Hans Wynberg

CATALYTIC ASYMMETRIC INDUCTION IN OXIDATION REACTIONS - SYNTHESIS OF OPTICALLY-ACTIVE EPOXIDES

The epoxide function plays an important role in metabolic processes. It is surprising therefore that the synthesis of optically active epoxides leaves much to be desired. We wish to report that the base-catalyzed hydrogen peroxide or t-butyl hydroperoxide mediated epoxidation of electron-poor olefins is subject to catalytic asymmetric induction. Using quaternary ammonium salts derived from alkaloids under phase-transfer conditions we have synthesized a number of optically active epoxides starting from chalcones, a quinone and similar electron-poor olefins. Chemical yields are excellent in all cases, while the enantiomeric excess, as determined in one case, amounted to 25%. No attempts have yet been made to optimize the enantioselectivity of this reaction. Initial experiments performed with quinine (2) as the catalyst in a mixture of 30% aq. H2O2 and ethanol showed that quinone 1 9 gave the corresponding epoxide 3 in 90% chemical yield. However, asymmetric induction was minimal and the results erratic. Tetrahedron Letters No. 21, pp 1831 1834, 1976. Pergamon Press. Printed in Great Britain.

BIOMIMETIC ASYMMETRIC OXIDATIVE COUPLING OF PHENOLS

Abstract This paper describes the first examples of asymmetric induction in the oxidative coupling of phenols using chiral oxidants. When chiral cupric-amine complexes were used as oxidants, low asymmetric induction was achieved in the coupling of naphthols. The formation of optically active d -dehydrogriseofulvin and l -Licarin A using the cupric- l-a -phenylethylamine complex perhaps mimics the action of copper-containing enzymes known to catalyze phenol coupling.

ENANTIOMERIC RECOGNITION AND INTERACTIONS

Abstract The consequences in a chemical reaction of the non-bonding interactions between molecules of one configuration on the one hand, and molecules of opposite configuration on the other hand are discussed. The reaction rates and product ratios in the reaction of a chiral compound depend upon the enantiomeric excess of the substrate. An antipodal interaction effect is observed in a phenol coupling reaction, a reductive camphor dimerization and a camphor reduction.

Nitroalkanes in C–C bond forming reactions: a crystal structure of a complex of a guanidine catalyst and a nitroalkane substrate

1,5,7-Triazabicyclo [4.4.0]dec-5-ene (TBD), a good catalyst for nitroalkanes in C–C bond forming reactions, forms a 1:1 α nitrotoluene as a substrate, as shown by a crystal structure determination.

RACEMATSPALTUNGEN MIT SUBSTANZFAMILIEN

Die Racematspaltung revolutionieren konnte das hier vorgestellte kombinatorische Verfahren, bei dem Gemische („Familien”) strukturell und stereochemisch verwandter Reagentien mit den zu trennenden racemischen Sauren oder Basen umgesetzt werden. Die in ublicherweise einem Schritt gebildeten diastereomeren Salze fallen fast immer rasch aus, Ausbeuten und Enantiomerenuberschusse sind gut bis hervorragend. Auch Reagensgemische mit racemischen oder achiralen Komponenten wurden entwickelt.

A New 31P NMR Method for the Enantiomeric Excess Determination of Alcohols, Amines and Amino Acid Esters.

Diastereoisomeric ester and amide derivatives of phosphoric acid chloride 2 show well separated signals in the 31P NMR spectra allowing accurate e.e. determination of chiral alcohols, esters of amino acids and amines.

Alkaloid catalysed asymmetric synthesis. The addition of selenophenols to 2-cyclohexen-1-ones and conversion to optically active allylic alcohols☆

Abstract The cinchona alkaloid catalyzed 1,4-addition reaction of selenophenols to cyclohexenols has been shown to proceed with asymmetric induction. Optically active selenium adducts are formed in quantitative chemical and up to 43% enantiomeric yields. The transformation to an optically active chiral allyl alcohol is described.

ENANTIOSELECTIVE AUTOINDUCTION IN THE ALDOL CONDENSATION OF ETHYL-ACETATE AND BENZALDEHYDE - SELECTIVE PRECIPITATION OF AN OPTICALLY INACTIVE LI-O-AGGREGATE

The influence of half an equivalent of optically active (–)-(S)-ethyl [3-2H]3-phenyl-3-hydroxypropanoate on the stereochemical course of the aldol condensation of ethyl acetate and benzaldehyde was studied; an enantioselective autoinductive effect on the formation of the second equivalent of hydroxyester of 18%(–100°C) was determined in favour of the opposite (+)-isomer; separate work-up of precipitated Li–O-aggregate revealed that the (+)–isomer is predominant in the solid phase [enantiomeric excess (e.e.) 68%]; the mother liquor is considerably enriched in the (–)-isomer (e.e. 59%).

SOLVENT EFFECTS IN HOMOGENEOUS ASYMMETRIC CATALYSIS

In a number of 1,4-addition reactions yielding chiral products through asymmetric catalysis the enantiomeric yield of products was found to be inversely proportional to the dielectric constant of the solvent.

The synthesis of a resolved triptycene : Spectroscopic evidence for transannular interaction

Abstract The synthesis of optically active 4,11-dihydro-4,11-o-benzenoanthra[2,3-b]thiophene (3) is reported. Resolution of 3 was achieved via the dehydroabietylamine salt of the 2-thiophene substituted carboxylic acid (4). Desulfurization of optically active 3 furnished optically active 5,12-dihydro-5,12-(13-ethyl)ethanonaphthacene (6). Comparison of the CD data of this compound and of 3 led to the conclusion that π-π interaction between all three rings in triptycene systems exists.