Chiral recognition in NMR spectroscopy using crown ethers and their ytterbium(III) complexes

Chiral crown ethers 1 and 5 are useful enantiomeric discriminating agents in ¹H NMR spectroscopy for neutral and protonated primary amines, amino acids, and amino alcohols. The presence of the carboxylic acid groups in 1 and 5 provide sites at which ytterbium(III) can bind. Adding ytterbium(III) nitrate to crown–substrate mixtures in methanol-d₄ causes shifts in the spectra of substrates and often enhances the chiral discrimination in the ¹H NMR spectrum. The enhancement in enantiomeric discrimination that occurs in the presence of ytterbium(III) allows lower concentrations of the crown ether to be used in chiral recognition studies. Several amide derivatives of 1 were prepared and evaluated as chiral NMR discriminating agents, although except for 1e, these were less effective than 1.     

Stereoselective synthesis of the cyclic ether core of (+)-laurenyne

A concise enantioselective synthesis of the cyclic ether core of the marine natural product (+)-laurenyne has been accomplished using ring-closing metathesis for medium-ring construction.     

Detecting the "afterglow" of 13C NMR in proteins using multiple receivers

We show that the weak signal that remains after (13)C-detected experiments (the (13)C "afterglow") can still be measured with high sensitivity by proton detection. This is illustrated by the incorporation of two experiments, 2D (HA)CACO and 3D (HA)CA(CO)NNH, into a single pulse sequence that makes use of two receivers in parallel. In cases where the sensitivity is not limiting, such as applications to small proteins, the inclusion of the projection-reconstruction method permits the recording of both spectra in only 15 min. High-quality data sets for the 143 residue nuclease A inhibitor (2 °C, correlation time 17.5 ns) were obtained in 3 h, illustrating the utility of the method even in studies of moderately sized proteins.     

Synthesis in ionic liquids: [Bi2Te2Br](AlCl4), a direct gap semiconductor with a cationic framework

The Lewis acidic ionic liquid EMIMBr-AlCl(3) (EMIM = 1-ethyl-3-methylimidazolium) allows a novel synthetic route to the semiconducting layered metal chalcogenides halide [Bi(2)Te(2)Br](AlCl(4)) and its Sb analogue. [Bi(2)Te(2)Br](AlCl(4)) is a direct band gap, strongly anisotropic semiconductor and consists of cationic infinite layers of [Bi(2)Te(2)Br](+) and [AlCl(4)](-) anions inserted between the layers.     

Natural-abundance 15N--13C correlation spectra of vitamin B-12

Two-dimensional nitrogen-carbon NMR correlation spectra have been derived by a new reconstruction technique based on standard two-dimensional HMQC and HMBC spectra, and operating with natural 15N and 13C isotopic abundances. Compared with conventional three-dimensional spectroscopy in which 15N and 13C spins must be present in the same molecule, the reconstruction method offers two orders of magnitude improvement in sensitivity. Vitamin B-12 serves as an illustrative example.     

The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ‐40, have been characterized. HIQ‐40 is made from 40 mol percent p‐hydroxybenzoic acid (HBA) and 30 mol % each of p‐hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution‐cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ‐40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T_g is observed in the range of 110–115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T_g increases rapidly until it is near the annealing temperature, then T_g increases more slowly at longer annealing times. In as‐cast films annealed at 120–135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 505–522, 1999     

Electrophile-mediated cyclizations in carbohydrate chemistry: synthesis of highly functionalized ribofuranose and ribopyranose compounds

Iodine-mediated cyclization of (Z)- and (E)-{=D}-ribohept-2-enonates $\text{1}$ and $\text{2}$ gave exclusively the β-ribofuranose and α-ribofuranose derivatives $\text{3}$ and $\text{4}$, respectively. Cyclization of the (Z)- and (E)-2-heptene-1-ol derivatives $\text{5}$ and $\text{6}$ gave ribofuranose products ($\text{7}$ and $\text{8}$) and a ribopyranose ($\text{9}$), respectively.