Synthesis of alkyl α- and β-<Emphasis Type="SmallCaps">d</Emphasis>-glucopyranoside-based chiral crown ethers and their application as enantioselective phase-transfer catalysts

Chiral monoaza-15-crown-5-type lariat ethers annelated to alkyl 4,6-O-benzylidene-α- and β-d-glucopyranosides have been synthesized. These macrocycles generated significant asymmetric induction as phase-transfer catalysts in a few two-phase reactions. The catalytic effect of the lariat ethers with methoxy, ethoxy, and i-propoxy substituents on C-1 of the sugar unit in both α and β positions was compared. In liquid–liquid two-phase reactions, the nature and position of the substituents did not have much effect. The α-anomers were somewhat more efficient in terms of enantioselectivity than the β forms. In asymmetric Darzens condensations, in the epoxidation of trans-chalcone, in the Michael addition of β-nitrostyrene and diethyl acetamidomalonate, and in the reaction of 2-benzylidene-1,3-indandione with diethyl bromomalonate, maximum enantioselectivities of 73, 94, 78, and 72%, respectively, were obtained in presence of glucopyranoside-based lariat ethers as catalysts.     

Über die Stereospezifität der α-Alkylierung von β-Hydroxycarbonsäureestern. Vorläufige Mitteilung

About the Stereospecific α-Alkylation of β-Hydroxyesters It was found, that dianions derived from β-hydroxyesters with lithium diisopropylamide (LDA) at ?50 to ?20° were alkylated stereospecifically (Scheme 1). The stereospecificity was 95–98%, the threo-compound (threo -2, -3 and -4) being the main product. This was proved for threo -2 and -3 by preparing the β-lactones 7 and 8 , respectively, which were pyrolyzed to trans-1, 4-hexadiene (9) and trans-1-phenyl-2-butene (10) , respectively (Scheme 2). Moreover, the acid threo -6 from threo -3 was converted by dimethylformamide-dimethylacetal to cis-1-phenyl-2-butene (11) (s. footnote 6). The alkylation of α-monosubstituted β-hydroxyesters also turned out to be stereospecific. Reduction of 16 and 18 with actively fermenting yeast furnished (+) -17 and (+) -2. respectively (Scheme 4), which were each mixtures of the (2R, 3S)- and the (2S, 3S)-isomers. Alkylation of (+) -17 with allyl bromide yielded after chromatography (2S, 3S) -19 and of (+) -2 with methyl iodide (2R, 3S) -19 , the oxidation of which finally gave (S)-(?) -20 and (R)-(+) -20 , respectively.     

Fundamental metal carbonyl equilibria: a quantitative infrared spectroscopic study of the equilibrium between dicobalt octacarbonyl and tetracobalt dodecacarbonyl under carbon monoxide pressure in hexane solution.

The equilibrium between Co₂(CO)₈ and Co₄(CO)₁₂ has been investigated in hexane solution in the temperature range 105–145°C under carbon monoxide pressure (6–14 bar). the data obtained by infrared analytical monitoring of the system in a high-pressure cell alow a reasonably precise extension of the calculated equilibrium concentration between —20 and 300°C. The thermo- dynamic parameters obtained for this system are: ΔH⁰ 29.5 ± 0.5 kcal mol^-1, ΔS⁰ 135 ± 3 cal mol^-1 K^-1. The stability regions of Co₂(CO)₈ and Co₄(CO)₁₂ in terms of p(CO) and temperature are discussed.     

Conformational dependence of the intrinsic acidity of the aspartic acid residue sidechain in N-acetyl- -aspartic acid-N′-methylamide

The sidechain conformational potential energy hypersurfaces (PEHS) for the γ_L, β_L, α_L, and α_D backbone conformations of N-acetyl- -aspartate-N′-methylamide were generated. Of the 81 possible conformers initially expected for the aspartate residue, only seven were found after geometric optimizations at the B3LYP/6-31G(d) level of theory. No stable conformers could be located in the δ_L, _L, γ_D, δ_D, and _D backbone conformations. The ‘adiabatic’ deprotonation energies for the endo and exo forms of N-acetyl- -aspartic acid-N′-methylamide were calculated by comparing their optimized relative energies against those found for the seven stable conformers of N-acetyl- -aspartate-N′-methylamide. Sidechain conformational PEHSs were also generated for the estimation of ‘vertical’ deprotonation energies for both endo and exo forms of N-acetyl- -aspartic acid-N′-methylamide. All backbone–sidechain (N–H⁻O–C) and backbone–backbone (N–HO=C) hydrogen bond interactions were analyzed. A total of two backbone–backbone and four backbone–sidechain interactions were found for N-acetyl- -aspartate-N′-methylamide. The deprotonated sidechain of N-acetyl- -aspartate-N′-methylamide may allow the aspartyl residue to form strong hydrogen bond interactions (since it is negatively charged) which may be significant in such processes as protein–ligand recognition and ligand binding. As a primary example, the molecular geometry of the aspartyl residue may be important in peptide folding, such as that in the RGD tripeptide.     

Thermal stability of new zinc acetate-based complex compounds

New zinc acetate based complex compounds (of general formula Zn(CH₃COO)₂·1?2L·nH₂O) containing one or two molecules of urea, thiourea, coffeine and phenazone were prepared namely: Zn(CH₃COO)₂·2.5H₂O, Zn(CH₃COO)₂·2u·0.5H₂O, Zn(CH₃COO)₂·tu·0.5H₂O, Zn(CH₃COO)₂·2tu, Zn(CH₃COO)₂·cof·2.5H₂O, Zn(CH₃COO)₂·2cof·3.5H₂O, Zn(CH₃COO)₂·2phen·1.5H₂O. The compounds were characterized by IR spectroscopy, chemical analysis and thermal analysis. Thermal analysis showed that no changes in crystallographic modifications of the compounds take place during (heating in nitrogen before) the thermal decompositions. The temperature interval of the stability of the prepared compounds were determined. It was found that the thermal decomposition of hydrated compounds starts by the release of water molecules. During the thermal decomposition of anhydrous compounds in nitrogen the release of organic ligands take place followed by the decomposition of the acetate anion. Zinc oxide and metallic zinc were found as final products of the thermal decomposition of the zinc acetate based complex compounds studied. Carbon dioxide and acetone were detected in the gaseous products of the decomposition of the compounds if ZnO is formed. Carbon monoxide and acetaldehyde were detected in the gaseous products of the decomposition, if metallic Zn is formed. It is supposed that ZnO and Zn resulting from Zn acetate complex compounds here studied, possess different degree of structural disorder. Annealing takes place by further heating above 600°C.     

Kinetics and mechanism of the reaction of tetracobalt dodecacarbonyl with carbon monoxide under pressure

The kinetics of the reaction of tetracobalt dodecacarbonyl with carbon monoxide to form dicobalt octacarbonyl in n-hexane have been investigated over a wide range of temperature and CO pressure. The reaction is first order in [Co₄(CO)₁₂]; the order in [CO] changes between one (at low pressures and high temperatures) and two (at high pressures and low temperatures).Activation parameters have been estimated and a mechanism involving initial reversible breaking of one CoCo bond, followed by irreversible breaking of a second, is proposed. The first step involves concerted addition of CO while the second can proceed with or without such addition.     

Synthesis of new guanidine derivatives from 2′,3′,4′,9′-tetrahydrospiro[piperidine-4,1′-[1H]pyrido[3,4-b]indole]

The reaction of the isothiourea derivative 2 with methylaminc or pyrrolidine resulted in guanidines 3a-3b . Using hydrazine under the same conditions the tetrazole derivative 4 was obtained. On reacting 2 with piperidine, morpholine, methylhydrazine, phenylhydrazine, hydroxylamine or sodium hydroxide, cycliza-tion took place leading to the novel 4-cyanimino-1,2,3,4,6,7,12,12b-octahydro-3,12b-ethanopyrim-ido[1′,6′:1,2]pyrido[3,4-b]indole ( 5 ). Some structural aspects of 5 and other model compounds were analysed mainly by ¹³C nmr spectroscopy.     

Effect of ion exchange by an organic cation on platinum immobilization on clays

Clay-supported platinum catalysts were prepared by ion exchange. The platinum precursor has been reduced either in the presence or in the absence of cinchonidinium cations introduced in the interlamellar space of the cationic clay. The samples were characterized by XRD and TEM. Reduction by NaBH₄ in the absence of cinchonidinium cations and the subsequent introduction of this latter ensured the formation of monodispersed, small platinum crystallites immobilized mostly between the clay sheets. On the other hand, reduction in the presence of the organic cations resulted in migration of the platinum particles on the external surface of the clay.     

Large Cliques in -Free Graphs

Dedicated to the memory of Paul Erdős A graph is called -free if it contains no cycle of length four as an induced subgraph. We prove that if a -free graph has n vertices and at least edges then it has a complete subgraph of vertices, where depends only on . We also give estimates on and show that a similar result does not hold for H-free graphs––unless H is an induced subgraph of . The best value of is determined for chordal graphs. Received October 25, 1999 RID="*" ID="*" Supported by OTKA grant T029074. RID="**" ID="**" Supported by TKI grant stochastics@TUB and by OTKA grant T026203.     

Biological UV dosimeters in quality control of tanning tubes

Although according to the International Radiological Protection Association-International Non-Ionizing Radiation Committee recommendation (1991) the use of sunbeds for cosmetic purposes is not recommended, tanning devices are used widely. Ten different types of commercially available sunbed tubes have been studied using a uracil biological UV dosimeter, and three of them were analyzed in detail. Dimerization effectiveness of the tubes was measured directly, whereas efficiency of erythema induction was calculated weighting the emission spectra by the Commission Internationale de l'Eclairage erythema action spectrum. The data obtained demonstrate that quality control of sunbed tubes has to include not only the determination of the UV doses administered but also the assessment of the health risk due to the UVB and UVA components of the lamp. A method of quality control using the uracil biological dosimeter was elaborated, and the estimation of the "acceptable" exposure time was checked/controlled on 15 volunteers by assessing individually the erythema induction threshold. A correct classification of the sunbed tubes is proposed by characterizing the erythema induction versus DNA-damaging effectiveness of tubes.