharmaceutically important molecules. Cheap and commercially available dihydroquinine effectively catalyzed conjugate additions of cyclic beta-ketoesters to ethylidenebisphosphonate esters, leading to optically active geminal bisphosphonates, bearing an all-carbon substituted quaternary stereocenter, in high yields and enantioselectivities of up to 99 % ee. Further elaborations of Michael adducts to the corresponding bisphosphonic acids or vinyl phosphonates have also been successfully performed, with conservation of optical purity.     

Mild electrophilic trifluoromethylation of beta-ketoesters and silyl enol ethers with 5-trifluoro methyldibenzothiophenium tetrafluoroborate

Cyclic and acyclic beta-ketoesters were efficiently trifluoromethylated with 5-trifluoromethyldibenzothiophenium tetrafluoroborate in the presence of a phase-transfer catalyst to afford the corresponding alpha-substituted alpha-trifluoromethyl beta-ketoesters in good to excellent yields. In a second approach, 5-trifluoromethyldibenzothiophenium tetrafluoroborate and tetrabutylammonium difluorotriphenylstannate were used for efficient electrophilic trifluoromethylation of various silyl enol ethers leading to the corresponding alpha-trifluoromethyl ketones in good to high yields.     

Organocatalytic asymmetric conjugate addition to allenic esters and ketones

The first example of an organocatalytic enantioselective conjugate addition of cyclic beta-ketoesters and glycine imine derivatives to electron-deficient allenes is described. We disclose that the corresponding chiral beta,gamma-unsaturated carbonyl compounds are formed exclusively under phase-transfer conditions using either cinchona-alkaloid-derived or biphenyl-based chiral quaternary ammonium salts as catalysts. The scope of the reaction for beta-ketoesters is outlined for allenes having a ketone or ester motif as electron-withdrawing group as well as different substituents in the 3-position, giving the optically active products in high yields and excellent diastereo- and enantioselectivities (90-96% ee). The conjugate addition also proceeds for a number of cyclic beta-ketoesters having different ring sizes, ring systems, and substituents in high yields and enantioselectivities. Glycine imine derivatives also undergo the asymmetric conjugate addition to electron-deficient allenes in high yields and with enantioselectivities in the range of 60-88% ee, thus providing a rapid entry to optically active alpha-vinyl-substituted alpha-amino acid derivatives. It is shown that the enantioselectivity is strongly dependent on the size of the ester moiety of the nucleophile in combination with the catalytic system used. The high synthetic value of the chiral products arising from these new catalytic processes is demonstrated by two straightforward transformations leading in one case to optically active hexahydrobenzopyranones and in the other to substituted pyroglutamates (gamma-lactames).     

Cascade synthesis of 3-quinolinecarboxylic ester via benzylation/ propargylation-cyclization

Reactions of 2-amino-aryl alcohols with beta-ketoesters catalyzed by a catalytic amount of FeCl3 via tandem benzylation-cyclization produce the corresponding 3-quinolinecarboxylic esters in good to high yields. Extending this methodology to propargylation-cyclization, 2-nitrophenyl propargyl alcohols with beta-ketoesters catalyzed by FeCl3 and SnCl2 also produce the 4-alkyne-3-quinolinecarboxylic esters. The mechanistic details of this benzylation/propargylation and cyclization cascade process are also discussed.     

Copper/silver-cocatalyzed Conia-ene reaction of linear beta-alkynic beta-ketoesters

A novel and general copper/silver catalytic system has been developed for the Conia-ene intramolecular reaction of linear beta-alkynic beta-ketoesters. In the presence of (CuOTf)2.C6H6 and AgSbF6 (or AgOAc), a variety of the linear beta-alkynic beta-ketoesters selectively underwent the Conia-ene intramolecular reaction in moderate to good yields.     

Highly diastereoselective heterogeneously catalyzed hydrogenation of enamines for the synthesis of chiral beta-amino acid derivatives

Pure (Z)-enamines readily prepared from beta-ketoesters and amides using (S)-phenylglycine amide were hydrogenated with very high diastereoselectivities (up to 200:1) using heterogeneous catalysis. Hydrogenolytic cleavage of the (S)-phenylglycine amide afforded the corresponding chiral beta-aminoesters and amides. The high geometrical purity of the (Z)-enamine and a simple activation procedure for the PtO2 catalyst are essential in achieving high selectivity.     

Friedel-Crafts type reaction of ferrocene with beta-ketoesters

The reaction of ferrocene with beta-ketoesters in the presence of BF3.Et2O or CF3SO3H affords beta-ferrocenyl-alpha,beta-unsaturated esters in 56-83% yield. In the case of alpha-unsubstituted-beta-ketoesters the reaction is totally E-stereoselective.     

Single molecule solvation and its effects on tautomeric equilibria in a self-assembled capsule

A self-assembled cylindrical capsule provides a nanoscale environment that affects keto-enol equilibria. The equilibrium constants for encapsulated beta-ketoesters show values that differ by an order of magnitude from that of the free tautomers in solution. For complexes with a single, large encapsulated guest, the inner surfaces of the capsule and the seam of the hydrogen bonds influence the equilibrium between the encapsulated keto and enol forms. For complexes of smaller beta-ketoesters, the coencapsulated solvent influences the equilibria. The solvent reduces the space available and affects the positioning of the ester in the capsule.     

Synthesis of a bulky and electron-rich derivative of SEGPhos and its application in Ru-catalyzed enantioselective hydrogenation of beta-ketoesters

The synthesis and resolution of a bulky and electron-rich derivative of SEGPhos and its application in Ru-catalyzed asymmetric hydrogenation reaction of beta-ketoesters are reported. Up to 99.5% ee was achieved. Under solvent-free reaction conditions, acetoacetates could be reduced with good enantioselectivity and high efficiency; a TON of 20 000 was obtained within 3.5 h. The results obtained were comparable to those when SEGPhos was applied.     

A new class of versatile chiral-bridged atropisomeric diphosphine ligands: remarkably efficient ligand syntheses and their applications in highly enantioselective hydrogenation reactions

A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed.     

A simple and stereospecfic route to 2,6-disubstituted 4-hydroxypiperidines. Synthesis of dendrobate alkaloid (+)-241D and formal synthesis of (-)-indolizidine 167B

The condensation of enantiopure beta-amino esters with beta-ketoesters followed by cyclization and decarboxylation afforded 2,3-dihydro-4-pyridones 3, which were selectively hydrogenated to provide 2,6-disubstituted 4-hydroxypiperdines.     

A new synthesis of 1-chloroalkynes

4-Unsubstituted isoxazolinones derived from the corresponding beta-ketoesters can be chlorinated and converted into 1-chloroalkynes upon treatment with sodium nitrite and ferrous sulfate in aqueous acetic acid.     

Titanium-catalyzed stereoselective geminal heterodihalogenation of beta-ketoesters

[reaction: see text] beta-Ketoesters can be effectively monofluorinated with F-TEDA using CpTiCl(3) as a catalyst. With the use of this catalyst, the extent of the competing difluorination does not reach 10%. [TiCl(2)(TADDOLato)] complexes catalyze the one-pot enantioselective heterodihalogenation of beta-ketoesters with F-TEDA and NCS to afford alpha-chloro-alpha-fluoro-beta-ketoesters in moderate to good yields. The sequence of addition of the halogenating agents determines the sense of chiral induction.     

The direct acyl-alkylation of arynes

An efficient and mild acyl-alkylation of arynes is described. The method is used to synthesize medium-sized carbocycles by the ring-expansion of cyclic beta-ketoesters.     

CuI-catalyzed intramolecular O-vinylation of carbonyl compounds

The first copper-catalyzed intramolecular O-vinylation of carbonyl compounds with vinyl bromides was reported, among which the efficient formation of 5-, 6- and even 7-membered cyclic alkenyl ethers was achieved with beta-ketoesters as nucleophiles.