Showing papers in "Tetrahedron-asymmetry in 1991"

Asymmetric synthesis of R-β-amino butanoic acid and S-β-tyrosine: Homochiral lithium amide equivalents for Michael additions to α,β-unsaturated esters.

Abstract: Michael addition of the lithium amide derived from R-N-(α-methylbenzyl)benzylamine to benzyl E-crotonate is highly stereoselective (95% d.e.) giving after debenzylation and crystallisation homochiral R-β-amino butanoic acid. A similar addition to methyl E-(p-benzyloxy)cinnamate is completely stereoselective giving after debenzylation and acid hydrolysis homochiral S-β-tyrosine as its HCl salt.

New chiral phospholanes; Synthesis, characterization, and use in asymmetric hydrogenation reactions

Abstract: We describe the practical synthesis of enantiomerically pure trans-2,5-disubstituted-1-phenylphospholanes which are then employed in the preparation of a new series of C2-symmetric bis- and C3-symmetric tris(phospholane) ligands. A versatile three-step route to the important chiral 1,4-diol intermediates, used in the phosphine syntheses, is outlined. Rhodium complexes bearing the new phosphine ligands were prepared and shown to act as efficient catalyst precursors for the enantioselective hydrogenation of various unsaturated substrates.

Enantioselective synthesis using chiral heterogeneous catalysts.

Abstract: The application of solid chiral catalysts for the enantioselective synthesis of chiral molecules is reviewed. An attempt has been made to discuss critically the scope and limitations of these catalytic systems and their value for the synthetic organic chemist. The different catalytic systems described in the literature are tabulated and the enantioselectivities observed for different reactions are summarized according to the functional group which is transformed. Conclusions concerning synthetic and commercial-scale applications and some ideas on the mode of action of chiral solid catalysts are presented.

Catalytic asymmetric epoxidation of unfunctionalized olefins using chiral (salen)manganese(III) complexes

Abstract: Several kinds of chiral (salen)manganese(III) complexes ( 2 and 3 ) having chiral salicylaldehyde and chiral ethylenediamine moieties were prepared and used for catalytic asymmetric epoxidation of unfunctionalized olefins with iodosobenzene as a terminal oxidant. Catalysts 2 and 3 were found to show the characteristic substrate specificity for the enantiofacial selection of olefins, respectively. Furthermore, the addition of donor ligands such as pyridine N -oxide or 2-methylimidazole to the epoxidation reaction system was found to alter the enantioselectivity. As a result, the highest enantioselectivity for nonenzaymatic catalytic epoxidation was achieved for ( E )-1-phenylpropene (56% ee, with 2c in the presence of 2-methylimidazole), ( E )-stilbene (48% ee, with 3a ), and dihydronaphthalene (83% ee, with 3a in the presence of pyridine N -oxide).

Enantioselective synthesis of β-amino acids based on BINAP—ruthenium(II) catalyzed hydrogenation

Abstract: BINAP—Ru(II) catalyzed hydrogenation of β-substituted ( E )-β-(acylamino)acrylic acids allows efficient enantioselective synthesis of β-amino acids. The Z double bond isomers which possess an intramolecular hydrogen bond between amide and ester groups are more reactive but are hydrogenated with poor enantioselectivity. BINAP—Rh(I) complexes afford only moderate stereoselectivity with the opposite sense of enantioselection.

Asymmetric hydroboration of styrenes catalyzed by cationic chiral phosphine-rhodium(I) complexes

Abstract: Reaction of styrene with catecholborane in the presence of 1 mol % of a cationic phosphine-rhodium catalyst prepared in situ from [Rh(COD)2]BF4 and 1,4-bis(diphenyl-phosphino)butane proceeded regioselectively to give, after oxidation, 1-phenylethanol in a quantitative yield. The regioselectivity forming benzylic alcohols was also observed in the reaction of substituted styrenes. Use of (R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) as a chiral ligand for the rhodium-catalyzed hydroboration of substituted styrenes (ArCHCH2) gave optically active (R)-1-arylethanols (ArCH(OH)Me) in high yields. The enantiomeric purities of the alcohols are 96% ee, 94% ee, 91% ee, 85% ee, 89% ee, and 82% ee for Ar = Ph, 4-MeC6H4,4-ClC6H4, 3-Cl6H4, 4-MeOC6H4, and 2-MeOC6H4, respectively.

New efficient methods for the synthesis and in-situ preparation of ruthenium(II) complexes of atropisomeric diphosphines and their application in asymmetric catalytic hydrogenations

Abstract: A new synthetically useful method for the synthesis of the diphosphine ruthenium dicarboxylato complexes (PP)Ru(O2CR)2 (R = CF3 and CH3) is presented, which uses the easily accessible complex (COD)2Ru2(μ-O2CCF3)4 as starting material. This complex as well as (COD)Ru(η2-O2CCH3)2 and (COD)2Ru2Cl4(NCCH3) have been shown to be suitable precursor complexes for the in-situ preparation of ruthenium(II) dicarboxylato and dichloro complexes of atropisomeric diphosphines, respectively. The high efficacy of the preformed and in-situ generated ruthenium complexes as precatalysts is demonstrated in asymmetric hydrogenations of allylic alcohols, enamides, and a β-keto ester.

Asymmetric synthesis. Practical production of D and L threonine. Dynamic kinetic resolution in rhodium and ruthenium catalyzed hydrogenation of 2-acylamino-3-oxobutyrates.

Abstract: Enantioselective syntheses of D and L threonine are described. Racemic methyl and ethyl 2-acylamino-3-oxobutyrate 1 were synthesized from the corresponding acetoacetates 6 and then hydrogenated stereoselectivety via dynamic kinetic resolution with various chiral P * P Rh (I) 8 and Ru (II) 10 catalysts to give syn optically active alcohols which could be converted by hydrolysis and treatment with propylene oxide into threonine. The best results were obtained using (−) CHIRAPHOS Ru and (+) BINAP Ru as catalysts, in the hydrogenation step leading respectively to D threonine (ee: 99 %) and L threonine (ee: 94 %) in 26–34 % overall yields.

Asymmetric synthesis of pipecolic acid derivatives using the aza-Diels-Alder reaction

Abstract: Imines of the type RNCHCO 2 Et can be coerced into undergoing a (4+2) cycloaddition with substituted dienes if the reaction is carried out in DMF in the presence of both water and acid: these reactions show extremely high regio- and diastereoselectivity. Use of the 1-phenylethyl group as a chiral auxiliary leads to moderate asymmetric induction (typical d.e. ca. 70%); moreover, the diastereoisomers are surprisingly easy to separate, giving a short general route to optically pure substituted pipecolic acid derivatives.

A trans-chelating chiral diphosphine ligand: Synthesis of 2,2″-bis[1-(diphenylphosphino)ethyl]-1,1″-biferrocene and its complexes with platinum(II) and palladium(II)

Abstract: A new chiral diphosphine ligand, ( R,R )-( S,S )-2,2″-bis[1-(diphenylphosphino)ethyl]-1,1″-biferrocene, which possesses both central and planar elements of chirality, was synthesized. The NMR studies and molecular weight determination indicated that the ligand chelates to platinum(II) and palladium(II) in trans -manner.

Polytryptophane terminated dendritic macromolecules

Abstract: The synthesis and characterization of dendritic macromolecules coated with tryptophan moieties are described. A direct relationship between the molecular ellipicity () and the number of surface tryptophans has been demonstrated.

Resolution of (±)-trans-sobrerol by lipase PS-catalyzed transesterification and effects of organic solvents on enantioselectivity

Abstract: Resolution of the mucolytic drug (±)- trans -sobrerol ( 1 ) was achieved by transesterification with vinyl acetate in organic media, catalyzed by free or immobilized Lipase PS. The enantioselectivity of the enzyme was markedly influenced by the nature of the organic solvent, but there was no correlation between enantiomeric ratio values (70–500) and either the hydrophobicity or the dielectric constant of the medium. With the enzyme immobilized onto Hyflo Super Cell and t -amyl alcohol as the solvent, the selectivity of Lipase PS for (−)- 1 was extremely high and, at 50 % conversion both (−)- trans -sobrerol and (+)- trans -sobrerol monoacetate were obtained in practically 100 % optical purity

Asymmetric allylic alkylation catalyzed by palladium-sparteine complexes

Abstract: The cationic complex [Pd(η 3 -C 3 H 5 )(sparteine)]PF 6 ( 6 ) was found to be a suitable catalyst precursor for the asymmetric alkylation of allylic acetates with Na[CH(COOMe) 2 ] as the nucleophile. This constitutes one of the first and still rare examples of a phosphine-free system for this type of Pd-catalyzed reaction. Using 5 mol % of 6 , alkylation products were obtained in up to 90% isolated yield and 85 % enandomeric excess. The alkylation reaction was shown to occur with overall retention of configuration, indicating an analogous mechanism to the one previously proposed for phosphine-containing catalysts. The reactivity of allylic acetates is strongly dependent upon the nature of the substituents, open-chain aliphatic substrates being unreactive.

On the way to chiral copper(I) arenethiolate catalysts for the enantioselective conjugate addition of methyl lithium and methyl magnesium iodide to benzylideneacetone

Abstract: Selective conjugate addition (0 % enantiomeric excess (e.e.)) of organo- arenethiolatocuprates (from methyl lithium and 23 l-(R)-(dimethylamino)ethyl)phenyl- thiolatocopper(I), CuSAr*) to benzylideneacetone (BA) is found up to a LiMe/CuSAr* ratio of 2/l indicating the potential of the chiral SAr*-anion as non-transferable group; at higher ratios only 1,2-addition occurs. Reactions of methyl magnesium iodide with BA in the presence of a catalvtic amount of CuSAr* (9 mol%) result in exclusive conjugate addition with 57% e.e..

Stereoselective synthesis of homochiral alpha substituted o-methoxybenzyl alcohols via nucleophilic additions to kinetically resolved homochiral tricarbonyl (η6-o-anisaldehyde)chromium(0).

Abstract: Tricarbonyl(η6-o-anisaldehyde)chromium(0) has been kinetically resolved via the selective hydrolysis of one of theL-valinol derived imine presence of deactivated alumina. An X-ray crystal structure analysis on the adduct formed between L-valinol and homochiral (+)-tricarbonyl(η6-o-anisaldehyde)chromium(0) unambiguously established the presence of the corresponding imine and not oxazolidine and confirmed the absolute configuration of the aldehyde complex. Addition of Grignard reagents to homochiral tricarbonyl(η6-o-anisaldehyde)chromium(0) occurs completely stereoselectively to give, following decomplexation, homochiral alpha substituted o-methoxybenzyl alcohols.

Highly diastereoselective Diels-Alder reactions with (R)-ethoxy p-tolyl vinyl sulfonium tetrafluoroborate

Abstract: p-Tolyl vinyl sulfoxide has been efficiently activated for Diels-Alder reactions by transformation into a sulfonium salt 3a or by addition of TMSOTf. Very high diastereomeric excesses have been achieved (>98%) at −20°C in many cases. Best results have been obtained with cyclopentadiene, furan and 2,3-dimethyl-1,3-butadiene. In this way enantiomerically pure oxanorbornenone 19a has been prepared. The reaction mechanism is briefly discussed.

General synthesis of novel chiral ruthenium catalysts and their use in asymmetric hydrogenation

Abstract: The first general synthesis of mononuclear hexacoordinate chiral ruthenium-complexes is presented. Four chiral ruthenium(II)(2-methylallyl) 2 complexes containing Diop, Chiraphos, Norphos, and Binap were prepared in 50–71% yield under mild conditions, and were found to be effective for asymmetric hydrogenation of unsaturated carboxylic acids to give the corresponding saturated derivatives attaining 90% optical purity.

Enantioselective Diels-Alder reactions of enals: Fighting species multiplicity of the catalyst with donor solvents

Abstract: Chiral Lewis acids obtained from reactions of N-arylsulphonylamino acids with borane adducts effectively catalyze Diels-Alder additions of α,β-unsaturated aldehydes. Maximum enantioselectivity (methacrylaldehyde: 86 %ee, crotonaldehyde: 81 %ee) was achieved in donor solvents. The first results with new chiral Lewis acids derived from C 2 -symmetric N-sulphonyl derivatives of 2,2′-diamino-1,1′-binaphthyl are also presented.

Asymmetric induction in the Baylis-Hillman reaction

Abstract: Asymmetric induction in the product of the Baylis-Hillman reaction between an aldehyde and an acrylic compound in the presence of a tertiary amine base, may be controlled by chirality in the aldehyde, acrylic ester, base or solvent; R-CHO + CH2=CH-COOR' (B:, solvent) →CH2=C(C∗HROH)-COOR' Examples of each are described but the best stereochemical result, 100% ee, is obtained from the high pressure reaction of benzaldehyde with (-)-menthyl acrylate.

Chiral titanium complex-catalyzed Diels-Alder reaction: A practical route to anthracycline intermediates

Abstract: Asymmetric Diels-Alder reactions of methacrolein and 1,4-naphthoquinone with 1,3-dienol derivatives catalyzed by the chiral binaphthol (BINOL)-derived titanium complex 1 are shown to provide the corresponding endo -adducts in high enantiomeric purity. The naphthoquinone adduct can serve as a synthetic intermediate of tetracycline antibiotics.

Synthesis of a novel type of chiral phosphinocarboxylic acids. The phosphine-palladium complexes catalyzed asymmetric allylic alkylation

Abstract: A novel type of chiral cycloalkylphosphines bearing the carboxy group at the β-position were developed, and used for palladium catalyzed asymmetric allylic alkylation of allylic substrates such as 2-cyclohexenylacetate and 1,3-disubstituted-propenyl acetates (R1CHCHCH(OAc)R2: R1=R2=Ph; R1=Ph, R2=(CH2)4OAc; R1=Ph, R2=(CH2)6OAc; R1=Ph, R2=(CH2)10OAc). Reaction of the propenyl acetates with soft carbon nucleophiles such as triethyl sodiophosphonoacetate and sodiomalonic acid esters in the presence of a palladium catalyst prepared in situ from Pd(OAc)2 and chiral (2-diphenylphosphino)cycloalkanecarboxylic acids (7a,b) gave high yields of alkylation products (PhCH=CHCH(X)Ph: > 77 %ee for X=CH(CO2Et)P(O)(OEt)2 and > 72 %ee for X=CH(CO2Me)2). The alkylation products 15 and 28a–c were converted into optically active α-methylene-γ-lactone and α-methylene macrolide derivatives. The high stereoselectivity demonstrated by the chiral phosphinocarboxylic acid-palladium catalyzed allylic alkylation suggested to be caused by an electronic repulsion between the carboxy group on the ligand and the incoming soft carbon nucleophile, which directs the nucleophilic attack on one of the π-allyl carbons.

Palladium-catalyzed asymmetric allylic alkylation using dimethyl malonate and its derivatives as nucleophile

Abstract: Palladlum-catalyzed asymmetric allylic alkylatlon of 1,3-dlphenyl-2-propenyl acetate with sodium salt of dimethyl malonate and its derivatives has been successfully carried out in the presence of optically active diphosphine, such as (S)-BINAP. High enantioselectivity of up to 90% ee was obtained with dimethyl malonate.

Asymmetric diels-alder reactions with 5-menthyloxy-2(5h)-furanone

Abstract: At the beginning of the reseach described in this thesis the catalytic asymmetric Diels-Alder reaction had scarcely been investigated. No good catalytic processes with high enantiomeric excess were known at that time. At the same time the Diels-Alder reactions with chiral dienophiles needed further improvement as the number of diastereoselective cycloaddition reactions was limited to a few cases only. In most cases no complete diastereoselectivity was obtained. Besides, most chiral auxiliaries which were used were rather expensive or had to be synthesized by a multistep syntheses. The inherent problem of the chiral dienophiles used sofar was their conformational freedom leading to low selectivities, although the results were considerably improved by means of Lewis acid complexation. The disadvantage of Lewis acids is that they also catalyze the polymerization of dienes. Therefore, in most catalyzed reactions the dienes are limited to the most simple and cheap ones like butadiene or cyclopentadiene. The general purposes of our research was: (i) to synthesize a chiral dienophile which would meet all the criteria described in Section 1.1, i.e. both enantiomers should be available, the absolute configuration of the Diels-Alder adducts should be predictable and the starting material should be a crystalline compound; (ii) to synthesize a number of substituted chiral dienophiles; (iii) to study these dienophiles in the thermal Diels-Alder reaction. Furthermore these dienophiles should have low conformational freedom, resulting in a high diastereoselectivity, and no Lewis acid catalysis should be needed in the cycloaddition reactions with these dienophiles.

Asymmetric amplification in nickel-catalyzed conjugate addition to chalcone

Abstract: Nickel complexes with optically active pyridine ligands catalyze the conjugate addition of diethyl zinc to chalcone. This catalytic alkylation gives products with ee's of up to 86%. Strong asymmetric amplification has been observed using ligands of low optical purity.

Enantioselective conjugate reduction of α,β-unsaturated carboxamides with semicorrin cobalt catalysts

Abstract: Chiral semicorrin cobalt complexes, prepared in situ from cobalt (II) chloride and the free ligands, are efficient, highly enantioselective catalysts for the conjugate reduction of α,β-unsaturated carboxamides with sodium borohydride. Enantiomeric excesses of up to 99%, essentially quantitative yields, and high substrate/catalyst ratios (1000–10 000:1) are attractive attributes of this catalytic process.

Enantioselective catalytic transfer hydrogenation of α,β-unsaturated carboxylic acids with formates catalyzed by novel ruthenium phosphine complexes☆

Abstract: Hydrogen transfer from the formic acid/triethylamme (5:2) azeotrope to α,β-unsaturated carboxylic acids is effectively catalyzed by ruthenium complexes of general formula [Ru(acac-F 6 )(η 3 - C 3 H 5 )(diphosphine)]. If a chiral diphosphine is employed, the saturated acids are obtained in up to 93% ee, the most active and selective catalyst being formed with BINAP.