Search results for "absolute configuration"

Synthesis and Isolation of Enantiomerically Enriched Cyclopenta[b]benzofurans Based on Products from Anodic Oxidation of 2,4-Dimethylphenol

2015

The anodic treatment of 2,4-dimethylphenol offers a powerful and direct method for the construction of a dehydrotetramer with four contiguous stereocentres on a multigram scale. The installation of propellanes on this scaffold using enantiomerically pure carbonyl compounds leads to a mixture of diastereomers. This mixture is easily separable using standard chromatography and gives rise to optically pure cyclopenta[b]benzofurans which are important scaffolds in a variety of natural products. The synthesis is easy to perform and allows a reliable access to chiral compounds with very high enantiomeric excess. It was possible to determine the absolute configuration of these compounds by compari…

A triclinic polymorph of (−)-(S)-N-benzyl-2-[(R)-6-fluorochroman-2-yl]-2-hydroxyethanaminium bromide

2013

The title salt, C18H21FNO2+·Br−, determined at 115 K, crystallizes in the triclinic space groupP1. The previously reported polymorph occurs in the monoclinic space groupP21and has two independent molecules in the asymmetric unit [Peeterset al.(1993).Acta Cryst.C49, 2157–2160]. In the title molecule, the pyran rings adopt half-chair conformations. The absolute configuration isSfor the hydroxy-bearing C atom andRfor the asymmetric C atom in the dihydropyran unit. In the crystal, the components are linked by N—H...Br and O—H...Br hydrogen bonds, forming chains along thec-axis direction. The crystal studied was refined as an inversion twin.

Timolol derivatives. I. X-ray, NMR and theoretical studies of the crystallization of (S)-timolol O,O-diacetyl-l-tartaric acid monoester

1993

Abstract The absolute configurations of (S)-timolol hemihydrate and (S)-timolol O,O-diacetyl-(R,R)-tartaric acid monoester were determined by single crystal X-ray diffraction. An NMR analysis based on the temperature dependence of vicinal coupling constants was carried out to characterize the conformational behaviour of the S,R,R- and R,R,R-forms in solution. The same conformation as in crystalline state was also found in solution, although with a rather low preference over some other conformations. Results of theoretical calculations using MNDO and AMBER force field methods are reported. An infinite chain of hydrogen bonds, along with other favourable inter- and intramolecular forces that …

Studien zum Vorgang der Wasserstoffübertragung, 57 Einige Beispiele zur diastereoselektiven Hydrierung von Verbindungen mit prochiralen CC-Bindungen

1979

(1R,2S)-N,N-Dimethylephedriniumchlorid, (1S,2S)-N,N-Dimethylpseudoephedriniumchlorid, (1R,2S)-N-Methylephedrin, (1S,2S)-N-Methylpseudoephedrin und [(2S)-2-Hydroxy-2-phenyl-ethyl]trimethylammoniumchlorid werden mit α- und β-Methylzimtsaure zu 1–4 und 7 verestert und an Raney-Nickel, Palladium, Platin und einer Reihe von Edelmetalltragerkatalysatoren hydriert. Die hydrierten Ester werden verseift und die optische Ausbeute der α- bzw. β-Methyldihydrozimtsaure bestimmt. Die optischen Ausbeuten bewegen sich in Abhangigkeit von der optisch aktiven Hilfskomponente zwischen 68 und 4%. Bei optischen Ausbeuten uber 10% wird die absolute Konfiguration der α- oder β-Methyldihydrozimtsaure von der Konfi…

Pterocarpol and triterpenes from Daemonorops draco

1981

Struktur-Wirkungs-Beziehungen bei Histaminanaloga, 24. Mitt. Absolute Konfiguration und histaminartige Wirkung der Enantiomere von 4-(2-Aminopropyl)-…

1982

Es werden die Racematspaltung von 4-(2-Aminopropyl)-5-methyl-imidazol (1), die Bestimmung der absoluten Konfiguration sowie die histaminartige Wirksamkeit der Enantiomere am Ileum (H1) und Atrium (H2) des Meerschweinchens beschrieben. Structure-Activity Relationships of Histamine Analogues, XXIV: Absolute Configuration and Histamine-Type Activity of the Enantiomers of 4-(2-Aminopropyl)-5-methylimidazole The resolution of 4-(2-aminopropyl)-5-methylimidazole (1), the determination of the absolute configuration and the histamine-type activity of the enantiomers on the guinea pig ileum (H1) and atrium (H2) are reported.

Synthesis of spongian diterpenes: (−)-spongian-16-oxo-17-al and (−)-acetyldendrillol-1

2001

Abstract An efficient diastereoselective synthesis of the spongian diterpenes (−)-spongian-16-oxo-17-al ( 1 ) and (−)-acetyldendrillol-1 ( 13 ) is described starting from (+)-podocarp-8(14)-13-one ( 6 ) via the ester-dialdehyde 11 as key intermediate. The absolute configuration at C-17 in synthetic compound 13 has conclusively been proved by NOE experiments.

Total Synthesis of (±)-Oxalicumone C and Chiral Resolution and Elucidation of Its Absolute Configuration

2014

The racemic total synthesis, the chiral resolution, and the elucidation of the absolute configuration of the natural product oxalicumone C were investigated. The goal was achieved by a synthetic strategy beginning with the synthesis of a 5-hydroxy-7-methylchromone precursor. Further modification possibilities were achieved by introduction of a vinyl moiety into the chromone, which was based on a selective iodination step in the 3-position. Further transformations furnished the racemic natural product. To determinate the absolute configuration of naturally occurring oxalicumone C, both enantiomers were separated by chiral HPLC. Comparison of the calculated and experimental vibrational circul…

Labdane Diterpenes from Stachys plumosa

2000

Three new labdane diterpenoids were isolated from the aerial parts of Stachys plumosa. The first two (1, 2) were the dextrorotatory enantiomers of the known 6-deoxyandalusol and 13-epijabugodiol. Structures were determined using NMR and MS techniques. The absolute stereochemistry of the third compound (3) was not experimentally proved.

Flavans and other chemical constituents of Crinum biflorum (Amaryllidaceae)

2019

Abstract The ethyl acetate extract from the whole plant of Crinum biflorum Rottb. Showed a moderate activity against Enterococcus faecalis. Its phytochemical investigation led to the isolation of a new flavan-3-ol derivative namely (2R,3R)-3-hydroxy-7-methoxy-3′,4′-methylenedioxyflavan, together with (2S)-7-hydroxy-3′,4′-methylenedioxyflavan, (2R,3R)-7-methoxy-flavan-3-ol, (2S)-7-hydroxy-3′,4′-dimethoxyflavan, 3′,7-dihydroxy-4′-methoxyflavan, 4′,7-dimethoxy-3′-hydroxyflavan, farrerol, β-sitosterol, β-sitosterol-3-O-β-D-glucopyranoside, oleanolic acid, kaempferol, pancratistatin, lupeol, aurantiamide acetate, Narciprimine and 2,3-dihydroxypropyl palmitate. Their structures were elucidated ma…