Showing papers in "Tetrahedron-asymmetry in 1996"

Heterogeneous asymmetric epoxidation of unfunctionalized olefins catalyzed by polymer-bound (salen)manganese complexes

Abstract: The synthesis of three different polymer-bound chiral Mn-salen complexes (Poly- 1, -2a, -2b ) is reported, along with their application as recyclable catalysts in heterogeneous asymmetric epoxidation with m CPBA/NMO of several unfunctionalized olefins. The introduction of a spacing group between the polymeric chain and the metal centre (Poly- 2a and -2b ) considerably increased the enantioselectivity of the process.

Synthesis of phosphinoferrocenyloxazolines. New ligands for asymmetric catalysis

Abstract: Condensation of β-hydroxy alcohols with ferrocenyl chloride gave the corresponding amides which were dehydrated to give ferrocenyl oxazolines in good overall yield. Subsequent lithiation with n -BuLi in the presence of TMEDA and addition of PPh 2 Cl led to the isolation of ( S )-( S )- i Pr-phosferrox and ( S )-( S )-Me-phosferrox ligands 1 and 2 . Their corresponding diastereoisomers, ( S )-( R )- i Pr-phosferrox and ( S )-( R )-Me-phosferrox 3 and 4 were obtained via the intermediacy of removable trimethylsilyl blocking groups. The CD spectra of 1 – 4 and their parent ferrocenyl oxazolines 9 and 10 were obtained.

Homochiral 2,2′-bis(oxazolyl)-1,1′-binaphthyls as ligands for copper(I)-catalyzed asymmetric cyclopropanation

Abstract: Homochiral 2,2′-bis[4-(alkyl)oxazol-2-yl]-1,1′-binaphthyls, which possess both binaphthyl axial chirality and carbon centred chirality, were prepared from 1,1′-binaphthyl-2,2′-dicarboxylic acid and 2-alkyl-2-aminoethanols in high yields. Asymmetric cyclopropanation of styrene with diazoacetates in the presence of 2 mol % of copper(I) triflate and ( S )-2,2′-bis[( S )-4-( t -butyl)oxazol-2-yl]-1,1′-binaphthyl gave optically active 2-(phenyl)cyclopropane-carboxylates of up to 97% ee.

Synthesis and resolution of 2,2'-bis-diphenylphosphino [3,3']biindolyl ; a new atropisomeric ligand for transition metal catalysis

Abstract: A synthesis of the title ligand is described in which the key step is the dilithiation of 1,1'- bis (dimethylaminomethyl)-3,3'-biindolyl and electrophilic trapping with ClPPh 2 . Concomitant N-deprotection and resolution with bis [(R)-dimethyl(1-(1-naphthyl) ethylaminato-C 2 ,N)palladium chloride] led to selective recovery of the S-enantiomer. The N-dimethylaminomethyl-substituted biindolyl, but not the parent compound, showed strongly second-order behaviour for 13 C NMR peaks in the region of the phosphorus atom due to a desymmetrising 13 C-induced isotope shift of the 31 P resonance.

Highly diastereoselective ortho-lithiation of 1,1′-bis-(oxazolinyl)ferrocene directed to C2-symmetric chiral ligands

Abstract: C 2 -Symmetric 1,1′,2,2′-tetrasubstituted ferrocene derivatives are prepared in high yields from chiral 1,1′-bis(oxazolinyl)ferrocenes via diastereoselective o , o '-dilithiation with sec -butyllithium followed by nucleophilic substitution. 1,1′-Bis(oxazolinyl)ferrocenes were prepared from 1,1′-ferrocenedicarbonyl dichloride and enantiomerically pure aminoalcohols via the corresponding bis(β-hydroxylamide)s and dimesylates as intermediates.

Approach to highly enantiopure β-amino acid esters by using lipase catalysis in organic media

Abstract: Ethyl esters of ten alicyclic β-aminocarboxylic acids were resolved by lipase catalysis in organic solvents. The resolution was based on acylation of the amino group at the R-stereogenic centre with various 2,2,2-trifluoroethyl esters. An increase in the hydrophobic nature of the acyl donor enhanced the enantioselectivity and reactivity in the case of lipase SP 526 from Candida antarctica, while the opposite effect was observed with lipase PS from Pseudomonas cepacia. An unexceptional enantioselectivity enhancement was observed when 2,2,2-trifluoroethyl chloroacetate was used in the case of lipase PS catalysis.

Oxindole alkaloids. A novel non-biomimetic entry to (−)-Horsfiline.

Abstract: A novel non-biomimetic synthesis of horsfiline has been developed The key pyrrolidine forming reaction is the 1,3-dipolar cycloaddition of the thermally generated N -methylazomethine ylide to a suitable 3-alkylidene-indolin-2(3 H )one The same strategy was also applied to the synthesis of pure ( R )-(−)-enantiomer

Synthesis of aryl 2-benzofuranyl and aryl 2-indolyl carbinols of high enantiomeric purity via palladium-catalyzed heteroannulation of chiral arylpropargylic alcohols

Abstract: 2-Iodophenol and 2-iodo-N-mesylaniline have been reacted with α-arylpropargylic alcohols of high enantiomeric purity under transition-metal catalysis leading to aryl 2-benzofuranyl and aryl 2-indolyl carbinols in good yield and high enantiomeric excess, thus constituting the first enantiospecific synthesis of these important classes of compounds.

Asymmetric aziridination by reaction of chiral N-sulfinylimines with sulfur ylides: Stereoselectivity improvement by use of tert-butylsulfinyl group as chiral auxiliary

Abstract: Chiral tert -butylsulfinyl group has been shown to be the chiral auxiliary of choice for the asymmetric aziridination of N-sulfinyliminas. Moreover, the sense of the asymmetric induction can be tuned in two ways: the chirality at the tert -butylsulfinyl Sulfur, or the nature of the methylene transfer reagent used. Thus, both aziridines 10( S s , S ) and 10( R s , R ), epimeric at C-2, were obtained in enantiomerically pure form by a single crystallisation (75% yield).

Transketolase from Escherichia coli: A practical procedure for using the biocatalyst for asymmetric carbon-carbon bond synthesis

Abstract: A practical procedure is reported for the use of the enzyme transketolase, from Escherichia coli , for asymmetric carbon-carbon bond synthesis. The reactions with the biocatalyst are conveniently carried out, on a gram scale, in unbuffered aqueous media by employing a pH autotitrator. An improved large scale synthesis of hydroxypyruvate is also reported.

Enantioselective acylation of primary and secondary alcohols catalyzed by lipase QL from Alcaligenes sp.: A predictive active site model for lipase QL to identify which enantiomer of an alcohol reacts faster in this acylation

Abstract: Lipase QL (from Alcaligenes sp.)-catalyzed acylation of alcohols using isopropenyl acetate as the acylating agent in diisopropyl ether converted preferentially primary alcohols with an S configuration and secondary alcohols with an R configuration into the corresponding homochiral acetates. On the basis of observed enantiomer selectivities, a predictive active site model for lipase QL is proposed for identifying which enantiomer of a primary or a secondary alcohol reacts faster in this acylation.

Highly diastereoselective asymmetric aldol reactions of chiral ni(ii)-complex of glycine with alkyl trifluoromethyl ketones

Abstract: Asymmetric aldol reactions between prochiral trifluoromethyl ketones and Ni(II)-complex of monochiral Schiff base of glycine with ( S )- o -[ N -( N -benzylprolyl)amino]benzophenone (BBP) are described. General stereodirecting features of the trifluoromethyl group controlling (2 S ,3 S )-absolute configuration (90–98 %de) of the resultant amino acids is demonstrated. New set of reaction conditions allowing preparative synthesis of diastereo and enantiomerically pure (2 S ,3 S )-3-trifluoromethyl-3-substituted serines of biomedicinal interest is presented.

Synthesis and some reactions of the first chiral tin hydride containing a C2-symmetric binaphthyl substituent

Abstract: Starting from pure (S)-2,2′-bis(bromomethyl)-1,1′-binaphthyl, the synthesis of a new chiral, enantiomerically pure dinaphthostannepin is described. Preliminary reactions of this hydride with a racemic organic halide under radical conditions gave the reduced product in up to 41 % enantiomeric excess, depending on the experimental conditions.

Efficient and practical polymeric catalysts for heterogeneous asymmetric dihydroxylation of olefins

Abstract: High enantioselectivity (>99% ee) and reactivity in heterogeneous catalytic asymmetric dihydroxylation (AD) of olefins have been achieved using new polymeric cinchona alkaloids containing 1,4-bis(9- O -quininyl)phthalazine ((QN) 2 -PHAL), which can be synthesized more economically than their homogeneous analogue, 1,4-bis(9- O -dihydroquininyl)phthalazine ((DHQ) 2 -PHAL)

Asymmetric trimethylsilylcyanation of aldehydes catalyzed by chiral (salen)Ti(IV) complexes

Abstract: A set of aromatic, aliphatic and α,β-unsaturated aldehydes has been asymmetrically trimethylsilylcyanated with e.e.'s ranging from 40% to 80% using a chiral (salen)Ti(IV) catalyst prepared in situ from titanium tetraisopropoxide and (1R,2R)[N,N′-bis(2′-hydroxy-3′-t-butyl-benzylidene)]-1,2-diaminocyclohexane or (1R,2R)-[N,N′-bis(2′-hydroxybenzylidene)]-1,2-diaminocyclohexane.

An improved procedure to homochiral cyclic statines

Abstract: Chiral tetramic acids were prepared in high yields by the reaction of N-protected amino acids with Meldrum's acid employing DCC as a carboxyl activating agent. Reduction or hydrogenation of these tetramic acids produced homochiral cyclic statines.

Indium trichloride-promoted tin-mediated carbonyl allylation in water: High simple diastereo- and diastereofacial selectivity

Abstract: Tin-mediated additions of allylic bromides to aldehydes and aldoses leads to adduct with high diastereo- and diastereofacial selectivity in the presence of indium trichloride in water.

Chiral t-butylphenylphosphinothioic acid: A useful chiral solvating agent for direct determination of enantiomeric purity of alcohols, thiols, amines, diols, aminoalcohols and related compounds

Abstract: Enantiomers of t-butylphenylphosphinothioic acid were found to be useful chiral solvating agents (CSAs) for 1H-NMR determination of enantiomeric excess (ee) of many classes of chiral organic compounds, such as alcohols, diols, thiols, mercaptoalcohols, amines, aminoalcohols, hydroxyacids and related compounds.

(1S,2S,7R,8aS)- and (1S,2S,7S,8aS)-trihydroxyoctahydroindolizine: Two new glycosidase inhibitors by nitrone cycloaddition strategy

Abstract: The two new epimeric (1 S ,2 S ,7 R ,8a S )- and (1 S ,2 S ,7 S ,8a S )-1,2,7-trihydroxyoctahydroindolizines 4 and 5 have been synthesized via methylenecyclopropane-nitrone cycloaddition-rearrangement methodology employing an enantiomerically pure l -tartaric acid derived nitrone 7b . Highly stereoselective reductions of the intermediate indolizidinone 10b and final deprotection furnished the two title indolizidinetriols 4 and 5 , the inhibiting abilities of which toward 24 commercially available glycosidases were tested. Both 4 and 5 are good competitive inhibitors of amyloglucosidases with K i values of ca. 6 and 75 μM, respectively. Compared with (+)-lentiginosine 3, 4 and 5 are less powerful inhibitors but, in contrast to 3 , the (7 R )-hydroxy analogue 4 possesses a weak inhibiting activity toward α- l -fucosidase from bovine epididymis. A model to rationalize the structure-activity relationship of (+)-lentiginosine and the two new 7-hydroxylentiginosines toward glucoamylases is proposed on the basis of their structural comparison with known inhibitors and with the natural enzyme's substrate amylose.

Enantioselective total syntheses of cyclopropane amino acids: Natural products and protein methanologs

Abstract: The syntheses of (−)-allo-coronamic acid, (−)-allo-norcoronamic acid, (−)-(Z)-2,3-methanohomoserine, (−)-(Z)-2,3-methanomethionine, and (2S,3R)-Cbz-cyclo-Asp-OMe have been achieved in 45–68% overall yields from suitable intermediates derived from homochiral aminopentenoates which were obtained, in turn, from D-glyceraldehyde. The key synthetic step involves the quantitative and highly diastereoselective cyclopropanation of such precursors. The factors dealing with the control of stereoselectivity are highlighted and the main features in side-chain functionalization to the respective target molecules are discussed.

Chloroperoxidase-catalyzed asymmetric synthesis of series of aromatic cyclic sulfoxides

Abstract: The chloroperoxidase (CPO) catalyzed oxidation of a series of rigid aromatic sulfides of well-defined geometry has been studied by the use of enantioselective gas chromatography for the determination of product composition and sulfoxide enantiomeric excess. The almost planar 1-thiaindane was found to be an excellent substrate, giving a quantitative yield of the (−)-(R)-1-oxide in 99% e.e. The sterically more demanding next higher homolog with a six-membered heterocyclic ring, 1-thiatetrahydronaphthalene (1-thiochroman), also gave a sulfoxide in high e.e (≥96%) but in a much lower yield, indicating a preserved stereorecognition ability but a lower turnover rate. A carbonyl group in the 4-position (1-thiochroman-4-one) had no further effect on the CPO-mediated sulfoxidation. Enantioselectivity was lost for the symmetric disulfide, 1,3-benzodithiol, indicating an equal accessibility of the FeO complex towards both sulfur atoms and an active site centred above the heme-iron atom. In all cases with pronounced enantioselectivity the reaction gave the (R)-sulfoxide as observed previously with alkyl aryl sulfides. A change of oxygen source from hydrogen peroxide to t-butyl hydroperoxide and a longer reaction time gave a higher chemical but a lower optical yield, most likely due to an increased competition by the uncatalyzed oxidation reaction in this case.

Asymmetric hetero diels-alder reactions of danishefsky's diene and glyoxylate esters catalyzed by chiral bisoxazoline derived catalysts.

Abstract: Asymmetric hetero Diels-Alder reactions of Danishefsky's diene and glyoxylate esters catalyzed by bis(oxazoline)-metal complex afforded the corresponding aldol adduct which upon treatment with trifluoroacetic acid furnished the hetero Diels-Alder product in 72% enantiomeric excess and 70% isolated yield.

A two-step asymmetric synthesis of pure trans-(R,R)-diaryl-epoxides

Abstract: Pure trans -( R , R )-diaryl-epoxides are obtained in two steps and under aprotic conditions using Eliel's oxathiane as the chiral auxiliary. The enantiomeric excesses, determined by chiral HPLC, are 97.9% to 99.9% and the oxathiane is recovered in 78% to 92% yield and may thus be re-used.

A new asymmetric entry to 2-substituted piperidines. A concise synthesis of (+)-coniine, (−)-pelletierine, (+)-δ-coniceine, and (+)-epidihydropinidine

Abstract: A new asymmetric route to 2-substituted piperidines involving the Sharpless asymmetric dihydroxylation (AD) of 5-hexenylazide 1 and an intramolecular aminocyclization as crucial steps and its application to the asymmetric synthesis of four piperidine alkaloids, (+)-coniine 2, (−)-pelletierine 3, (+)-δ-coniceine 4, and (+)-epidihydropinidine 5 is presented.

Enantioselective hydrogenation of α-keto esters over cinchona-PtAl2O3 catalyst. Kinetic evidence for the substrate-modifier interaction in the liquid phase

Abstract: The hydrogenation of ethyl pyruvate was studied over cinchonidine- Pt Al 2 O 3 catalyst. Contrary to earlier results it has been found that the initial enantiomeric excess extrapolated to zero conversion is close to zero. Based on kinetic analysis the results are considered as indirect evidence for the substrate-modifier interaction taking place in the liquid phase. The above interaction leads to the formation of a weak substrate-modifier complex. The formation of the complex in the liquid phase is the key step to control both the rate acceleration and the induction of enantio-differentiation in the hydrogenation of α-keto esters in the presence of cinchona- Pt Al 2 O 3 catalysts. The character of interactions in the substrate-modifier complex is discussed. By using molecular modelling the possible form of the complex is also given.

Accurate assay of enantiopurity of 1-hydroxy- and 2-hydroxyalkylphosphonate esters

Abstract: Enantiomerically pure (Rp)-tert-butylphenylphosphinothioic acid and quinine were successfully used for the direct determination of the enantiomeric purity of diethyl 1-hydroxyalkylphosphonates. Only quinine was effective as a chiral solvating agent for the determination of the enantiomeric excess of diethyl 2-hydroxyalkylphosphonates. Copyright (C) 1996 Elsevier Science Ltd

Highly stereoselective molecularly imprinted polymer synthetic receptors for cinchona alkaloids

Abstract: Molecularly imprinted polymer (MIP) receptors for the cinchona alkaloids (−)-cinchonidine 1 and (+)-cinchonine 2 have been prepared and characterised using the polymers as HPLC chiral stationary phases (CSPs). Stereoseparation factors of up to 31 were obtained. The resolution of the β-adrenergic blocking agent propranolol 8 and studies with related structures of pharmaceutical significance has permitted conclusions to be drawn regarding the origin of recognition phenomena.