László Fenichel

Asymmetric Michael addition and deracemization of enolate by chiral crown ether

Abstract Crown ethers anellated to glucose units have been used to catalyse the enantioselective Michael addition of methyl phenylacetate to methyl acrylate in high chemical yields in up to 84% enantiomeric excess. A novel CH-acid deracemization has also been discovered and the mechanistic rationale of the asymmetric induction is discussed. The proposed mechanism of the addition was also substantiated by molecular mechanics calculations.

Asymmetric Michael reaction. Deracemization of enolate by chiral crown ether

Abstract Crown ethers anellated to sugar units have been used to catalyse the enantioselective carbon-carbon bond forming reaction of methyl phenylacetate with methyl acrylate. A novel CH-acid deracemization has also been discovered in this study.

Cleavage of the acetal rings in bis(methyl 4,6-O-benzylidene-α-d-glucopyranosido)-18-crown-6

Abstract Treatment of bis(methyl 4,6- O -benzylidene-2,3-dideoxy-α- d -glucopyranosido [2,3- b ][2′,3′- k ])-1,4,7,10,13,16-hexaoxacyclo-octadecane ( 1 ) with aqueous acetic acid gave bis(methyl 2,3-dideoxy-α- d -glucopyranosido[2,3- b ][2′,3′- k ])-1,4,7,10,13,16-hexaoxacyclo-octadecane ( 2 ). With LiAlH 4 AlCl 3 , 1 gave a mixture of three O -benzyl derivatives in which bis(methyl 4- O -benzyl-2,3-dideoxy-α- d -glucopyranosido[2,3- b ][2′,3′- k ])-1,4,7,10,13,16-hexaoxacyclo-octadecane preponderated. Methylation and butylation of 2 in a two-phase system gave the tetramethoxy and tetrabutoxy crowns. Bis(methyl 4- O -acetyl-2,3-dideoxy-6- O -trityl-α- d -glucopyranosido[2,3- b ][2′,3′- k ])-1,4,7,10,13, 16-hexaoxacyclo-octadecane ( 5 ) was obtained from 2 by tritylation and acetylation. Detritylation of 5 with acetic acid-hydrogen bromide gave bis(methyl 4- O -acetyl-2,3-dideoxy-α- d -glucopyranosido[2,3- b ][2′,3′- k ])-1,4,7,10,13, 16-hexaoxacyclo-octadecane. Treatment of 1 with N -bromosuccinimide gave the 4-benzoyl-6-bromo-6-deoxy compound which was suitable for making a new ring with a trans -annular connection. The complex stability constant of each new crown has been measured and evaluated.

The complexing ability of crown ethers incorporating glucose

The complexing (in CHCl3) and extracting abilities of 18-crown-6 type compounds (1–15) were measured with Li, Na, K and NH4 cations. The substituents on the sugar part affected these properties significantly (Ka=103–107). Some substituents, like acetyloxy groups (3) decreased whereas others, like tosyloxy groups (10, 11) significantly increased the complexing ability and thus changed the selectivity. The compound with four tosyloxy groups (11) shows an excellent picrate salt extracting ability in a CH2Cl2-water system.

Novel azacrown ethers incorporating sugars: synthesis and complex formation

Azacrown ethers derived from amino-deoxy sugars in which the sugar moiety forms part of the macro-cycle, or is pendant from it, have been synthesised and their complexing ability towards cations compared.

Methylation of cyclodextrins by phase-transfer catalysis

A new, simple method has been developed for the methylation of cyclodextrins. The reaction proceeds in the heterogeneous phase with dimethyl sulphate, using a solvent in which the original cyclodextrins and the bases used are poorly soluble or insoluble. However, in the presence of phase transfer catalysts, methylation proceeds with good yields. The products are mixtures of randomly methylated cyclodextrins (RAMEB), containing 60–70% of heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB), 10–15% of heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) and some monomethylated isomers. These methylated products have proved to be excellent detergents; e.g., they are able to significantly increase the water solubility of hydrocortisone, methyltesterone, etc. On repeating methylation twice, the amount of TRIMEB increases, and a pure product (28% yield) can be obtained by crystallisation.

Reaction of penta-O-benzoyl-β-d-glucopyranose with titanium tetrachloride and preparation of complexes of peracylated, mixed esters of glucose

Abstract Preparative experiments and kinetic measurements have shown that penta- O -benzoyl-β- d -glucopyranose reacts instantaneously with titanium tetrachloride to give tetra- O -benzoyl-β- d -glucosyl chloride, which is anomerized to the α isomer by the catalytic action of benzoyloxytitanium trichloride, formed in the reaction as a by-product. Experiments with mixed acetic-benzoic esters indicate that complex formation, essential for the reaction to occur, takes place selectively, probably at the acyl group in position 1.

Synthesis and complex-forming properties of crown ethers incorporating glucuronic acid moieties

Crown ethers1 and4 of the 18-crown-6 type containing two glucose units have been oxidised by KMnO4 into mono- and dicarboxylic acid derivatives (5 and11), and derivatives with different lipophilicities of the above crown ethers, namely the acetyl, benzyl and butyl derivatives (8–10, 13, 14) and methyl esters (6 and12) have been synthesizedThe association constants (Ka) with Li, Na, K and NH4 cations measured in CHCl3 indicate that complexing ability increases on introduction of carboxy groups, and selectivity changes in favour of the Na cation. These compounds were able to transport alkyl-ammonium salts through a CHCl3 liquid membrane, displaying, however, no chiral recognition ability.