Thioether macrocycles of the microbisporicins via reductive desulfurization

The microbisporicins are the most potent lantibiotics isolated to-date. Cyclic tetra-, hexa- and octapeptides, inspired by this family of antimicrobial agents, have been synthesized from linear peptides. Generalized reaction conditions are reported for the two-step conversion of linear peptides to thioether macrocycles: formation of a disulfide followed by reductive desulfurization. ¹H NMR analysis of the reduction reaction mixture indicated the intermediacy of a dehydroalanine when excess hexamethylphosphorus triamide (HMPT) was employed for the reduction. Maintaining a stoichiometric amount of HMPT, in dilute methanolic solution, gave the corresponding thioethers, retaining stereochemical integrity.     

Electron delocalization in cross-conjugated p-phenylenevinylidene oligomers

The synthesis, structure, and electronic properties of a series of cross-conjugated p-phenylenevinylidene oligomers with one to four double bonds are reported. The X-ray crystal structure of the compound with two double bonds reveals a nonplanar conformation with torsion angles about the C(phenylene)-C(vinylidene) and C(phenyl)-C(vinylidene) formal single bonds of 39.5(2) degrees and 30.5(2) degrees, respectively. Admixture of quinoid character in the ground state is observed. Infrared and Raman spectroscopy do not provide a clear picture of the degree of electron delocalization in the series, since the C=C stretching mode does not adequately reflect the C=C bond order and has a local nature. In contrast, electronic spectra and electrochemical data, as well as AM1 and PPP/SCF calculations, reveal that the cross-conjugated compounds basically behave as linearly pi-conjugated systems in the sense that molecular orbitals are delocalized over the entire structure and systematically change in energy. The electronic interaction between the repeating units is, however, not very strong, which has the consequence that spatial extension of the molecular orbitals does not lead to a red shift of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) electronic transition. This is related to the feature that the modest narrowing of the HOMO-LUMO gap with the chain length is accompanied by a relatively large reduction of electron repulsion. This finding implies that care should be taken in the use of electronic spectra for the evaluation of conjugation phenomena.     

Na2Mg(SO3) · 2H2O. Ein neues ternäres Magnesiumsulfit Kristallstruktur,schwingungsspektroskopische und thermoanalytische Untersuchungen

Na₂Mg(SO₃)₂ · 2H₂O. A New Ternary Magnesium Sulfite. Crystal Structure, Thermoanalytical, I.R., and Raman Data Single crystals of the hitherto unknown Na₂Mg(SO₃)₂ · 2 H₂O have been obtained by crystallization from Mg(HSO₃)₂ solutions saturated with NaCl and with the technique of gel crystallization. The crystal structure of the triclinic Na₂Mg(SO₃)₂ · 2 H₂O (P1 , Z = 1, a = 752.4(1), b = 590.3(1), c = 517.8(1) pm, α = 106.25(1), β = 109.80(1), and γ = 101.49(1)°) has been determined using single crystal X-ray diffraction data. The Mg? O distances of the nearly regular MgO₆ octahedra are between 206.6 and 210.5 pm. The MgO₆ octahedra are connected by sulfite bridges forming chains in [001], which are held together by strong hydrogen bridges. The SO₃^2? ions have nearly C_3v symmetry. The results of thermoanalytical and I.R. and Raman spectroscopic measurements are reported and discussed. The O? D stretching modes of HDO molecules in partially deuterated samples show that the water molecules differ strongly from C_2v symmetry.     

Beitr?ge zur Chemie des Schwefels. 84

Zusammenfassung Durch Zonenschmelzen konnte hochreiner Schwefel dargestellt werden, dessen Verunreinigungsgehalt (Kohlenstoff: weniger als 2,4 · 10^–4%, Silicium 1,3 · 10^–6%, Magnesium: 4,0 · 10^–6%) wesentlich kleiner war als bei allen nach anderen Verfahren gereinigten Schwefelproben. Ein Verfahren zur Bestimmung von Silicium, Eisen und Magnesium in kleinen Schwefeleinwaagen durch Spektralanalyse ohne vorherige Anreicherung wurde ausgearbeitet.

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Summary High-purity sulphur was prepared by zone melting. The percentage of impurities (carbon < 2.4×10^–4%, silicon 1.3×10^–6%, magnesium 4.0×10^–6%) was considerably smaller than in sulphur purified by other procedures. A method is described for the determination of silicon, iron and magnesium in sulphur samples by direct spectroscopic analysis without enrichment of the elements to be analysed.

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83. Mitteilung: Lutz, H. D., u. Lj. Kosti: Z. Naturforsch. 21b, 892 (1966).

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Der Deutschen Forschungsgemeinschaft danken wir für die Förderung dieser Arbeit.     

Metal ion distribution and solubility of Mn1−xCux(HCOO)2 · 2 H2O Mixed Crystals [1–5]

The metal ion distribution on the two metal sites of monoclinic Mn_1?xCu_x(HCOO)₂ · 2(H,D)₂O mixed crystals are studied by infrared and Raman spectroscopic methods. The spectral regions 3 200–3 400 cm^?1 (v_OH), 2 875–2 990 cm^?1 (v_CH), 2 330–2 500 cm^?1 (v_OD of matrix isolated HDO molecules), 1 350–1 400 cm^?1 (symmetric CO₂ stretching modes), 570–950 cm^?1 (H₂O librations), and 490 cm^?1 (M? O lattice modes) are mostly sensitive to the metal ions present. The frequency shifts of these bands with increasing content of copper show that Cu²⁺ prefers the M(1) site, coordinated by HCOO^? only. The strengths of the hydrogen bonds increase on going from manganese to copper formate, due to the increased synergetic effect of Cu²⁺. Solubility and X-ray data of the mixed crystals are included. Irrespective of the same crystal structure, two series of mixed crystals are formed: eutonic area at 0.65 ≥ x ≥ 0.5.     

Electrochemical C−H Amination by Cobalt Catalysis in a Renewable Solvent

Syntheses of substituted anilines primarily rely on palladium‐catalyzed coupling chemistry with prefunctionalized aryl electrophiles. While oxidative aminations have emerged as powerful alternatives, they largely produce undesired metal‐containing by‐products in stoichiometric quantities. In contrast, described herein is the unprecedented electrochemical C?H amination by cobalt‐catalyzed C?H activation. The environmentally benign electrocatalysis avoids stoichiometric metal oxidants, can be conducted under ambient air, and employs a biomass‐derived, renewable solvent for sustainable aminations in an atom‐ and step‐economical manner with H₂ as the sole byproduct.