Enzyme-Catalyzed Synthesis of O-Glycopeptide Building Blocks

Abstract

In 1988, MOSBACH et aL¹ reported the synthesis of a-D-mannopyranosy1-L-serine and Q -D-N-acetylgalactosamino-L-serine by reversing the hydrolytic activity of a -mannosidase from jack beans and a -N-acetylgalactosaminidase from beef liver, respectively. The yields in these equilibrium-controlled syntheses² using high concentrations of monosaccharide and serine were highly dependent on the enzyme concentration and ranged from 5 to 10 percent in both cases.     

Photoemission of LaI2 and CeI2

The electronic structure of the layered compounds LaI₂ and CeI₂ was investigated by photoemission and electron energy loss spectroscopy. From the experimental results we are able to confirm the metallic nature of these compounds, and by using photon energy dependent measurements of the valence band we can identify the orbital character of the conduction band as essentially 5d¹-like. A detailed analysis of the Ce 3d and 4f spectra yields a remarkably small 4f-5d hybridization strength, almost completely decoupling the f-electron from the conduction band, which makes CeI₂ a somewhat unusual system compared to other metallic Ce compounds. Band structure calculations by Jepsen and Andersen [1] confirm these experimental results.     

Double-quantum filtered MAS NMR in the presence of chemical shielding anisotropies and direct dipolar and J couplings

Double-quantum filtered MAS NMR spectra of an isolated homonuclear spin-1/2 pair are considered, at and away from rotational resonance conditions. The pulse sequence used is the solid-state NMR equivalent of double-quantum filtered COSY, known from solution-state NMR. The 119Sn spin pair in [(chex3Sn)2S] is characterized by a difference in isotropic chemical shielding smaller than the two chemical shielding anisotropies and by direct dipolar and isotropic J-coupling constants of similar magnitudes. At rotational resonance, one-dimensional double-quantum filtered 119Sn lineshapes yield the relative orientation of the two 119Sn chemical shielding tensors. Good double-quantum filtration efficiencies are found at and away from rotational resonance conditions, despite the presence of large chemical shielding anisotropies. Numerical simulations illustrate the interplay of the direct dipolar and J-coupling pathways and identify the latter as the main pathway even at rotational resonance conditions.     

Optical activity induced by a velocity gradient in a monatomic gas

In a pair of atoms in collision, the polarizability of the system depends on the relative velocity. The mechanism introduced here implies the time dependence of the effective field acting on the partners. The linear term of the expansion with respect to the velocity suggests the existence of an optical activity for a monatomic fluid submitted to a non-uniform flow.     

Übergangsmetall-substituierte phosphane,arsane und stibane : XXV. C5H5(CO)2[P(OMe)3]M-substituierte dioxaphospholane und dioxarsolane; stereochemische evidenz für den intermolekularen ablauf einer isomerisierung unter methylgruppenverschiebung

2-C₅H₅(CO)₂[P(OMe)₃]Mo(W)-substituted 4,4,5,5-tetramethyl-1,3,2-dioxarsolanes and -dioxaphospholanes (Ia–Ic) are obtained from C₅H₅(CO)₃M derivatives via CO/P(OMe)₃ exchange. In all cases the phosphite ligand appears trans to the σ-bonded arsenic heterocycle, which prefers a conformation with the transition metal group in an axial position. The dioxaphospholane-metal (Ia) undergoes isomerization with alkyl migration at 0°C, which due to stereochemical reasons can only occur by an intermolecular mechanism.     

Enantioselective Synthesis of Planar Chiral [Cr(η6-Arene)(CO)3] complexes via nucleophilic addition/hydride abstraction. Preliminary communication

Non-racemic, planar chiral 1, 2-disubstituted [Cr(η⁶-arene)(CO)₃] complexes were obtained via external chiral ligand-controlled nucleophilic addition of alkyl-, vinyl-, and aryllithium reagents to monosubstituted complexes followed by an endo-hydride abstraction with trityl cation. The reactions with [Cr(CO)₃(η⁶-phenyloxazoline)], [Cr(CO)₃(η⁶-phenylmethaneimine)], and [Cr(CO)₃ (η⁶-phenylmethanenydrazone)] took place with complete ortho-selectivity and a high degree of enantioselectivity (up to 98% ee).