Search results for "EOS"

An approach to erythrophleum alkaloids. Synthesis of methyl (−)-4-epi-cassamate

1986

Abstract A synthetic approach to Erythrophleum alkaloids, involving as a key-step the regiospecific deprotonation of the ketone 5a, is described.

Stereospecific CC-bond formation with rabbit muscle aldolase - A chemoenzymatic synthesis of (+)-exo-brevicomin

1990

Abstract (+)-(1S,5R,7S)-Exo-brevicomin 9, a sex pheromone of the western pine bark beetle, is synthesized using an aldol reaction catalyzed by fructose-1,6-diphosphate aldolase (EC 4.1.2.13) from rabbit muscle as the key step by which the absolute configuration of the target is established.

Photoconductivity of Poly(Arylenevinylene)S and Poly(Hetarylenevinylene)S

1994

Abstract The intramolecular condensation of aromatic Schiff bases leads under strong alkaline conditions to the poly(arylenevinylene)s and poly(hetarylenevi-nylene)s A - H. Due to the extreme stereoselectivity the resulting products are rigid, (E)-configurated polymers. The broad band gap of the unsubstituted intrinsic insulators is decreased by alkoxy substituents. Investigations of the photoconductivity and the photoreactivity, as well as of the AC-conductivity of some undoped, doped and quaternized systems are performed.

Regio- and diastereoselective fluorination of alicyclic β-amino acids.

2011

A regio- and stereoselective approach to fluorinated β-aminocyclohexene or cyclohexane esters has been developed, starting from a bicyclic β-lactam (1). The procedure involves six or seven steps, based on regio- and stereoselective iodolactonization, lactone opening and hydroxy–fluorine exchange. The method has been extended to the synthesis of fluorinated amino ester enantiomers.

2013

Selective alkylation of RNA nucleotides is an important field of RNA biochemistry, e.g. in applications of fluorescent labeling or in structural probing experiments, yet detailed structure-function studies of labeling agents are rare. Here, bromomethylcoumarins as reactive compounds for fluorescent labeling of RNA are developed as an attractive scaffold on which electronic properties can be modulated by varying the substituents. Six different 4-bromomethyl-coumarins of various substitution patterns were tested for nucleotide specificity of RNA alkylation using tRNA from Escherichia coli as substrate. Using semi-quantitative LC-MS/MS analysis, reactions at mildly acidic and slightly alkaline…

On the structure of passifloricin A: asymmetric synthesis of the delta-lactones of (2Z,5S,7R,9S,11S)- and (2Z,5R,7R,9S,11S)tetrahydroxyhexacos-2-enoi…

2003

Stereoselective syntheses of the delta-lactone of (2Z,5S,7R,9S,11S)-tetrahydroxyhexacos-2-enoic acid, the structure reported for passifloricin A, and of its (5R)-epimer are described. The creation of all stereogenic centers relied upon Brown's asymmetric allylation methodology. The lactone ring was created via ring-closing metathesis. The NMR data of both synthetic products, however, were different from those of the natural product. The published structure of passifloricin A is thus erroneous and will require further synthetic work to be unambiguously assigned. [structure: see text]

Stereoselective synthesis of carane-based chiral β- and γ-amino acid derivatives via conjugate addition

2015

Abstract Michael addition of dibenzylamine to (−)-tert-butyl isochaminate, prepared in three steps from (−)-perillaldehyde, furnished a carane-based β-amino acid derivative in a highly stereospecific reaction. The resulting amino ester was transformed to the bicyclic amino acid, a promising building block for the synthesis of 1,3-heterocycles and peptidomimetics. The conjugate addition of nitromethane to α,β-unsaturated methyl ester likewise resulted in nitro esters in stereospecific reactions. Catalytic reduction of the nitro group yielded a γ-amino ester. Under acidic conditions, the hydrolysis of the methyl ester resulted in an unexpected aminolactone-type product through rearrangement o…

The role of Zn2+ in enhancing the rate and stereoselectivity of the aldol reactions catalyzed by the simple prolinamide model

2011

Abstract The aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by single l -prolinamide and its zinc complexes has been studied. An increase in the rate and the stereoselectivity of the reaction has been shown by using zinc derivatives. A mechanistic proposal, based on NMR and ESI studies, has been put forward to explain the experimental data: zinc–prolinamide complexes catalyze the reaction following the general mechanism of stereoselective enamine nucleophilic addition to the acceptor aldehyde. Zn2+ prevents the nonspecific base-catalyzed reaction by diminishing the basicity of the amine nitrogen of prolinamide.

The nucleophilic addition of nitrones to carbonyl compounds: insights on the nature of the mechanism of the l-proline induced asymmetric reaction fro…

2004

Abstract The mechanism of the l -proline induced asymmetric nitrone–aldol reaction of N -methyl- C -ethylnitrone with dimethyl ketomalonate has been studied by using density functional theory at B3LYP/6-31G** level. The reaction was initialized by the addition of l -proline to the nitrone to form an aminal, which by elimination of the hydroxylamine gave a chiral enamine. The nucleophilic addition of this chiral enamine to dimethyl ketomalonate corresponds to stereoselective C–C bond-formation step. Further nucleophilic addition of hydroxylamine to the zwitterionic intermediate formed in the enamine addition gave a second aminal, which by l -proline elimination afforded the corresponding β-h…

Carbohydrates as chiral templates: asymmetric ugi-synthesis of alpha-amino acids using galactosylamines as the chiral matrices

1988

Abstract In the presence of Lewis acid catalysts O-acetyl- (1) and O-pivaloyl- (2) protected β-D-galactopyranosylamines react with aldehydes, isocyanides and carboxylic acids in Ugi-four-component-condensations to give the corresponding N-galactosyl-amino acid amide derivatives 3,5 in almost quantitative yields. Zinc chloride is the most effective Lewis acid catalyst. At 0°C or even at room temperature the (2R,β-D)-diastereomers of the amino acid derivatives 3,5 are formed with high diastereoselectivity. If the sterically more demanding O-pivaloyl galactosylamine 2 is used at -78°C to -25°C the stereoselectivity often exeeds 20:1 favouring the (2R,β-D) diastereomers 5. After one recrystalli…