Rm Kellogg

Cyclic bis-urea compounds as gelators for organic solvents

The gelation properties of bis-urea compounds derived from opti- cally pure trans-1,2-diaminocyclohexane and 1,2-diaminobenzene, with pendant aliphatic, aromatic, or ester groups, as well as the structure of the resulting gels, have been studied by differential scan- ning calorimetry, infrared spectroscopy, small-angle X-ray diffraction, and elec- tron microscopy. These compounds have been found to be very potent gelators for organic solvents, such as aliphatic and aromatic hydrocarbons, esters, ke- tones, and alcohols, at concentrations well below 1 (w/v)%. Gelation by these compounds is completely thermorever- sible, with melting temperatures up to 1208C, and many of the gels display thixotropic properties. Even at low con- centrations these compounds self-as- semble into elongated and very thin fibers, which in turn form a three-dimen- sional network in the solvent. Infrared studies showed that aggregation is ac- companied by the formation of a hydro- gen-bonded network between urea moi- eties, and a single-crystal X-ray structure of one of the compounds showed that in crystals the molecules assemble into one-dimensional chains, which are sta- bilized by the formation of eight hydro- gen bonds between the urea groups and adjacent molecules. The molecular ar- rangement in gels is most likely very similar to that in the crystal, but the complete elucidation of the molecular arrangement in gels is complicated be- cause aggregation of these compounds is prone to polymorphism. It is concluded that the very efficient aggregation of these molecules and the elongated shape of the fibers most likely arise from the highly anisotropic hydrogen-bonding properties of these molecules, which is due to the presence of two coplanar oriented urea moieties in a single mol- ecule. Since the bis-urea compounds presented in this paper are very easy to synthesize and many structural varia- tions are possible without loss of the gelation ability, they are excellent build- ing blocks for the construction of func- tional gels.

Di-urea compounds as gelators for organic solvents

Abstract Simple diurea compounds form thermoreversible gels with several organic solvents. These gels are stable up to temperatures of 100°C, and can be stored for months. Electron microscopy reveals that in these solvents the gelation agents assemble into very thin rectangular sheets which are several tens of micrometers long.

THE SCOPE AND LIMITATIONS OF LIQUID-CRYSTALLINE BEHAVIOR IN MONOSACCHARIDE AMPHIPHILES - COMPARISON OF THE THERMAL-BEHAVIOR OF SEVERAL HOMOLOGOUS SERIES OF D-GLUCOSE DERIVED COMPOUNDS WITH AN AMINO-LINKED ALKYL CHAIN

Abstract A short review of the scope and limitations of liquid-crystalline behaviour in carbohydrate derivatives is presented. In order to investigate the influence of structure variations on the thermal behaviour of monosaccharide amphiphiles, six homologous series of D-glucose and 2-deoxy-D-glucose derivatives with an aminolinked n-alkyl chain were prepared. The observed thermal behaviour could be readily explained with the aid of a refinement of the qualitative model that was presented earlier [1]. All compounds were found to be mesogenic, and the observed mesophase was smectic Ad (i.e. a partially overlapping bilayer structure) in all cases.

SYNTHESIS AND MESOGENIC PROPERTIES OF SEVERAL HOMOLOGOUS SERIES OF ALDOSE DIALKYL DITHIOACETALS - A MODEL FOR THEIR BEHAVIOR

Abstract Ten homologous series (n-butyl through n-decyl) of aldose S,S-acetals (D-glucose, D-galactose, D-mannose, L-rhamnose, 2-deoxy-D-glucose, D-xylose, D-lyxose, D- or L-arabinose, D-ribose and 2-deoxy-D-ribose) have been prepared. Most of these compounds form thermotropic liquid crystals, the exceptions being the entire L-rhamnose series and some of the derivatives with the shortest alkyl chains. All of the compounds have been investigated with differential scanning calorimetry and polarization microscopy. Some temperature dependent powder X-ray data are also presented. A model is proposed which correlates the carbohydrate configuration with the melting behaviour. On the basis of now available behaviourial characteristics, visual inspection, mixing experiments and precedent, the mesophase is identified as smectic Ad, the partially overlapping carbohydrate moieties being in the centre and the aliphatic chains pointing outward at an angle of about 62°. Despite the intrinsic chirality of all the carbohy...

A new synthetic route to symmetrical photochromic diarylperfluorocyclopentenes

Abstract Symmetrical and photochromic diarylperfluorocyclopentene has been prepared by reaction of 3-lithio-5-chloro-2-methylthiophene with the ethyl ester of hexafluoroglutaric acid, followed by ring closure via a McMurry coupling. Compound 7 is a versatile intermediate for the development of photochromic materials.

Optically active 6-acetyloxy-2H-pyran-3(6H)-one obtained by lipase catalyzed transesterification and esterification

Abstract Kinetic resolution of 6-acetyloxy-2 H -pyran-3(6 H )-one ( 1 ) is achieved by immobilized lipase PS on Hyflo Super Cell in organic solvents. Transesterification in hexane/n-butanol yields enantiomerically pure R -(−)-6-acetyloxy-2 H -pyran-3(6 H )-one, whereas esterification of 6-hydroxy-2 H -pyran-3(6 H )-one ( 2 ) with vinyl acetate by immobilized lipase PS gives the S -enantiomer with e.e.s up to 76%.

CHEMISTRY OF DIHYDROPYRIDINES .5. HYDRIDE TRANSFER FROM 1,4-DIHYDROPYRIDINES TO PYRIDINIUM SALTS - ASSESSMENT OF STRUCTURAL AND ENERGETIC FACTORS WITH HANTZSCH ESTER DERIVED COMPOUNDS

Abstract Hydride exchange occurs between 3,5 - di(alkoxycarbonyl) - 1,4 - dihydropyridines and their corresponding pyridinium salts. For the case of 1,2,6 - trimethyl - 3,5 - di(ethoxycarbonyl) - 1,4 - dihydropyridine in the presence of the structurally corresponding pyridinium perchlorate, hydride is transferred to the 4-position of the pyridinium salt in a reversible “blind” reaction as revealed by deuterium labeling experiments and to the 2,6-positions irreversibly to afford 1,2,6 - trimethyl - 3,5 - di(ethoxycarbonyl) - 1,2 - dihydropyridine as final product. Removal of the methyl groups at the 2,6-positions, i.e. 1 - methyl - 3,5 - di(methoxycarbonyl) - 1,4 -dihydropyridine and its structurally corresponding pyridium perchlorate, causes hydride transfer to become completely reversible. Substitution of the 4-position with Me, i.e. 1,2,4,6 - tetramethyl - 3,5 - di(methoxycarbonyl) -1,4- dihydropyridine and its corresponding pyridinium perchlorate leads to cessation of hydride transfer: the same is true for the analogous 4-phenyl (and substituted phenyl) compounds. However, these 1,4-dihydropyridines are capable of transferring hydride at reasonable temperatures to less highly substituted pyridinium salts. Activation parameters for some of these hydride transfers have been determined, mechanistic conclusions are presented, and the consequences of these observations for experiments with “model” NADH compounds are discussed.

EFFECT OF CONFORMATION OF CYCLIC SULFIDE LIGANDS ON THE MODE OF COMPLEXATION TO SILVER SALTS

The cyclic trisulfide 1,3,5-trithiane has a flattened chair conformation. It forms with AgSO3CF3 a dimeric complex wherein each silver is tetracoordinated bearing three sulfides and one triflate ion as ligands. On the other hand 1,4-dithiane, which also has a chair conformation, forms an infinite polymer with AgO2CCF3 wherein each silver is coordinated to two different dithianes and two different CF3CO2 ligands.

Non-amphiphilic carbohydrate liquid crystals containing an intact monosaccharide moiety

Abstract A chiral rigid moiety which forms the basis of a new class of non-amphiphilic carbohydrate liquid crystals has been developed. This moiety contains a fully intact glucopyranose ring embedded in a trans-decalin structure. The original carbohydrate is substituted so that only two hydroxyl groups are left, resulting in derivatives with reduced hydrophilicity. The substituents R and X-R′ on the 4,6-O-ylidene s-D-glucopyranoside are in the equatorial position and can be varied extensively, using straightforward synthetic procedures. Investigations as to the requirements for R and X-R′ for inducing liquid-crystalline behavior have shown that at least one of the substituents should contain a large, polarizable aromatic moiety. An aromatic Schiff base fulfils this requirement.

The crystal structure of heptyl 1-thio-α-d-glucopyranoside, a member of a new homologous series of mesogenic carbohydrate derivatives

Abstract The crystal structure of heptyl 1-thio-α- d -glucopyranoside, C 13 H 26 O 5 S, is monoclinic, space group P 2 1 , with Z = 2, a = 6.013(1), b = 7.324(1), c = 17.536(4) A, β = 98.21(2)°, V = 764.4 A 3 , D calc = 1.279 g.cm −3 . The crystal involves a bilayer head-to-head molecular packing with interdigitizing alkyl chains. The carbohydrate moieties are hydrogen-bonded in finite chains which include all the oxygen atoms and which are linked 1 into a network by the minor component of the three-center bond from HO-6. The crystal structure is transformed into a smectic A liquid crystal phase at 96.8° (Δ H 34.6 kJ.mol −1 ) that shows a periodicity of 23.1 A. The clearing point is at 138.3° (Δ H 2.2 kJ.mol −1 ).