Ferroelectrically switchable columnar liquid crystal

Abstract

The synthesis of a new disc-like mesogenic compound permitted, for the first time, ferroelectric electrooptical switching in a tilted columnar liquid crystal. The spontaneous molecular dipole moment is induced by the bend between the dibenzopyrene core and part of the eight O-hexyllactic acid chains attached to it. Two similar new compounds displayed a weak electrooptical effect in the columnar phase.     

Lie geometry of linear Weingarten surfaces

We show how linear Weingarten surfaces appear as special Ω-surfaces and give a characterization of those linear Weingarten surfaces that allow a Weierstrass type representation.     

Total and Semi‐Syntheses of Antimicrobial Thuggacin Derivatives

The total and semi‐synthesis of 13 new macrolactones derived from thuggacin, which is a secondary metabolite from the myxobacterium Sorangium cellulosum, are reported. The thuggacins have attracted much attention due to their strong antibacterial activity, particularly towards Mycobacterium tuberculosis. This study focuses on 1) thuggacin derivatives that cannot equilibrate by transacylation between the three natural thuggacins A–C, 2) the roles of the thiazole ring, and 3) the hexyl side chain at C2. Semi‐synthetic O‐methylation at C17 suppressed the transacylations without a substantial loss of antibacterial activity. Exchanging the C17–C25 side chain for simplified hydrophobic chains led to complete loss of antibacterial activity. Exchange of the thiazole by an oxazole ring or removal of the hexyl side chain at C2 had no substantial effect on the biological properties.     

A Facile Route to Backbone‐Tethered N‐Heterocyclic Carbene (NHC) Ligands via NHC to aNHC Rearrangement in NHC Silicon Halide Adducts

The reaction of 1,3‐diisopropylimidazolin‐2‐ylidene (iPr₂Im) with diphenyldichlorosilane (Ph₂SiCl₂) leads to the adduct (iPr₂Im)SiCl₂Ph₂ 1 . Prolonged heating of isolated 1 at 66 °C in THF affords the backbone‐tethered bis(imidazolium) salt [(^aHiPr₂Im)₂SiPh₂]²⁺ 2 Cl^? 2 (“^a” denotes “abnormal” coordination of the NHC), which can be synthesized in high yields in one step starting from two equivalents of iPr₂Im and Ph₂SiCl₂. Imidazolium salt 2 can be deprotonated in THF at room temperature using sodium hydride as a base and catalytic amounts of sodium tert‐butoxide to give the stable N‐heterocyclic dicarbene (^aiPr₂Im)₂SiPh₂ 3 , in which two NHCs are backbone‐tethered with a SiPh₂ group. This easy‐to‐synthesize dicarbene 3 can be used as a novel ligand type in transition metal chemistry for the preparation of dinuclear NHC complexes, as exemplified by the synthesis of the homodinuclear copper(I) complex [{^a(ClCu?iPr₂Im)}₂SiPh₂] 4 .     

A comparison of the structure and bonding in the donor–acceptor complexes H3N → BR(OH)2 and H3N → BRH(OH) (R = H; NH2, OH,and F): a computational investigation

Structural Chemistry - Boronic acids, R–B(OH)2, play an important role in synthetic, biological, medicinal, and materials chemistry. Borinic acids, R–BH(OH), find relevance in similar...     

The Potential of O‐MMT as a Reinforcing Filler for Uncured and Dynamically Cured PVC/XNBR Composites

Organic montmorillonite modified with quaternary ammonium (O‐MMT) was compounded with uncured and dynamically cured poly(vinyl chloride)/carboxylated nitrile butadiene rubber (PVC/XNBR) composites, using a Brabender Plasticorder at 130°C and 50 rpm rotor speed. The reinforcing efficiency of the O‐MMT was investigated in the uncured PVC/XNBR composite and the dynamically cured PVC/XNBR counterpart. Mixing and dynamic curing of the composites were monitored by typical torque‐time curves derived from a Brabender internal mixer. The torque‐time curves revealed that the dynamic curing process was successful and the incorporation of O‐MMT has no adverse effect on the processibility of the composites. It has been found that the introduction of crosslinks within the rubbery phase in the presence of the O‐MMT has further improved the tensile properties. DMA studies revealed that dynamically cured composite with O‐MMT showed higher storage modulus than the composite without O‐MMT. Furthermore, a one‐step tensile modulus vs. temperature curve and a related one peak tensile loss modulus vs. temperature curve were obtained, consequently, both are characteristics of a miscible polymers system. Further evidence on the composite miscibility was purchased by thermal scans from DSC, which showed a single glass transition temperature of PVC/XNBR composites. This claim was further supported by ATR‐IR spectra which revealed that hydrogen bonding is extensively involved in PVC/XNBR composites. This evidence unveiled the exact nature of the specific interactions responsible for miscibility and hence, enhanced mechanical properties. Furthermore, we proved in our studies the reinforcing role played by layered clay due to better dispersion, as well as improved interactions.     

Synthesis Of Isomaltose Analogues

ABSTRACT

Preparation of the α-glucosides 11, 12, 13 and 14 were accomplished through glycosylation of racemic trans-1-hydroxy-2-(hydroxymethyl)cyclohexane using 2-thiopyridyl tetra-O-benzyl-glucoside as the glycosyl donor in acceptable overall yield for α-selectivity, but with poor regioselectivity. Glycosylation under thermodynamic control using tetrabenzyl glucopyranose acetate and trimethylsilyl triflate as the promotor gave similar results. The unprotected glucosides 12 and 13 were separated and characterized by NMR spectroscopy. Similarly methyl 4-deoxy-α-isomaltoside (5a) was prepared through halide catalyzed glycosylation of methyl 2,3-di-O-benzoyl-4-deoxy-α-D-glucopyranoside (15) in acceptable yield and the unprotected compound characterized by NMR spectroscopy. Compounds 5a, 12a, 13a and the mixture 11a and 14a were all tested as substrates for the enzyme glucoamylase from Aspergillus niger and proved to be very poor substrates for the enzymic hydrolysis.