Totally selective ring-opening of amino epoxides with ketones: a general entry to enantiopure (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols

[Reaction: see text] Transformation of enantiopure diastereoisomers (2R,1'S)- and (2S,1'S)-2-(1-aminoalkyl)epoxides into the corresponding 4-(1-aminoalkyl)-1,3-dioxolanes is achieved by reaction with different ketones in the presence of BF3.Et2O. The conversion takes place in very high yields, total selectivity, and without epimerization. A mechanism to explain this transformation is proposed. The obtained 1,3-dioxolanes can be deprotected, and (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols were isolated.     

Cycloaddition reactions of 5-deutero-1-phenyl-4-vinylpyrazole. Stereoselectivecis “ene” reaction and simplification of NMR spectra of mixtures of cycloadducts

Summary The reaction of methyl propiolate (MP) with 1-phenyl-4-vinylpyrazole deuterated in the five position has clarified the¹H NMR spectrum of the mixture of the adducts 1:2 which are obtained as a result of a Diels-Alder cycloaddition followed by a non-regioselective ene reaction. The deuterated substrate allowed at the same time to establish the stereochemistry of the ene reaction as a concertedcis process.

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Cycloadditionsreaktionen von 5-Deutero-1-phenyl-4-vinylpyrazol. Stereoselektivecis-en-Reaktion und Vereinfachung von NMR-Spektren von Mischungen der Cycloaddukte (Kurze Mitt.)

Zusammenfassung Die Reaktion von Methylpropiolat (MP) mit an der 5-Stellung deuteriertem 1-Phenyl-4-vinylpyrazol wurde zur vereinfachten Interpretation der¹H-NMR-Spektren einer 1:2 Mischung von Addukten aus der Diels-Alder-Cycloaddition mit nachfolgender nicht-regioselektiver en-Reaktion eingesetzt. Das deuterierte Substrat erlaubte zugleich auch die Bestimmung der Stereochemie der en-Reaktion als einen konzertiertencis-Prozeß.

     

Determination of lead in the urine of exposed and unexposed adults by extraction and flow-injection/atomic absorption spectrometry

Lead is extracted from urine with ammonium pyrrolidien dithiocrbamate into methyl isobutyl ketone, and 20 μl of the extract is injected into a water carrier stream in a flow-injection/atomic absorption system. The procedure is simple, quick, accurate and reproducible. Recoveries are 96–105%. The mean urine lead contents of 20 unexposed and 20 printing-press lead exposed workers was found to be 39.0 ± 8.3 and 71.7 ± 26.3 μg Pb l^?1, respectively.     

Structural characterization of single-walled carbon nanotube bundles by experiment and molecular simulation

A procedure, combining molecular simulation, Raman spectroscopy, and standard nitrogen adsorption, is developed for structural characterization of single-walled carbon nanotube (SWNT) samples. Grand canonical Monte Carlo simulations of nitrogen adsorption are performed on the external and internal adsorption sites of homogeneous arrays of SWNTs of diameters previously determined by Raman spectroscopy of the sample. The results show the importance of the peripheral grooves of a nanotube bundle at low relative pressure and the insensitivity of nanotube diameter toward adsorption on the external surface of the bundle at higher pressures. Simulations also reveal that samples containing thin nanotubes have less internal adsorption capacity that saturates at lower pressure than those comprising large diameter nanotubes. The fraction of open-ended nanotubes in a sample can be estimated by scaling the simulated internal adsorption inside nanotubes to obtain a near perfect fit between simulated and experimental isotherms. This procedure allows extrapolation of adsorption properties to conditions in which all nanotubes in the sample are open-ended.     

Stereochemical analysis of 3,4-methylenedioxymethamphetamine and its main metabolites by gas chromatography/mass spectrometry

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) is consumed as the racemate but some metabolic steps are enantioselective. In addition, chiral properties are preserved during MDMA biotransformation. A quantitative analytical methodology using gas chromatography/mass spectrometry (GC/MS) to determine enantioselective disposition in the body of MDMA and its main metabolites including 3,4-methylenedioxyamphetamine (MDA), 4-hydroxy-3-methoxymethamphetamine (HMMA), and 4-hydroxy-3-methoxyamphetamine (HMA) was developed. Plasma and urine samples were collected from a male volunteer. The analysis of MDMA, MDA, and 4-hydroxy-3-methoxy metabolites by GC/MS required a two-step derivatization procedure. The first step consisted of derivatization of the amine with enantiomerically pure Mosher's reagent ((R)-MTPCl). Triethylamine was used as a base to neutralize hydrochloric acid formed during the reaction allowing quantitative derivatization, which resulted in a substantial improvement in the sensitivity of the method compared with other previously described techniques. Further treatment with ammonium hydroxide was required since both amine and hydroxyl groups underwent derivatization in the reaction. Ammonium hydroxide breaks bonds formed with hydroxyl groups without affecting amine derivatives. The second derivatization step using hexamethyldisilazane was needed for metabolites containing phenol residues. This derivatization method permitted the stereochemically specific study of MDMA and its main monohydroxylated metabolites by GC/MS. A detailed study of the chemical reactions involved in the derivatization steps was indispensable to develop a straightforward, sensitive, and reproducible method for the analysis of the parent drug compound and its metabolites.     

Synthesis and 13C-NMR spectroscopic investigations of rhamnobioses

A convenient method has been developed for the synthesis of all mono- and di-O-benzyl ethers of methyl α-L-rhamnopyranoside applying a new stereoselective method for the hydrogenolytic ring-cleavage of benzylidene acetals. Using the prepared dibenzyl ethers as aglycones, the (1→2)-, (1→3)- and (l→4)-linked rhamnosyl-rhamnose derivatives (13–15) were synthesised. Hydrogenolysis of the latter compounds and subsequent acetylation gave the pentaacetates (16–18) of methyl dirhamnosides, which on saponification furnished the free methyl dirhamnosides (19–21). Acetolysis of 16–18 gave the corresponding dirhamnose-hexaacetates which were transformed into the three disaccharides by saponification. The structure of each product was investigated by ¹³C-NMR spectroscopy, and for the purpose of ¹³C-NMR studies the mono-O-methyl ethers of methyl α-L-rhamnopyranoside, the diacetates and di-O-benzyl ethers of the latter compounds, and, also the diacetates of methyl α-L-rhamnopyranoside were synthesised.It has been established that, for ¹³C-NMR investigations of oligosaccharides, the benzyl ethers of monosaccharides are more suitable model compounds than the currently used monosaccharide methyl ethers.     

Coordination solids via assembly of adaptable components: systematic structural variation in alkaline earth organosulfonate networks

A family of alkaline earth organosulfonate coordination solids is reported. In contrast to more typical crystal engineering approaches, these solids are sustained by the assembly of building blocks that are coordinatively adaptable rather than rigid in their bonding preferences. The ligand, 4,5-dihydroxybenzene-1,3-disulfonate, L, progressively evolves from a 0D, 1D, 2D, to a 3D microporous network with the Group II cations Mg(2+), Ca(2+), Sr(2+), and Ba(2+), (compounds 1-4), respectively. This trend in dimensionality can be explained by considering factors such as hard-soft acid-base principles and cation radii, a rationalization which follows salient crystal engineering principles. The selective gas sorption properties of the microporous 3D network [Ba(L)(H(2)O)].H(2)O, 4, with different gaseous guests are also presented.     

Structural characterization and magnetic properties of sandwich-type tungstoarsenate complexes. Study of a mixed-valent VIV2/VV heteropolyanion

Complexes K11Na1[As2W18(Mn(H2O))3O66]x27H2O (1) and Na12[As2W18(Co(H2O))3O66]x34H2O (2) have been characterized. 1 crystallizes in the orthorhombic space group Pnma, with a = 30.6484(4) A, b = 14.9946(2) A, and c = 19.17080(10) A (Z = 4), while 2 crystallizes in the monoclinic space group C2/c, with a = 14.124(2) A, b = 23.294(3) A, c = 32.247(3) A, and beta = 98.935(10) degrees (Z = 4). Structures of the anions of 1 and 2 are similar, the divalent metals adopting a square pyramidal environment. K11[As2W18(VO)3O66]x23H2O (3) crystallizes in the orthorhombic space group Pnma, with a = 30.6240(5) A, b = 14.9861(2) A, and c = 19.2651(3) A (Z = 4). The structure has revealed a disorder on two of the three metals linking the [alpha-AsW9O33]9- parts. For these two vanadium atoms, the V=O bonds are directed alternatively toward the inside or the outside of the [alpha-AsW9O33]9- cavity. The remaining vanadium shows a V=O bond always directed toward the outside of the cavity. Titration of VIV by CeIV revealed that 3 is the mixed-valent VIV2VV species. Magnetic measurements are in agreement with this formulation. The high-temperature molar magnetic susceptibility of a powdered sample of 3 clearly confirms the presence of two d1 centers. The two VIV are antiferromagnetically coupled, with J = -2.9 cm-1 and g = 1.93. Crystallographic data do not permit the location of the two VIV to be distinguished from the location of the VV. As expected, the Mn(II) are very weakly antiferromagnetically coupled in compound 1. The complex Na8[Ni(H2O)6]2[As2W18(Ni(H2O))3O66]x20H2O (4) has been synthesized. The anion crystallized with two octahedral [Ni(H2O)6]2+ as counterions. Magnetic data have been fitted assuming that the only exchange-coupled centers are the nickels of the polyanion. 4 exhibits an antiferromagnetic coupling with J = -1.7 cm-1, g = 2.27, and theta = -1.5 K.     

Crystal structure of potassium tetramethylammonium hexacyanocobaltate(III) trihydrate: K3(Me4N)3[Co(CN)6]2·3H2O

The structure of K₃(Me₄N)₃[Co(CN)₆]₂·3H₂O has been determined from three-dimensional X-ray diffraction data. The unit cell is formed by parallel layers of cobalt octahedra [CoC₆] and potassium octahedra, [K(1)N₅O(1)], separated byc/2. In each layer both types of octahedra are located alternatively. The [MeN₄]⁺ tetrahedra are located in the cavities between the two layers of octahedra. The crystal structure of this compound is the first example of its type. TMC 2483     

Trehalose-based cyclodextrin analogs: cyclotrehalans (CTs)

A new concept for the de novo synthesis of artificial glyconanocavities is presented. The use of alternating α,α′-trehalose building blocks and (thio)urea segments allows the efficient synthesis of a new family of cyclooligosaccharides, namely cyclotrehalans (CTs), featuring a convex-shaped cavity with an apolar environment. CTs are designed to exhibit molecular inclusion abilities similar to that of cyclodextrins (CDs). Contrary to CDs, CTs expose the monosaccharide β-face to the inner cavity, while the (thio)urea tethers provides some conformational adaptability. High-yielding syntheses of a series of CTs and a preliminary evaluation of their inclusion properties are reported.