Synthesis and characterization of Mn-, La-Mn- and La-Ca-Mn-citrates as precursors for LaMnO<Subscript>3</Subscript> and La<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

Mn-, LaMn- and LaCaMn-citrates were synthesized at 60–120°C in ethylene glycol medium using chlorides or nitrates as metal sources. Their composition, IR spectra and thermal decomposition were studied. Equimolar La/Mn ratio has been established in the complex, prepared from chloride solution with the same initial composition of the metals. In the isolated three-metallic complex the molar ratio of the metals deviates from the composition in the initial solution. The final products of the heating of Mn- and mixed-metal LaMn-citrates at 1000°C are phase-homogeneous Mn₃O₄ (hausmannite) and LaMnO₃ respectively. Parasitic phase(s) are observed in La_xCa_1−xMn_yO₃, produced from LaCaMn-citrate.     

Theoretical analysis of the porphyrin-porphyrin exciton interaction in circular dichroism spectra of dimeric tetraarylporphyrins

Chiral bis-porphyrins are currently the subject of intense interest as chiral receptors and as probes in the determination of structure and stereochemistry. To provide an improved framework for interpreting the circular dichroism (CD) spectra of bis-porphyrins, we have calculated the CD spectra of chiral bis-porphyrins from three classes: I, where porphyrins can adopt a relatively wide range of orientations relative to each other; II, porphyrins have a fixed relative orientation; III, porphyrins undergo pi-stacking. The calculations primarily utilized the classical polarizability theory of DeVoe, but were supplemented by the quantum mechanical matrix method. Class I was represented by three isomers of the diester of 5alpha-cholestane-3,17-diol with 5-(4'-carboxyphenyl)-10,15,20-triphenylporphin (2-alphabeta, 2-betaalpha, 2-betabeta). Careful analysis of the torsional degrees of freedom led to two to four minimum-energy conformers for each isomer, in each of which the phenyl-porphyrin bonds had torsional angles near 90 degrees. Libration about these bonds is relatively unrestricted over a range of +/-45 degrees. CD spectra in the Soret region were calculated as Boltzmann-weighted averages over the low-energy conformers for each isomer. Three models were used: the effective transition moment model, in which only one of the degenerate Soret components is considered, along the 5-15 direction; the circular oscillator model, in which both Soret components are given equal weight; and the hybrid model, in which the 10-20 oscillator is given half the weight of the 5-15 oscillator, to mimic the effect of extensive librational averaging about the 5-15 direction. All three models predict Soret exciton couplets with signs in agreement with experiment. Quantitatively, the best results are given by the hybrid and circular oscillator models. These results validate the widely used effective transition moment model for qualitative assignments of bis-porphyrin chirality and thus permit application of the exciton chirality model. However, for quantitative studies, the circular oscillator or hybrid models should be used. The simplified effective transition moment and hybrid models are justified by the librational averaging in the class I bis-porphyrins and should only be used with such systems. Two class II bis-porphyrins were also studied by DeVoe method calculations in the circular oscillator model, which yielded good agreement with experiment. Class III bis-porphyrins were represented by 2-alphaalpha, for which the calculations gave qualitative agreement. However, limitations in the conformational analysis with the close contacts and dynamic effects in these pi-stacked systems preclude quantitative results.     

Enantioselective binding of an 11-cis-locked cyclopropyl retinal. The conformation of retinal in bovine rhodopsin

[formula: see text] The conformation of the retinal chromophore in rhodopsin is central for understanding the visual transduction process. The absolute twist around the 12-s bond of the chromophore in rhodopsin has been determined by studies with 11-cis-locked 11,12-cyclopropylretinal analogues (11S,12R)-2 and (11R,12S)-3, enantioselectively synthesized with the aid of an enzyme. The finding that enantiomer 2 binds to opsin while the other 3 does not defines the absolute sense of twist around the 12-s bond.     

Combining [Arene–Ru] with Azocarboxamide to Generate a Complex with Cytotoxic Properties

Azocarboxamide (azcH) has been combined for the first time with [Ru–Cym] to generate metal complexes with N,N‐ and N,O‐coordination mode, [(Cym)Ru(azc)Cl] and [(Cym)Ru(azcH)Cl]⁺[PF₆]^?. Geometric and electronic structures of the complexes are reported along with their in vitro activities against different tumour cell lines and preliminary results on solution chemistry. Compound [(Cym)Ru(azc)Cl] exhibited remarkable cytotoxic properties. It was cell‐type specific and had comparable IC₅₀ values towards both cancer cells and their drug‐resistant subline. A tenfold increase in the sensitivity towards [(Cym)Ru(azc)Cl] was noted for the tumour cells with depleted intracellular glutathione (GSH) level, suggesting the essential role of GSH in cell response to this compound.     

Circular Dichroism of Some 2-(Phenylmethyl)pyridine Derivatives

The absolute configuration of the 2-(phenylmethyl)pyridine derivatives 1 – 9 had been established by X-ray diffraction and chemical correlation. Their CD spectra have been studied in different solvents for the free and protonated forms. It has now been found that, from the sign of the strong CD couplet between 270 and 220 nm, which was observable for all these compounds besides 7 and 9 , their absolute configuration can be determined much quicker.     

Stereochemical correlation of diastereomeric 3-amino- with 3-arylamino acids,and their derivatives via stereospecific N-phenylation by diphenylhalonium salts

Diphenylhalonium salts were used in stereospecific N-phenylation of racemic and optically active 3-amino-2,3-diphenyl-1-propanols leading to 3-anilino-2,3-diphenyl-1-propanols. The absolute configurations of the latter and of genetically related compounds were established.

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Stereochemische Korrelation von diastereomeren 3-Amino- mit 3-Arylaminosäuren bzw. ihrer Derivate durch stereospezifische N-Phenylierung mit DiphenylhaloniumsalzenKurze Mitteilung

Zusammenfassung Durch stereospezifische N-Phenylierung mit Hilfe von Diphenylhaloniumsalzen werden racemische sowie optische aktive 3-Amino-2,3-diphenyl-1-propanole in die entsprechenden 3-Anilino-2,3-diphenyl-1-propanole übergeführt. Auf Grund dieser Umwandlung wird die absolute Konfiguration der Aminoalkohole und davon abgeleiteter Derivate ermittelt.

     

Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 2. Structural studies of host-guest complexes with chiral alcohol and monoamine conjugates.

A structural study of complexes formed between a dimeric zinc porphyrin tweezer (host) and chiral monoalcohols and monoamines derivatized by a bidentate carrier molecule (guest) confirmed that their CD couplets arise from the preferred porphyrin helicity of 1:1 host-guest complexes. NMR experiments and molecular modeling of selected tweezer complexes revealed that the preferred conformation is the one in which the L (larger) group protrudes from the porphyrin sandwich; this preferred helicity of the complex determines the CD of the complexes. It was found that the porphyrin ring-current induced (1)H chemical shifts and molecular modeling studies of the complex lead to the assignments of relative steric size of the L (large)/M (medium) substituents attached to the stereogenic center. The assignments, in turn, are correlated with the sign of the CD exciton couplet that establishes the absolute configuration at the stereogenic center. Variable-temperature NMR experiments proved that the observed increase in CD amplitude at lower temperatures derives from conformational changes in the preferred offset geometry between two porphyrin rings.