Conformational analysis of polymethylated derivatives of piperidine by the method of molecular mechanics

The conformational (steric) energies of different conformers and thus the type of dominant conformation were determined by the method of molecular mechanics for polysubstituted derivatives of piperidine. The chair conformation dominates for polymethylated 4-piperidines. The half-chair conformation is preferred for 1-methyl-3-methylene-4-piperidone, and the sofa conformation is preferred for analogous polymethylated enones.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 91–97, January, 1991.     

A Study of the Acid Properties of Structurally and Compositionally Different Heteropoly Acids in Acetic Acid

The acid properties of heteropoly acids of the following three structure types were studied by conductometry in acetic acid: Keggin (H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiW₁₂O₄₀, H₃PW₁₁ThO₃₉; and H₅PW₁₁XO₄₀, where X(IV) = Ti or Zr), Dawson (-H₆P₂W₁₈O₆₂and -H₆P₂Mo₁₈O₆₂), and H₆P₂W₂₁O₇₁(H₂O)₃. These compounds are electrolytes that dissociate in only the first step of this solvent. The thermodynamic dissociation constants of the heteropoly acids were calculated by the Fuoss–Kraus method. The Hammett acidity functions H ₀of the solutions of H₅PW₁₁XO₄₀, H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, and H₆P₂W₂₁O₇₁(H₂O)₃in 85% acetic acid at 25°C were determined by the indicator method. All of the test heteropoly acids were found to be strong acids.     

Esterification of n-Butanol with Acetic Acid in the Presence of Heteropoly Acids with Different Structures and Compositions

The esterification reaction of n-butanol with acetic acid ([BuOH] : [HOAc] = 1 : 15 mol/mol; 55°C, 5% H₂O) was studied in the presence of tungsten heteropoly acids of the Keggin (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₅PW₁₁TiO₄₀, H₅PW₁₁ZrO₄₀, and H₃PW₁₁ThO₃₉) and Dawson structure (-H₆P₂W₁₈O₆₂, H₆P₂W₂₁O₇₁(H₂O)₃, H₆As₂W₂₁O₆₉(H₂O), and H₂₁B₃W₃₉O₁₃₂). The reaction orders with respect to H₆P₂W₂₁O₇₁(H₂O)₃, H₃PW₁₂O₄₀, and H₆P₂W₁₈O₆₉are equal to 0.78, 1.00, and 0.97, respectively. It was found that the reaction rate depends on the acidity, as well as on the structure and composition of heteropoly acids. The H₂₁B₃W₃₉O₁₃₂heteropoly acid is most active, whereas the Keggin-structure heteropoly acids exhibit the lowest activities. Of the Keggin structure heteropoly acids, H₅PW₁₁ZrO₄₀exhibits the highest activity because of the presence of a Lewis acid site in its structure.     

Reactions of the 2,2′,6,6′-tetra-tert-butyl-4,4′-bispyrylium cation with bases

From 2,2,6,6-tetra-tert-buty1-4,4-bispyrylium perchlorate by the action of aqueous solutions of sodium acetate or caustic soda we have obtained a spiran containing pyran and dihydrofuran rings the structure of which has been established by the x-ray structural method. It has been shown that under the same conditions sodium sulfide reduces the bispyrylium salt to 2,2,6,6-tetra-tert-butyl-4,4-bispyranylidene. For the product of the interaction of the bispyrylium salt with aqueous ammonia the structure of an unsaturated tetraketone is suggested. Possible mechanisms of the formation of the compounds mentioned are discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 461–467, April, 1980.     

Conformation of the rings and relative stabilities of isomers of trans,trans- and trans,cis-eudesm-6,12-olides

An investigation has been made by the method of molecular mechanics of a number of isomers of trans,trans- and trans,cis-eudesm-6,12-olides. Questions of the energetic advantageousness of particular isomers, the influence of the orientation of the substituents in the mechanism on their conformation, and the relative stability of the conformers with different methods of linking the rings are considered.A. M. Nesmeyanov Institute of Organometallic Compounds, USSR Academy of Sciences, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 193–197, March–April, 1990.     

Conformations of the rings and relative stabilities of isomers of trans,trans- and trans,cis-eudesman-8,12-olides

A series of isomers of trans,trans- and trans,cis-eudesman-8,12-olides has been investigated by the method of molecular mechanics (MMM). Questions of the energetic suitability of particular isomers, of the influence of the orientation of substituents attached to the basic skeleton on their conformation, and of the relative stabilities of the conformers with different methods of linking the rings are considered.A. N. Nesmeyanov Institute of Heteroorganic Compounds, USSR Academy of Sciences, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 334–338, May–June, 1990.     

Conformational flexibility of the twelve-membered siloxane ring

Conclusions The conformational analysis of the molecules of twelve-membered cis- and trans-2,8-dihydroxycyclohexasiloxanes showed that intramolecular dehydration with the formation of a bicyclic system is possible for the cis isomer with the approach of the OH groups on account of the large conformational flexibility of the ring. For the trans isomer this approach is only possible with cleavage of the bonds in the ring at the intermediate stages.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1871–1874, August, 1984.     

Effect of molecular mass on the optical activity of chitosan

The optical activity of chitosan has been examined by the optical rotary dispersion method in a wide molecular mass range, beginning from low-molecular analogs of the monomer unit and oligomers. In the spectral range from 300 to 710 nm, all samples demonstrate smooth curves of the optical rotary dispersion. For the polymers and oligomers, these curves lie in the region of negative values of specific optical rotation [α], while for monosaccharides, these curves are in the positive region. High-molecular-mass samples are characterized by maximal ?[α] values that are practically independent of molecular mass. For partially depolymerized samples with M < 2 × 10⁵ and for oligomers, the smaller the molecular mass of the sample, the greater the decrease in the modulus of ?[α]. The passage to D-glucosamine chlorohydrate and to N-acetyl-D-glucosamine is characterized by inversion of the sign of rotation and achievement of high +[α] values.     

An EPR and molecular mechanics study of radical adducts of phosphonyl radicals with 2-hydroperfluoro-4-methyl-and 4,4-dimethyl-2-pentene

EPR spectra of radical adducts of phosphonyl radicals with 2-hydroperfluoro-4-methyl- and 4,4-dimethyl-2-pentene have been studied. The molecular mechanics method has been used to determine the preferred conformation of the (CF₃)₂CF-CF-HCF₃P(O)(OMe)₂ radical. The eclipsed conformation of the C–P bond and 2p _z -orbital of an unpaired electron is stabilized due to steric factor and hyperconjugation.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2550–2554, November, 1992.