Ben de Lange

Asymmetric 1,3-dipolar cycloadditions to 5-(R)-menthyloxy-2(5H)-furanone

Various diazo compounds, nitrile oxides, nitrones and azomethine ylides were examined in 1,3-dipolar cycloadditions to enantiomerically pure 5-(R)-menthyloxy-2(5H)-furanone 1a. Pyrazoline 9 was obtained in 100% c.y. as a mixture of 2 diastereoisomers in ratios up to 72 : 28, whereas pyrazoline 16 was obtained in 100 % c.y. as a single enantiomer. Photochemically pyrazolines 9 and 10 have been converted to cyclopropanes 11 and 13. Under thermal conditions pyrazoline 9 is converted to 4-methyl-5-menthyloxy-2(5H)-furanone. Isoxazoles 21a-24a were obtained enantiomerically pure via nitrile oxide addition to 1a in 64-67% yield. Nitrone addition afforded isoxazolidines 27, 28 and 34 with complete anti-facial- and regiochemistry, but with endo-exo selectivities up to 76%. Enantiomerically pure isoxazolidines were obtained in 25-75% yield. Pyrrolidine 36 was obtained diastereomerically pure in 81% c.y. Pyrrolidines 42 and 45, however, were obtained as diastereomeric mixtures in 37% resp. 6% yield.

Asymmetric 1,3-Dipolar Cycloadditions to 5-Menthyloxy-2[5H]-furanones.

Abstract Cycloadditions of various 1,3-dipolar reagents to chiral butenolides 1 and 5 proceed with diastereomeric excess of 20–100%.

Resolution of sterically overcrowded ethylenes ; a remarkable correlation between bond lengths and racemization barriers

The synthesis and resolution of 2-methyl-9H-thioxanthene-9-(9H-thioxanthen-9-ylidene) 1 and related structures 2-5, being the first examples of thermally stable optically active sterically overcrowded ethylenes, are reported. Substitution of the sulfur atom in the thioxanthene part of 1 with an X group (2, X = C(CH3)2; 3, NCH3; 4, O) results in a change in racemization barrier (1, 27.4 kcal.mol-1; 2, 25.1 kcal.mol-1; 3, 21.3 kcal.mol-1; 4, <20 kcal.mol-1) depending upon the aryl-X bond lenghts.

Catalytic kinetic resolution of 5-alkoxy-2(5h)-furanones

Abstract The kinetic resolution of racemic 5-alkoxy-2(5H)-furanones, using a chiral aminoalcohol catalyzed 1,4-addition of arylthiols, was examined. Using various butenolides it was shown that a γ-alkoxy substituent appears to be essential to reach high enantioselectivities whereas electron-donating substituents in the arylthiols also increase the selectivity. Cinchona alkaloids are the preferred catalysts for the kinetic resolution, with quinine and quinidine leading to the most efficient and selective thiol additions. A remarkable dilution effect and a strong dependency on the mode of addition of reactants were observed. Optimization studies are presented of the kinetic resolution of 5-methoxy-2(5H)-furanone 2 resulting in (R)-2 or (S)-2 with enantiomeric excesses exceeding 90%. A mechanism for the quinine (quinidine) catalysed kinetic resolution is given.

Sterically Overcrowded Alkenes : Synthesis, Resolution and Circular Dichroism Studies of Substituted Bithioxanthylidenes

Abstract Bithioxanthylidenes with small substituents at positions 2 and 2′ have been synthesized and resolved by chiral HPLC. The X-ray molecular structure of 9-(2′-methyl-9′H-thioxanthene-9′-ylidene)-9H-xanthene ( 17 ) confirms the folded form of these molecules. UV and CD studies of several bithioxanthylidenes are presented. The CD spectra of these molecules can be described qualitatively by subtracting the CD spectra of both differently sensed helices (M and P) present in the molecule, which can be distinguished by the substituents at positions 2 and 2′.

Chiroptical Molecular Switches 2; Resolution, Properties and Applications of Inherent Dissymmetric Alkenes.

Abstract The concept and the synthesis of the basic molecules for a chiroptical molecular switch are described. This molecular switch is based on photochemical interconversion of two bistable forms of chiral sterically overcrowded olefins. A large variety of these alkenes with different properties have been prepared by the use of a coupling reaction between a thioketone and a diazo compound.

Polystyrene based azo-dyes for non-linear optics; a new polymer-diazo coupling approach

A new direct functionalization of N-methylanilino-methylpolystyrene (α= 0.31) with p-nitrobenzenediazonium chloride yields an azastilbene modified polystyrene with excellent solubility properties.