Two-point Padé approximants for formal Stieltjes series

We prove that certain two-point Padé approximants occupying the diagonal of the Padé table form monotone sequences of lower and upper bounds uniformly converging to a Stieltjes function. The results can be applied to the theory of inhomogeneous media for the calculation of the bounds on the effective transport coefficients of heterogeneous materials.     

Crystal and molecular structure of the sesquiterpene humulane alcohol juniferol

The spatial structure of the sesquiterpene humulane alcohol juniferol, which is represented in the crystal by two conformers, IA and IB, differing by the conformation of the C2...C9 section, has been established by the x-ray structural method. The strain energies of the conformers have been calculated by the method of molecular mechanics. The barrier to the interconversion of the conformers by rotation of the C2...C9 section has been calculated; it amounts to 7–8 kcal/mole. This value of the transformation barrier permits the assumption that the two conformers also exist in solution, their interconversion being hindered.Institute of Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Institute of Crystallography, Academy of Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 757–762, November–December, 1990.     

Synthesis of tetrakis(triethylsiloxy)titanium and tetrakis(triethylsiloxy)tin

Summary By the metathetic reaction of sodium triethylsiloxide with titanium tetrachloride and with tin tetrachloride, tetrakis(triethylsiloxy)titanium and tetrakis(triethylsiloxy)tin were synthesized.     

Vibrational spectra and stereoisomerism of the hexopyranose ring in epoxysaccharides

Journal of Structural Chemistry - Investigation of the potential surface of methyl 2,3-anhydro-2,3,4-trideoxy-β-D-lyxohexopyranoside has shown that the molecule has seven conformers with...     

Coordination of N-Substituted Porphyrins with Simple and Chelate Zinc Salts in DMSO

The effect of peripheral substitution in the porphyrin macrocycle (₂OEtP, H₂TPP) of the nature of the N-substituent (X = Me, Ph) and of the anion in the salt solvate (A = ^–, NO₃ ^–, ^–) on the complexation of the N-substituted porphyrins with zinc salts in DMSO is studied. The positions of the bands in the electronic absorption spectrum of the (A)M(N-X)P metal complex were found to depend only slightly on the nature of the A anion in the composition of the coordination sphere of the complex. An inhibition of coordination of the N-substituted ₂TPP derivatives by zinc nitrate was discovered that is not typical of porphyrins proper. The type of hybridization of the substituted nitrogen atom in the porphyrin molecule is discussed. A mixed type of hybridization is suggested: the sp ²- and p ³-forms in the ligands and an almost complete transition to the p ³ form in the N-substituted metal complexes.     

Rearrangements of 1-oxa-2-azoles. 2. Structure and isomerization of pentamethylene-amidoximes of 4-aminofurazan-3-carboxylic acid

The Z-oxime of the pentamethyleneamide of 4-aminofurazan-3-carboxylic acid isomerizes — rapidly in acid media, slowly in the absence of acid — to the E-oxime which at 120–140C undergoes alkali-catalyzed rearrangement to 4-piperidinofurazan-3-carboxyamidoxime.For Communication 1 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 122–123, January, 1991.     

Nucleophilic reactions of polydialkylcyclosilazanes

A study is made of the catalytic condensation polymerization of polydialkylcyclosilazanes [RR'SiNH]_n (n=3,4; R=CH₃, C₂H₅; R'=C₂H₅, C₄H₉, C₆H₁₃, C₉H₁₉) in the presence of small amounts of nucleophilic reagents, to give compounds of the general type R₅R₆'Si₆N₃(NH)₂ and oligomers. The reactivities of polydialkylsilazanes decrease with increase in aliphatic group chain length.     

Synthesis and properties of derivatives of 4-aminofuroxan-3-carboxylic acid

4-Amino-3-cyanofuroxan, obtained by the oxidation of aminocyanoglyoxime, reacts with hydrazine and hydroxylamine to form the amidrazone and amidoxime respectively of 4-aminofuroxan-3-carboxylic acid. The reaction of the amidoxime with triethyl orthoformate can lead to closure of both the pyrimidine and the 1,2,4-oxadiazole ring.Latvian Institute of Organic Synthesis, Riga. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1115–1119, August, 1997.