Substrate and Positional Selectivity in Electrophilic Substitution Reactions of Pyrrole, Furan, Thiophene, and Selenophene Derivatives

The relative reactivity (substrate selectivity) of five-membered heterocycles on electrophilic substitution (pyrrole >> furan > selenophene > thiophene) and their positional selectivity (furan > selenophene thiophene > pyrrole) are not consistent. The indicated differences in positional selectivity ( : ratios) of the parent heterocycles show up essentially in orientation on electrophilic substitution reactions of their derivatives and the corresponding benzannelated systems. It was shown that the positional selectivity was reduced in a sequence corresponding to the change in relative stability of the onium states of the elements (O⁺ < Se⁺ S⁺ < N⁺) and reflects the predominant role of the heteroatoms in stabilizing the -complexes formed on -substitution, in which the positive charge is distributed between the heteroatom and one -carbon atom (in -isom ers four atoms of the ring participate in delocalization of the charge). This interpretation was confirmed by quantum-chemical calculations carried out by the RHF/6-31G(d), MP2/6-31G(d)//RHF(6)-31G(d), and B3LYP/6-31G(d) ab initio methods.     

A Thermal-Desorption Surface-Ionization Indicator of Narcotics and Other Drug Preparations

The results of the development of a new method and instrument for determining trace narcotics and other abused drugs in extracts of biological samples (urine, blood, and postmortem specimens) from users of the above substances are reported. The method is based on the unique selectivity (up to 10⁵–10⁸ with respect to organic solvents or up to 10¹⁰ with respect to simple gases) and high efficiency of the surface ionization of nitrogen-containing organic bases and on the use of the temperature-programmed evaporation of sample molecules from an evaporator to the surface of an ionizer (emitter). This method provides not only the detection of trace substances (for example, opiates and amitriptyline at a picogram level) but also their identification based on the most probable evaporation temperature.     

Efficient Replication Bypass of Size‐Expanded DNA Base Pairs in Bacterial Cells

Supersize me! Size‐expanded DNA bases (xDNA) are able to encode natural DNA sequences in replication. In vitro experiments with a DNA polymerase show nucleotide incorporation opposite the xDNA bases with correct pairing. In vivo experiments using E. coli show that two xDNA bases (xA and xC, see picture) encode the correct replication partners.

     

Retention and Thermodynamic Studies of Piperazine Diastereomers in Reversed-Phase Liquid Chromatography

The effect of organic modifier concentration on retention and selectivity of two piperazine diastereomers in a typical n-octadecyl-bonded silica (ODS) column was investigated at pH 6.4 and pH 3.0 using phosphate-buffered acetonitrile (MeCN/H₂O) and methanol (MeOH/H₂O) mobile phases. The results show the logarithmic retention factors decrease with increasing organic concentration in a less rectilinear fashion in the MeCN/H₂O system than in the MeOH/H₂O system at high organic concentrations at both pHs. At pH 6.4, the MeOH/H₂O system provided significantly higher diastereomer selectivity than the MeCN/H₂O system, which can be ascribed to the hydrogen bonding interaction of methanol (as a hydrogen donor) with the piperazine amine moiety of the solute (as a hydrogen acceptor). At pH 3.0, both mobile phases provided high selectivity, in which both acetonitrile and methanol acted as hydrogen acceptors, while the protonated amine acted as the hydrogen donor. The effect of temperature on retention and selectivity was also studied in the two mobile phase systems at both pHs. It was found that at pH 6.4 the retention and selectivity were enthalpically driven in the MeOH/H₂O system, while entropically driven in the MeCN/H₂O system. However, the retention was entropically driven and the selectivity enthalpically driven in both systems at pH 3.0. Locally preferential solvating and hydrogen bonding effects are proposed to explain the anomalous retention and selectivity behaviors.     

Metal‐Ion Selectivity of Chemically Modified Uracil Pairs in DNA Duplexes

DNA duplexes containing 5‐modified uracil pairs (5‐bromo, 5‐fluoro, and 5‐cyanouracil) bind selectivity to metal ions. Their selectivity is sensitive to the pH value of the solution (see picture), as the acidities of the modified uracil bases vary according to the electron‐withdrawing properties of the substituents.

     

Synthesis of aryl(γ-pyridyl)methanes and their conversion to substituted benzo[g]isoquinolines

New substituted pyridine bases of the aryl(-pyridyl)methane type were obtained from 1,2,5-trimethyl-4-piperidone by successive transformations. The new substituted pyridine bases were catalytically dehydrocyclized to di(tri)methylbenzo[g]isoquinolines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1077–1084, August, 1976.     

Biomimetic Oxidations Using Transition Metal Complexes Encapsulated in Zeolites

An account of biomimetic oxidations by metal phthalocyanines, tri- and tetra-aza macrocycles, Schiff bases (salen and saloph), dimeric Cu-acetate and Co-Mn-acetate complexes encapsulated in zeolite-Y and molecular sieves is reported. The selective oxidation reactions investigated include epoxidation of styrene, hydroxylation of phenol, oxidation of p-xylene to terephthalic acid, ethylbenzene to acetophenone and cyclohexane, cyclohexanol and cyclohexanone to adipic acid. In all these reactions, the encapsulated metal complexes exhibit enhanced activity or selectivity compared to the neat complexes. The reasons for the enhanced activity of metal complexes upon encapsulation in zeolites are reported.     

Reaction of 2-chloroacetyl-1,3-cyclohexanediones with cyclic Schiffs bases. A new approach to 8-aza-D-homogone-1,3,5(10),9(11),13-pentaene-12,17A-dione

We have studied the reaction of 3,4-dihydroisoquinolines and their hydrochlorides with 2-chloroacetyl-1,3-cyclohexanediones. We have shown that the reaction in alcoholic solutions of HCl leads to annelation of the cyclic Schiff's bases by -triketones with formation of ABCD-tetracyclic 8-aza-D-homogonanium chlorides, treatment of which with bases gives 9,11-dehydro derivatives of 8-aza-D-homogona-1,3,5(10),13-tetraene-12,17a-diones.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1368–1373, October, 1993.     

Kinetics of the Hydrogenation of α-Pinene to cis- and trans-Pinanes on Pd/C

The liquid-phase hydrogenation of -pinene on a Pd/C catalyst at 0–100° and hydrogen pressures of 1–11 atm was studied. It was found that the order of reaction with respect to pinene increased with hydrogen pressure and did not depend on temperature, whereas the selectivity of cis-pinane formation decreased with temperature and increased with hydrogen pressure. A mechanism was proposed for the hydrogenation of -pinene. According to this mechanism, the selectivity of cis-pinane formation depends on the following two factors: (a) a temperature-dependent equilibrium between adsorbed -pinene species, which are cis- and trans-pinane precursors, and (b) competition between the hydrogenation and -H-elimination of surface -pinanyl complexes. The ratio between the rates of these reactions depends on the concentration of surface hydride species, and this concentration depends on the pressure of hydrogen.     

Chiral separation on achiral stationary phases with different functionalities using β-cyclodextrin in the mobile phase and application to bioanalysis and coupled columns

Summary -cyclodextrin was used in the mobile phase as chiral selector for separating the enantiomers of terbutaline, chlorthalidone and oxazepam. The effect on chiral resolution using e.g. hydrophobic, polar or cation exchanging stationary phases was investigated. Both the chiral separation factor and retention level were affected by the concentration of methanol and -cyclodextrin. The stationary phase had no effect on the chiral separation only on the level of retention. By tuning the concentration of -cyclodextrin and methanol in the mobile phase chiral separation could be obtained on most stationary phases. By changing the stationary phase while adjusting the mobile phase composition to maintain the chiral selectivity, improvements of the selectivity towards e.g. endogenous compounds can be obtained when separating enantiomers in complex matrixes as biological fluids. Further improvement on selectivity can be obtained if coupled columns are used. This is examplified for separation of chlorthalidone and terbutaline enantiomers in biological fluids by coupling an achiral column to another achiral column and using a mobile phase containing -cyclodextrin on the last column.