Novel discotic boroxines : synthesis and mesomorphic properties

Discotic liquid crystals have attracted the attention of many research groups worldwide. Due to the one dimensional charge and ion transport in the columnar mesophase, the ability of liquid crystals (LCs) to self-heal structural defects by thermal annealing and the ease of processing via spin coating, drop casting and other solution processing methods highly useful applications could be realized. Aryl boronic acids are valuable reagents for a number of metal-catalyzed reactions, and boroxines, the cyclic trimers of organoboronic acids, may serve as useful building blocks. However, surprisingly little is known about liquid crystalline boroxines. We described a new synthetic approach to highly substituted triphenylboroxines and investigated their liquid crystalline properties. The tris(3,4,5-trialkyloxy)phenyl functionalized derivatives showed broad mesophases for a minimum alkyl chain length of C9. Textures observed under polarizing optical microscope indicated a columnar hexagonal (Colh) mesophase geometry that was confirmed by X-ray diffraction experiments

Enzymatic resolution of tropinone derivatives

The lipase-mediated kinetic resoln. of racemic scopoline and 6-hydroxytropinone is reported. Whereas (±)-scopoline is difficult to resolve due to steric hindrance of the hydroxy function, the resoln. of (±)-6-hydroxytropinone with vinyl acetate as acyl donor gave alc. (+)-6-hydroxytropinone and the corresponding acetate (-)-6-acetoxytropinone with high enantiomeric excess. In case of the optimal result of 99% ee for (+)-6-hydroxytropinone and 80% ee for (-)-6-acetoxytropinone, an E-value of 35 was calcd. The preparative lipase-catalyzed resoln. of (±)-6-hydroxytropinone resulted in almost enantiopure alc. (+)-6-hydroxytropinone with >99% ee after one recrystn

Chiral Phosphites and Phosphoramidites Based on the Tropane Skeleton and Their Application in Catalysis

Chiral BINOL phosphorochloridites were esterified and amidated with enantiopure 6-hydroxytropinone [6-hydroxy-8-azabicyclo[3.2.1]octan-3-one] cyclic acetal to yield chiral bidentate and monodentate phosphite-phosphoramidite ligands for copper-catalyzed asym. conjugate addn. of diethylzinc to cyclic enones. Condensation of N-protected pyrrole with tetrabromoacetone with subsequent acetalization and asym. hydroboration gave (6S)-6-hydroxy-8-azabicyclo[3.2.1]octan-3-one ethylene glycol acetal (8), which was treated with (2S)- and (2R)-BINOL phosphorochloridite esters (7a,b), affording chiral bidentate phosphite-phosphoramidite ligands, (S,S)-9a and (R,R)-9a; their octahydro analogs (S,S)-9b and (R,R)-9b were also prepd. Monodentate chiral phosphoramidite (6S)-6-methoxymethoxy-N-[(2S)-BINOL-phosphino]-8-azabicyclo[3.2.1]octan-3-one acetal [(S)-13] and 8-(2-methylthioethyl)-substituted tropinone acetal (2S)-BINOL phosphite esters (11a, b) were prepd. by phosphination of the corresponding N- or O-protected 8 with BINOL- or octahydro-BINOL phosphorochloridites. When applied to Cu-catalyzed asym. 1,4-addns. of dialkylzinc to cyclic enones, ranging from 2-cyclopentenone to 2-cyclooctenone (14a-d) and acyclic enones RCH:CHCOR1 (16a-d; R = Bu, Ph, 2-thienyl; R1 = Me, Ph), the bidentate phosphoramidite (S,S)-9a bearing two (S)-binaphthol units worked selectively in most cases affording the alkylation products with up to 90% ee. Ligand (S,S)-9a was also tested in Rh-mediated asym. hydrogenation of di-Me itaconate and Me acetamidocinnamate, exhibiting 85% ee and 95% ee selectivity superior to the majority of other ligands. In both reactions, a matched/mismatched case was obsd. for (S,S)-9a and its diastereomer (R,R)-9a

Gallic Esters of 4,5-Dinitrocatechol as Potential Building Blocks for Thermotropic Liquid Crystals

A series of unsubstituted and 1,4-disubstituted gallic catecholates 1, 6 and 7 as possible candidates for wedge-shaped mesogens were prepared starting from the respective benzene derivatives 2a–c and gallic esters 5a–h. The mesomorphic properties were investigated by DSC. However, only the 4,5-dinitro derivatives 1d,f–h with C8H17 and C10H21 to C12H25 alkyl side chains displayed mesophases, as evaluated by fluidity and optical anisotropy

Enantioselective desymmetrization of tropinone derivatives by hydroboration

N-Protected tropenone derivs. I (R = CO2Me, CBZ, Ts) prepd. from the resp. pyrroles and tetrabromoacetone, were used as starting materials for desymmetrization by hydroboration of the C-C double bond. Hydroboration of I with (-)-(Ipc)2BH followed by oxidn., however, gave the desired 6-hydroxylated product only in low yield due to byproduct formation. After acetalization of the carbonyl group in I, the corresponding acetals were desymmetrized with (Ipc)2BH and oxidative workup to chiral alcs. II in good yields with excellent enantiomeric excesses in most cases

Enantioselective total synthesis of cylindramide

Key steps in the convergent enantioselective synthesis of the cytotoxic natural product cylindramide are tandem Michael addn./electrophilic trapping reactions, Sonogashira coupling, Julia-Kocienski olefination, and macrocyclization. For example, the Sonogashira coupling of pentalene I with iodoacrylate II followed by Julia-Kocienski olefination with sulfone III gave an intermediate which underwent macrocyclization. Formation of the tetramic acid completed the synthesis

Chiral bicyclo[3.3.0]octa-2,5-dienes as steering ligands in substrate-dependent rhodium-catalyzed 1,4-addition of arylboronic acids to enones

The synthesis of disubstituted chiral diene ligands (3aR,6aR)- and (3aS,6aS)-I (R1 = Ph, PhCH2) with a pentalene backbone from the corresponding bicyclo[3.3.0]octa-1,4-diones II is described. The compds. II were in turn accessible by enzymic resoln. of the corresponding racemic diol and subsequent Swern oxidn. The efficiency of the ligands I in the rhodium-catalyzed 1,4-addn. of arylboronic acids ArB(OH)2 (Ar = Ph, 4-MeC6H4) to cyclic and acyclic enones, namely, 2-cyclopentenone, 2-cyclohexenone, 2-cycloheptenone, and R2CH:CHCOMe (R2 = Me, 2-furyl), was demonstrated. In the case of cyclic enones, the enantiomeric di-Ph dienes (3aR,6aR)- and (3aS,6aS)-I (R1 = Ph) were more selective than the corresponding dibenzyl analogs. When acyclic enones were employed, the result was, however, reversed: the ligands I (R1 = Ph) were nearly inactive whereas I (R1 = PhCH2) afforded the corresponding conjugate addn. products with enantioselectivities up to 91 %

Self-assembly and biological activities of ionic liquid crystals derived from aromatic amino acids.

The self-assembly of amino acid-derived ionic liquid crystals (ILCs) into lamellar or micellar-like aggregates suggests that they might interact with biological membranes. To get some insight, guanidinium chlorides derived from the natural l-amino acids phenylalanine (Phe), tyrosine (Tyr) and 3,4-dihydroxyphenylalanine (DOPA) were synthesized and their mesomorphic properties were investigated via polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (SAXS, WAXS). Mesophase types depended on the number of alkoxy side chains. Phe- and Tyr-based ILCs with one and two side chains, respectively, self-assembled into smectic A bilayers (SmA2), while Dopa-derived ILCs with three side chains formed columnar (Colh) mesophases. The mesophase ranges for Phe ILCs increased steadily with side chain length, for Tyr- and Dopa-based ILCs, however, size matching effects were observed. To clarify whether the mesomorphic behaviour has an impact on biological properties, cytotoxic and antibacterial activities of the ILCs were studied. Phe and Tyr ILCs exhibited much higher cytotoxicities (against the L-929 mouse fibroblast cell line) and/or antibacterial activities (against Staphylococcus aureus) than Dopa ILCs, which were mostly inactive. Furthermore, within each series, the side chain length largely influenced the biological activity. Thus, the bulk mesophase behaviour appeared to correlate with the biological properties, in particular, the interactions with membranes, as shown by measuring the intracellular Ca2+ concentration in human monocytic U937 cells after treatment with the amino acid-based ILCs

Fluorenone imidazolium salts as novel de Vries materials

International audienceIn ionic liquid crystals (ILCs) tilted mesophases such as SmC required for electro-optic devices are quite rare. We report a design concept that induced the SmC phase and enabled de Vries-like behaviour in ILCs. For this purpose, we synthesized and characterized a library of ILC derivativesImR(On,Ym)Xwhich consist of a rigid central fluorenone core containing an alkoxy or thioether side chain and connectedviaa flexible spacer to an imidazolium head group. The mesomorphic properties were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Temperature-dependent measurements of smectic layer spacingdby small-angle X-ray scattering (SAXS) and of optical tilt angles by POM demonstrate that ILCsImR(On,Ym)Xundergo SmA-SmC phase transitions with maximum layer contraction values between 0.4% and 2.1%. The lowest reduction factorRof 0.2 at the reduced temperatureT-T-AC= -10 K was calculated forIm(O12,S14)Br. Electron density calculations indicated a bilayer structure. Furthermore, temperature dependent emission studies show that self-assembling has a strong influence on the emission intensity of these ILCs

Total synthesis and biological evaluation of (-)-pectinatone employing a methyl-branched wax ester as key building block.

Unnatural (-)-pectinatone ((-)-3) was prepared in five steps starting from the highly methyl-branched wax ester 4, employing bromination of the ester enolate and subsequent base-induced elimination to the enoate 6 as the key step. Both (-)-3 and the amides 8b and 8c, which were isolated as by-products in the reaction sequence, displayed antimicrobial activity and cytotoxicity