Stereoselective total synthesis (±)-trimethylsequirin-B

Abstract

Sunzvnary Stereoselective total synthesis of ( *)-trimethylsequirin-B (16) and the formation ( -J-)-trimethylsequirin-C (14) in an intermediate stage is described.

Refinement of the Solution Conformation of Valinomycin with the Aid of Coupling Constants from the 13C-Nuclear-Magnetic-Resonance Spectra

“…Its torsion angles obtained from the spin-spin couplings are summarized in Table 7. The proposed conformation is not contradicted by the long-range five-bond proton coupling 'J(H-C"C'NC"-H) recently obtained by Davies and Khaled [6] for valinomycin in C2HC13 solution. Calculations according Eqn (i) in [6] with the 4 and $ torsion angles for the A, conformation (Table 7) Table 7 The most important feature of this conformation is its general similarity to that of valinomycin-K+ complex.…”

“…The proposed conformation is not contradicted by the long-range five-bond proton coupling 'J(H-C"C'NC"-H) recently obtained by Davies and Khaled [6] for valinomycin in C2HC13 solution. Calculations according Eqn (i) in [6] with the 4 and $ torsion angles for the A, conformation (Table 7) Table 7 The most important feature of this conformation is its general similarity to that of valinomycin-K+ complex. Both conformation possess the A, 'bracelet' type of depsipeptide backbone in which the amino acid carbonyls are directed towards the symmetry axis and they both have practically the same population distribution among the Ca-Cfl rotamers of the side chains.…”

“…The spatial structures of valinomycin and of its alkali metal complexes have been studied by a variety of spectral methods (circular dichroism, NMR, infrared and Raman spectroscopy) (see reviews [l -31 and references cited therein and also [4][5][6]), and by theoretical conformational analysis [7] ; also the crystalline structure has been determined by X-ray diffraction methods solvents such as heptane, CCl, etc., in which all six NH groups participate in 4 -+ 1 hydrogen bonds with the hydroxy acid (amide) carbonyls; form B ('propeller'), predominant in solvents of medium polarity with only three intramolecular 4-1 hydrogen bonds; form C, characteristic of polar solvents, whose molecules apparently solvate all the NH groups of the valinomycin molecule.…”

Mixed Titanium–Hafnium Chloridometallate Complexes

“…For instance, the solid-state HfCl 4 structure displays hexagonal packing of chloride ions; the Hf 4+ ions occupy 1/4 of the octahedral sites, which results in bioctahedral moieties that are arranged pairwise and share an edge or a triangular face. [27,28] The well-defined dinuclear [29,30] may be considered to be derived from the solid-state structure of HfCl 4 by chopping off "Hf 2 Cl 8 " pieces and stabilizing them by addition of controlled amounts of additional chloride ions to the resulting unsaturations, whereas the addition of excess Cl -leads to the mononuclear octahedral [HfCl 6 ] 2-complex [31] (Scheme 1). Ti IV edge-sharing and face-sharing bioctahedral analogues ([Ti 2 Cl 10 ] 2-and [Ti 2 Cl 9 ] -) also exist, [32] and we have recently reported spectroscopic evidence for the existence of [Ti 2 Cl 11 ] 3-.…”

Extraction of niobium and tantalum isotopes using organophosphorus compounds. Part I – Extraction of “carrier-free” metal concentrations from HCl solutions

“…Based upon previous experiments [3][4][5][6][7][8], Pershina has postulated that, in hydrochloric acid (HCl), the change from cationic to anionic species occurs between 5 and 7 M HCl for Nb and between 6 and 8 M HCl for Ta (See Fig. 1 in [2]).…”