LiTiCl3: Synthesis and thermal behaviour

LiTiCl₃ is obtained as one example within an ample solid solution, Li_24–2nTi_nCl₂₄ (?4?n?10), by synpropotionation of TiCl₃ and Ti in the presence of LiCl (2:1:3 molar ratio) in sealed tantalum tubes at 750°C. It crystallizes with the inverse spinel-type structure according to (Li_0.67)^[4](Li_0.67Ti_1.33)^[4]Cl₄ with, at 25°C, a = 1048.62(4) pm, space group Fd3m. Thermal expansion is linear with α = 4.85 × 10^?5K^?1 up to about 300°C and thereafter, when the migration of Li⁺ from tetrahedral to octahedral interstices becomes increasingly important, it exhibits a relative decrease resulting, finally, in the phase transition to a NaCl-type structure that is observed for the first time at about 575°C.     

CZE separations of basic proteins at low pH in fused silica capillaries with surfaces modified by silane derivatization and/or adsorption of polar polymers

Mixtures of several basic proteins have been used to test CZE capillaries with surfaces modified by new pretreatment procedures; the performance obtained has been compared with that achieved using capillaries treated by procedures described in the literature. It has been shown that addition of non-ionic polyvinylalcohols (PVA) to CZE buffer solutions deactivates even bare, i.e. untreated, fused silica surfaces and renders them suitable for separations of basic proteins. The performance obtained from such surfaces was comparable with that of capillaries modified by the more elaborate procedures of etching, silanol derivatization, and/or adsorptive coating (again with polymers). A home-made device is described which enables derivatization and coating reactions to be performed on fused silica capillaries under an inert atmosphere, i.e. one free from oxygen and water.     

Thermodynamic and kinetic data for adduct formation, cis-trans isomerization and redox reactions of ML4 complexes: a case study with rhodium- and iridium-tropp complexes in d8, d9 and d10 valence electron configurations (tropp=dibenzotropylidene phosphanes)

The formation of adducts of the square-planar 16-electron complexes trans-[M(tropp(ph))(2)](+) and cis-[M(tropp(ph))(2)](+) (M=Rh, Ir; tropp(Ph)=5-diphenylphosphanyldibenzo[a,d]cycloheptene) with acetonitrile (acn) and Cl(-), and the redox chemistry of these complexes was investigated by various physical methods (NMR and UV-visible spectroscopy, square-wave voltammetry), in order to obtain some fundamental thermodynamic and kinetic data for these systems. A trans/cis isomerization cannot be detected for [M(tropp(ph))(2)](+) in non-coordinating solvents. However, both isomers are connected through equilibria of the type trans-[M(tropp(ph))(2)](+)+L<==>[ML(tropp(ph))(2)](n)<==>cis-[M(tropp(ph))(2)](+)+L, involving five-coordinate intermediates [ML(tropp(ph))(2)](n) (L=acn, n=+1; L=Cl(-), n=0). Values for K(d) (K(f)), that is, the dissociation (formation) equilibrium constant, and k(d) (k(f)), that is, the dissociation (formation) rate constant, were obtained. The formation reactions are fast, especially with the trans isomers (k(f)>1x10(5) m(-1) s(-1)). The reaction with the sterically more hindered cis isomers is at least one order of magnitude slower. The stability of the five-coordinate complexes [ML(tropp(ph))(2)](n) increases with Ir>Rh and Cl(-)>acn. The dissociation reaction has a pronounced influence on the square-wave (SW) voltammograms of trans/cis-[Ir(tropp(ph))(2)](+). With the help of the thermodynamic and kinetic data independently determined by other physical means, these reactions could be simulated and allowed the setting up of a reaction sequence. Examination of the data obtained showed that the trans/cis isomerization is a process with a low activation barrier for the four-coordinate 17-electron complexes [M(tropp(ph))(2)](0) and especially that a disproportionation reaction 2 trans/cis-[M(tropp(ph))(2)](0)-->[M(tropp(ph))(2)](+)+[M(tropp(ph))(2)](-) may be sufficiently fast to mask the true reactivity of the paramagnetic species, which are probably less reactive than their diamagnetic equilibrium partners.     

A novel synthesis of substituted 4H-pyrazolo[3,4-d]pyrimidin-4-ones

A novel synthesis of 4H-pyrazolo-[3,4-d]pyrimidin-4-ones is described. This approach utilizes an in situ generated iminochloride as a key precursor for amidine formation, with subsequent base-catalyzed ring closure. This method represents a mild and efficient entry into this ring system which is amenable to diversification of the core template.     

Chlor-dimethylaminosulfonium-Salze Darstellung und IR-Spektren

Chloro-dimethylaminosulfonium Salts. Preparation and I. R. Spectra By reaction of bis-dimethylamino-mono- resp. -disulfane with a mixture of chlorine/antimony(V) chloride the chloro dimethylaminosulfonium hexachloroantimonates(V) [(CH₃)₂N]₂SCI^⊕SbCl₆^? (I) and (CH₃)₂NSCI₂^⊕SbCl₆^? (II) are yielded. The i. r. spectra of these compounds were measured and assigned.     

Chemoenzymatic approach to enantiopure streptogramin B variants: characterization of stereoselective pristinamycin I cyclase from Streptomyces pristinaespiralis

Streptogramin B antibiotics are cyclic peptide natural products produced by Streptomyces species.In combination with the synergistic group A component, they are "last line of defense" antimicrobial agents against multiresistant cocci. The racemization sensitivity of the phenylglycine (Phg(7)) ester is a complex challenge in total chemical synthesis of streptogramin B molecules. To provide fast and easy access to novel streptogramin antibiotics, we introduce a novel chemoenzymatic strategy in which diversity is generated by standard solid phase protocols and stereoselectivity by subsequent enzymatic cyclization. For this approach, we cloned, overproduced, and biochemically characterized the recombinant thioesterase domain SnbDE TE of the pristinamycin I nonribosomal peptide synthetase from Streptomyces pristinaespiralis. SnbDE TE catalyzes regioselective ring closure of linear peptide thioester analogues of pristinamycin I as well as stereoselective cyclization out of complex in situ racemizing substrate mixtures, enabling synthesis of Streptogramin B variants via a dynamic kinetic resolution assay. A remarkable substrate tolerance was detected for the enzymatic cyclization including all the seven positions of the peptide backbone. Interestingly, SnbDE TE was observed to be the first cyclase from a macrolactone forming NRPS which is additionally able to catalyze macrolactamization of peptide thioester substrates. An N-methylated peptide bond between positions 4 and 5 is mandatory for a high substrate turnover. The presented strategy is potent to screen for analogues with improved activity and guides our understanding of structure--activity relationships in the important class of streptogramin antibiotics.     

Acyl- und Alkylidenphosphane. XXII [1]. Synthese und Struktur des 1, 3-Dimethyl-2,2,4,4-tetrakis-(trimethylsilylsulfano)-1,3-diphosphetans

Acyl- and Alkylidenephosphines. XXII. Synthesis and Structure of 1, 3-Dimethyl-2,2,4,4-tetrakis(trimethylsilylsulfano)-1,3-diphosphetane At ?30°C methylbis(trimethylsilyl)phosphine reacts with carbon disulfide to give a red adduct first which rearranges to [bis(trimethylsilylsulfano)methylidene]methylphosphine 1a . In contrast to the thermally stable phenyl derivative 1b [2], this compound with its insufficiently shielded P?C group dimerizes fast with increasing temperature. 1,3-Dimethyl-2,2,4,4-tetrakis(trimethylsilylsulfano)-1,3-diphosphetane 2a formed by this reaction, crystallizes in the triclinic space group P1 with following dimensions of the unit cell, determined at a temperature of measurement of ?80 ± 3°C: a = 1024.7(3); b = 1360.2(5); c = 1326.3(6)pm; α = 117.85(4); ß = 111.05(3); γ = 72.09(3)°; Z = 2. Due to ring folding at the P1? P2 axis of 149.1°, the molecule shows pseudosymmetry C_s. Characteristic averaged bond lengths and angles obtained at an R_w-value of 0.030, are: P? C(endocyclic) 188 and 191; P? CH₃ 184; C? S 183; S? Si 216 pm; C? P? CH₃ 105; P? C? S 113; S? C? S 114; C? S? Si 108; P? C? P 90 and C? P? C 86°.     

Solid-solute phase equilibria in aqueous solution. VI. Solubilities,complex formation,and ion-interaction parameters for the system Na+−Mg2+−ClO 4 − −CO2−H2O at 25°C

The stoichiometric solubility constant of eitelite (NaMg _0.5 CO ₃ +2H⁺ ⇄ Na⁺+0.5Mg ²⁺ +CO ₂ (g)+H ₂ O, log^*K _pso ^I =14.67±0.03 was determined at I=3 m (mol kg⁻¹) (NaClO ₄ ) and 25°C. The stability of magnesium (hydrogen-)carbonato complexes in this ionic medium was explicitely taken into account. Consequently, trace activity coefficients of free ionic species, calculated from the Pitzer model with ion-interaction parameters from the literature, were sufficient for an evaluation of the thermodynamic solubility constants and Gibbs energies of formation for eitelite (−1039.88±0.60), magnesite (−1033.60±0.40), hydromagnesite (−1174.30±0.50), nesquehonite (−1724.67±0.40), and brucite (−835.90±0.80 kJ-mol ⁻¹ ). The increasing solubilities of nesquehonite and eitelite at higher sodium carbonate molalities were explained by invoking a magnesium dicarbonato complex (Mg²⁺+2CO ₃ ²⁻ ⇄ Mg(CO₃) ₂ ²⁻ , log β_z = 3.90 ± 0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic acid in 1 to 3.5 m NaClO ₄ media which reproduce these constants to 0.02 units in log K. The following Pitzer parameters are consistent with the previously studied formation of magnesium (hydrogen-)carbonato complexes in 3m NaClO ₄ . The model and Gibbs functions of solid phases derived here reproduce original solubility data (−log [H⁺], [Mg ²⁺ ] _tot ) measured in perchlorate medium within experimental uncertainty. Presented at the XXII International Conference on Solution Chemistry, July 14–19, 1991, Linz, Austria.     

Salvinicins A and B, new neoclerodane diterpenes from Salvia divinorum

[reaction: see text] Two new neoclerodane diterpenes, salvinicins A (4) and B (5), were isolated from the dried leaves of Salvia divinorum. The structures of these compounds were elucidated by spectroscopic techniques, including (1)H and (13)C NMR, NOESY, HMQC, and HMBC. The absolute stereochemistry of these compounds was assigned on the basis of single-crystal X-ray crystallographic analysis of salvinicin A (4) and a 3,4-dichlorobenzoate derivative of salvinorin B.     

Efficient, highly regioselective, and stereospecific conversion of glycidol systems into C2-O-acylated vicinal halohydrins

Glycidyl esters and ethers undergo a regioselective and stereospecific opening of the oxirane ring upon treatment in chloroform in the presence of pyridine with trimethylsilyl halide (TMSX, X = Cl, Br, or I) and a mixture of carboxylic acid (CA)-trifluoroacetic anhydride (TFAA), to produce the corresponding C2-O-acylated vicinal halohydrins in high yields.