Search results for "Selectivity"

Cross-Metathesis Reactions as an Efficient Tool in the Synthesis of Fluorinated Cyclic β-Amino Acids

2009

The synthesis of enantiomerically pure, cyclic, gamma,gamma-difluorinated beta-amino acids with various ring sizes has been carried out with a cross-metathesis (CM) reaction being one of the key steps, followed by a Dieckmann-type condensation to bring about the cyclization. Subsequent catalytic hydrogenation under microwave irradiation with (-)-8-phenylmenthol as a chiral auxiliary led to the successful chemo- and diastereoselective chemical reduction of the resulting cyclic beta-enamino esters. The efficiency and scope of the CM reaction with different types of fluorinated imidoyl chlorides and unsaturated esters has also been studied in order to determine the optimal reaction conditions …

New Strategy for the Stereoselective Synthesis of Fluorinated β-Amino Acids

2002

Racemic and chiral nonracemic alpha-substituted and alpha-unsubstituted beta-fluoroalkyl beta-amino acid derivatives 6 and 9 have been synthesized in two steps starting from fluorinated imidoyl chlorides 1 and ester enolates. This approach is based on the chemical reduction of previously obtained gamma-fluorinated beta-enamino esters 4 by using ZnI(2)/NaBH(4) in a nonchelated aprotic medium (dry CH(2)Cl(2)) as the reducing agent. A metal-chelated six-membered model has been suggested to explain the stereochemical outcome of the reduction reaction. The process takes place with high yields and with moderate to good diastereoselectivity. The best results related to diastereoselective reduction…

Stereoselective synthesis of benzomorphan derivatives with perpivaloylated galactose as the chiral auxiliary.

2006

Asymmetric Synthesis of New β,β-Difluorinated Cyclic Quaternary α-Amino Acid Derivatives

2006

The synthesis of new beta,beta-difluorinated cyclic quaternary alpha-amino acid derivatives 1 in which a ring-closing metathesis reaction (RCM) constitutes the key step is described. The approach employs imidoyl chlorides 3 as fluorinated building blocks, and the overall process involves the stereoselective creation of a quaternary stereocenter. Complete selectivity was achieved when (R)-phenylglycinol methyl ether was used as chiral auxiliary, allowing for the preparation of new six-membered cyclic fluorinated alpha-amino acids as single enantiomers.

ChemInform Abstract: Carbohydrate Auxiliaries in Stereoselective Syntheses of Decahydroquinoline Alkaloids.

2010

Using tetra-O-pivaloyl-β-D-galactopyranosylamine as the chiral auxiliary, both trans- and cis-annelated decahydroquinoline alkaloids can be synthesized stereoselectively. This methodology of asymmetric synthesis is based on the effect that both enantiomers of 2,6-disubstituted piperidin-4-ones are selectively and alternatively accessible using the auxiliary as the identical stereodifferentiating tool. In addition, the carbohydrate auxiliary controls the stereoselective protonation of enolates formed by conjugate addition of cuprates to N-galactosyl octahydroquinolin-4-ones. The syntheses of trans-4a-epi-pumiliotoxin C and cis-4a-epi-perhydro-219A illustrate this concept of asymmetric synthe…

ChemInform Abstract: Enantioselective Syntheses of Dopaminergic (R)- and (S)-Benzyltetrahydroisoquinolines.

2010

Optically pure (1S,R)- and (1R,S)-benzyltetrahydroisoquinolines (BTHIQs), 12a,b as the major diastereomers, were prepared by stereoselective reduction of the isoquinolinium salt possessing (R)- and (S)-phenylglycinol as the chiral auxiliary, respectively. The absolute configurations of (1S,R)-13a hydrochloride (O-debenzoylated derivative from 12a) and (1R,S)-12b diastereomers were unambiguously determined by single-crystal X-ray analysis. Reductive removal of the chiral auxiliary group, subsequent N-propylation, and cleavage of the methylenedioxy group furnished the optically active catecholamines (1S)-16a and (1R)-16b in good overall yield. We have separately prepared for the first time pa…

Carbohydrate Auxiliaries in Stereoselective Syntheses of Decahydroquinoline Alkaloids

2002

Using tetra-O-pivaloyl-β-D-galactopyranosylamine as the chiral auxiliary, both trans- and cis-annelated decahydroquinoline alkaloids can be synthesized stereoselectively. This methodology of asymmetric synthesis is based on the effect that both enantiomers of 2,6-disubstituted piperidin-4-ones are selectively and alternatively accessible using the auxiliary as the identical stereodifferentiating tool. In addition, the carbohydrate auxiliary controls the stereoselective protonation of enolates formed by conjugate addition of cuprates to N-galactosyl octahydroquinolin-4-ones. The syntheses of trans-4a-epi-pumiliotoxin C and cis-4a-epi-perhydro-219A illustrate this concept of asymmetric synthe…

Enantiomer Separations Using Designed Imprinted Chiral Phases

2003

Regio- and stereoselective synthesis of the enantiomers of monoterpene-based β-amino acid derivatives

2007

Abstract The regio- and stereospecific addition of chlorosulfonyl isocyanate to cis-δ-pinene enantiomers has furnished monoterpene-fused β-lactams. The observed regioselectivity can be explained by ab initio DFT modeling of transition state structures. In contrast with the less reactive α-pinane-fused β-lactam 4, the resulting β-lactams 5 and 13 containing an amino group connected to a secondary carbon possess similar reactivity to the cycloalkane-fused analogues and can be easily converted to the β-amino acid and its protected derivatives. The base-catalyzed isomerization of the cis-amino ester afforded the corresponding trans-amino ester in moderate yield.

1-Hexyl-3-methyl imidazolium tetrafluoroborate: an efficient column enhancer for the separation of basic drugs by reversed-phase liquid chromatograph…

2012

Abstract Ionic liquids are dual modifiers composed by a large anion and a large cation, which interact with both the hydrophobic alkyl-bonded phase and the anionic residual silanols in C18 columns. The deactivation of the silanol groups has important implications on the chromatographic analysis of basic drugs, being the improvement of peak profiles and shorter retention times the most noticeable features. However, other characteristics as selectivity or resolution are not usually considered, or are only examined for selected chromatographic conditions. In this work, the effect of the addition of the ionic liquid 1-hexyl-3-methyl imidazolium tetrafluoroborate to acetonitrile–water mixtures i…