The synthesis of a 2-azabicyclo[3.1.0]hexane by rearrangement of a spirocyclic epoxide

An unusual rearrangement of a 1-oxa-6-azaspiro[2.5]octane derivative giving access to novel 5-substituted 2-azabicyclo[3.1.0]hexanes is described. The synthetic application of the reaction is demonstrated by the synthesis of N-Boc-2,3-methano-β-proline. The amino acid was prepared through a three-step synthesis from easily available reagents in an overall yield of 15%.     

Complexation of thiacalix[4]arene methylphosphonic and sulphonic acids with amino acids

In this paper, host–guest complexation process of thiacalix[4]arene tetrakis–methylphosphonic and tetrakis–sulphonic acids with amino acids by HPLC and molecular modelling methods has been studied. It was shown that thiacalix[4]arene tetrakis–methylphosphonic acid due to transformability of macrocyclic skeleton and flexibility of methylphosphonic substituents can adopt its conformation for strong multicentre binding of the amino acids with association constant values 530–10,140 M^? 1 in water.     

Dibutylphosphinoylmethyloxythiacalix[4]arenes. Synthesis,structure, americium,europium and technetium extraction

A series of thiacalix[4]arenes bearing one, two or four chelating dibutylphopshinoylmethoxy groups have been synthesised and studied in the context of this paper. The synthesis consisted of precise Williamson alkylation of thiacalixarene tetrols with tosylate of dibutylhydroxymethyl phosphine oxide in the presence of alkali metal carbonates or sodium hydride. Stereochemical yield of the reaction (cone or 1,3-alternate conformer) depends on the nature of alkali metal. Small-sized ‘hard’ sodium cation organises the macrocyclic platform in the cone conformation, but larger and ‘soft’ potassium and cesium cations stabilise the macrocycle in the 1,3-alternate conformation. All synthesised compounds (except monophosphine monoxide) possess either moderate or high extraction ability towards pertechnetate ion. The cone-shaped thiacalix[4]arene tetraphosphine tetraoxide due to cooperative (macrocyclic) effect of eight oxygen atoms of the phosphinoylmetoxy-binding groups effectively extract spherical americium, europium cations due to cooperative (macrocyclic) effect of eight oxygen atoms of the phosphinoylmetoxy-binding groups. The extraction ability is very similar to that of calix[5]arene pentaphosphine pentaoxide existing in the cone conformation.     

Closed-vessel microwave digestion technique for lichens and leaves prior to determination of trace elements (Pb,Zn, Cu) and stable Pb isotope ratios

A reliable and robust procedure using closed-vessel microwave digestion of lichens and leaves for precise and accurate determination of trace elements (Pb, Zn and Cu) and stable Pb isotope ratios is presented. The method was developed using certified reference material CRM 482 Pseudovernia furfurea (Lichens), NIST 1515 (Apple Leaves) and NIST 1547 (Peach Leaves) and tested on lichens from a mining site in Russia. A mixture of 3?mL of HNO₃, 3?mL of H₂O₂, 2?mL of H₂O and 0.8?mL of HF ensured complete sample dissolution with 100?±?5% recovery for Pb, Zn and Cu at a maximum temperature of 210°C and pressure of 350?psi. The amount of HF and microwave pressure significantly influenced Pb, Zn and Cu recovery. Comparison between EMMA-XRF and ICP-AES showed a good correlation between Pb, Zn and Cu concentrations. Using the newly developed digestion method, Pb isotopes in lichens from the mining site were determined with an internal precision better than 0.02%.     

Perquinolines A–C: Unprecedented Bacterial Tetrahydroisoquinolines Involving an Intriguing Biosynthesis

Metabolic profiling of Streptomyces sp. IB2014/016‐6 led to the identification of three new tetrahydroisoquinoline natural products, perquinolines A–C ( 1 – 3 ). Labelled precursor feeding studies and the cloning of the pqr biosynthetic gene cluster revealed that 1 – 3 are assembled by the action of several unusual enzymes. The biosynthesis starts with the condensation of succinyl‐CoA and l ‐phenylalanine catalyzed by the amino‐7‐oxononanoate synthase‐like enzyme PqrA, representing rare chemistry in natural product assembly. The second condensation and cyclization events are conducted by PqrG, an enzyme resembling an acyl‐CoA ligase. Last, ATP‐grasp RimK‐type ligase PqrI completes the biosynthesis by transferring a γ‐aminobutyric acid or β‐alanine moiety. The discovered pathway represents a new route for assembling the tetrahydroisoquinoline cores of natural products.     

Cascadic multilevel methods for ill-posed problems

Multilevel methods are popular for the solution of well-posed problems, such as certain boundary value problems for partial differential equations and Fredholm integral equations of the second kind. However, little is known about the behavior of multilevel methods when applied to the solution of linear ill-posed problems, such as Fredholm integral equations of the first kind, with a right-hand side that is contaminated by error. This paper shows that cascadic multilevel methods with a conjugate gradient-type method as basic iterative scheme are regularization methods. The iterations are terminated by a stopping rule based on the discrepancy principle.