Squaric acid is a suitable building-block in 4C-Ugi reaction: access to original bivalent compounds

The 4C-Ugi reaction requires an acidic component as one of the building-blocks. We report here the first use of squaric acid for this reaction to access original symmetrical compounds. The scope and conditions of the reaction were explored.     

Strategies for excluding false Y-chromosomal SNP entries from human genome databases

Current human genome databases for public single nucleotide polymorphisms (SNPs) still contain a substantial fraction of false entries. The main reasons for errors include sequencing or assembly errors, paralogous sequence-, and private variants. In the course of our studies on the Y chromosome, we established a set of internal laboratory guidelines for reliably identifying false SNP entries in databases.     

Preparation and Radical Oligomerization of anFe(II) Complex without Loss of Spin-Crossover Properties

Summary.  11-(4H-1,2,4-Triazol-4-yl)-undecylmethacrylate (1), a new ligand for Fe(II) spin-crossover (SCO) complexes containing a polymerizable group, was synthesized and characterized. The complex [Fe·1 ₃](BF₄)₂ (2) was obtained by reaction of 1 with Fe(BF₄)₂·6H₂O (molar ratio 1/Fe(II) = 3/1) in THF. Complex 2 showed a gradual spin-crossover between 80 and 230 K. The methacrylate units in the ligands of complex 2 could be oligomerized radically in solution (initiator: azoisobutyronitrile) without loss of the spin-crossover behaviour. Received May 30, 2000. Accepted December 10, 2000     

Heterogeneous Metal–Organic-Framework-Based Biohybrid Catalysts for Cascade Reactions in Organic Solvent

In cooperative catalysis, the combination of chemo- and biocatalysts to perform one-pot reactions is a powerful tool for the improvement of chemical synthesis. Herein, UiO-66-NH₂ was employed to stepwise immobilize Pd nanoparticles (NPs) and Candida antarctica lipase B (CalB) for the fabrication of biohybrid catalysts for cascade reactions. Distinct from traditional materials, UiO-66-NH₂ has a robust but tunable structure that can be utilized with a ligand exchange approach to adjust its hydrophobicity, resulting in excellent catalyst dispersity in diverse reaction media. These attractive properties contribute to the formation of MOF-based biohybrid catalysts with high activity and selectivity in the synthesis of benzyl hexanoate from benzaldehyde and ethyl hexanoate. With this proof-of-concept, we reasonably expect that future tailor-made MOFs can combine other catalysts, ranging from chemical to biological catalysts for applications in industry.     

Open covers and the square bracket partition relation

An open cover of an infinite separable metric space is an -cover of if and for every finite subset of there is a such that . Let be the collection of -covers of . We show that the partition relation holds if, and only if, the partition relation holds.

     

Comparison of different extraction techniques for the determination of chlorinated pesticides in animal feed

The performances of Soxhlet extraction, dive-in Soxhlet extraction, microwave-assisted extraction (MAE), accelerated solvent extraction (ASE), fluidized-bed extraction (FBE), and ultrasonic extraction (UE) for the analysis of organochlorine pesticides in animal feed have been investigated. ASE and MAE provided significantly better extraction efficiency than Soxhlet extraction. The concentrations were 126.7 and 114.8%, respectively, of the values obtained by classical Soxhlet extraction, whereas the results from FBE and dive-in Soxhlet were comparable with those from the standard Soxhlet procedure. The reproducibility of FBE was the best, with RSDs ranging from 0.3 to 3.9%. Under the investigated operation conditions UE was not efficient, with the recoveries of target compounds being about 50% less than Soxhlet. Additionally, the performances of Soxhlet, dive-in Soxhlet, MAE, ASE and FBE were validated by determination of the certified reference material BCR-115. The results from the extraction techniques were in good agreement with the certified values.     

Reaktionen von Undecacarbonyl(acetonitril)trieisen mit Alkinethern

Reactions of Undecacarbonyl(acetonitrile)triiron with Alkyne Ethers (CO)₁₁(CH₃CN) 1 reacts with the alkyne ethers H₃C? C?C? OC₂H₅ 2a , H? C?C? OC₂H₅ 2b , H₃C? O? CH₂? C?C? CH₂? O? CH₃, 2c and H₃C? O? C(CH₃)H? C?C? C(CH₃)H? O? CH₃ 2d forming different cluster products depending on the substituents and the reaction conditions. The product obtained with 2a is the bisalkylidyne cluster Fe₃(CO)₉(m?₃-C? CH₃)(m?₃-C? OC₂H₅) 3 which results from the cleavage of the carbon carbon triple bond. The alkyne 2b however yields the vinylidene cluster Fe₃(CO)₁₀(m?₃-η²-C? C(H)OC₂H₅) 4 by 1,2 proton shift. The alkyne clusters Fe₃(CO)₁₀(m?₃-η²-C? C(H)OC₂H₅) 4 by 1,2 proton shift. The alkyne clusters Fe₃(CO)₁₀(m?₃-η²- H₃ C? O? CH₂? C?C? CH₂? O? CH₃) 6 and Fe₃(CO)₉(m?-η²-H₃C? O? CH₂? C?C? CH₂? O? CH₃) 7 are the isolated products obtained from 2c . Thermolysis of 7 results in the formation of the dinuclear butatrien complex Fe₂(CO)₆ (H₂C? C? C? CH₂) 8a . The analogous compound Fe₂(CO)₆[H(H₃C)C ? C ? C ? C(CH₃)H] 8b is the only product of 2d and 1 . The structures of 4, 5 , and 6 have been determined by crystal structure determinations.     

Bidirectional electron transfer in photosystem I: direct evidence from high-frequency time-resolved EPR spectroscopy

Efficient charge separation occurring within membrane-bound reaction center proteins is the most important step of photosynthetic solar energy conversion. All reaction centers are classified into two types, I and II. X-ray crystal structures reveal that both types bind two symmetric membrane-spanning branches of potential electron-transfer cofactors. Determination of the functional roles of these pairs of branches is of fundamental importance. While it is established that in type II reaction centers only one branch functions in electron transfer, we present the first direct spectroscopic evidence that both cofactor branches are active in the type I reaction center, photosystem I.     

Synthesis and structural characterization of the type‐I clathrates K8AlxSn46–x and Rb8AlxSn46–x (x ≃ 6.4–9.7)

Exploratory studies in the systems A–Al–Sn (A = K and Rb) yielded the clathrates K₈Al_xSn_46–x (potassium aluminium stannide) and Rb₈Al_xSn_46–x (rubidium aluminium stannide), both with the cubic type‐I structure (space group Pmn, No. 223; a ? 12.0 Å). The Al:Sn ratio is close to the idealized A₈Al₈Sn₃₈ composition and it is shown that it can be varied slightly, in the range of ca ±1.5, depending on the experimental conditions. Both the (Sn,Al)₂₀ and the (Sn,Al)₂₄ cages in the structure are fully occupied by the guest alkali metal atoms, i.e. K or Rb. The A₈Al₈Sn₃₈ formula has a valence electron count that obeys the valence rules and represents an intrinsic semiconductor, while the experimentally determined compositions A₈Al_8±xSn_38?x suggest the synthesized materials to be nearly charge‐balanced Zintl phases, i.e. they are likely to behave as heavily doped p‐ or n‐type semiconductors.     

Design of sulfides with a locked conformation as promoters of catalytic and asymmetric sulfonium ylide epoxidation

[reaction: see text] A new generation of 2,5-dimethylthiolanes with a locked conformation was developed to promote the asymmetric addition of chiral sulfonium ylides to aldehydes. The novel chiral sulfur derivative 4 succeeded the synthesis of trans-stilbene oxide derivatives with enantiomeric ratios ranging from 95:5 to 98:2. This user-friendly organocatalytic process proved to be efficient with 20-10% of sulfide 4 in 1 or 2 days of reaction. An insight into the ylide intermediate conformation is given on the basis of a computational ab initio study.