Experimental and theoretical characterization of the aromatization, epimerization, and fragmentation reactions of bi-2H-azirin-2-yls prepared from 1,4-diazidobuta-1,3-dienes

1,4‐Diazidobuta‐1,3‐dienes (Z,Z)‐ 10 , 17 , and 21 were photolyzed and thermolyzed to yield the pyridazines 13 , 20 , and 23 , respectively. To explain these aromatic final products, the generation of highly strained bi‐2H‐azirin‐2‐yls 12 , 19 , and 22 and their valence isomerization were postulated. In the case of meso‐ and rac‐ 22 , nearly quantitative formation from diazide 21 , isolation as stable solids, and complete characterization were possible. On the thermolysis of 22 , aromatization to 23 was only a side reaction, whereas equilibration of meso‐ and rac‐ 22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25 . The change of the configuration at C‐2 of the 2H‐azirine unit was observed not only in the case of bi‐2H‐azirin‐2‐yls 22 but also for simple spirocyclic 2H‐azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac‐ 22 to give alkyne 24 and two molecules of acetonitrile was also studied by high‐level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS‐ 30 – 31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one‐step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.     

Formation and stability of prolinol and prolinol ether enamines by NMR: delicate selectivity and reactivity balances and parasitic equilibria

Enamine key intermediates in organocatalysis, derived from aldehydes and prolinol or J?rgensen-Hayashi-type prolinol ether catalysts, were generated in different solvents and investigated by NMR spectroscopy. Depending on the catalyst structure, trends for their formation and amounts are elucidated. For prolinol catalysts, the first enamine detection in situ is presented and the rapid cyclization of the enamine to the oxazolidine ("parasitic equilibrium") is monitored. In the case of diphenylprolinol, this equilibrium is fully shifted to the endo-oxazolidine ("dead end") by the two geminal phenyl rings, most probably because of the Thorpe-Ingold effect. With bulkier and electron-withdrawing aryl rings, however, the enamine is stabilized relative to the oxazolidine, allowing for the parallel detection of the enamine and the oxazolidine. In the case of prolinol ethers, the enamine amounts decrease with increasing sizes of the aryl meta-substituents and the O-protecting group. In addition, for small aldehyde alkyl chains, Z-configured enamines are observed for the first time in solution. Prolinol silyl ether enamines are evidenced to undergo slow desilylation and subsequent rapid oxazolidine formation in DMSO. For unfortunate combinations of aldehydes, catalysts, solvents, and additives, the enamine formation is drastically decelerated but can be screened for by a rapid and facile NMR approach. Altogether, especially by clarifying the delicate balances of catalyst selectivity and reactivity, our NMR spectroscopic findings can be expected to substantially aid synthetically working organic chemists in the optimization of organocatalytic reaction conditions and of prolinol (ether) substitution patterns for enamine catalysis.     

Optical coherence tomography in biomedical research

Optical coherence tomography (OCT) is a noninvasive, high-resolution, interferometric imaging modality using near-infrared light to acquire cross-sections and three-dimensional images of the subsurface microstructure of biological specimens. Because of rapid improvement of the acquisition speed and axial resolution of OCT over recent years, OCT is becoming increasingly attractive for applications in biomedical research. Therefore, OCT is no longer used solely for structural investigations of biological samples but also for functional examination, making it potentially useful in bioanalytical science. The combination of in vivo structural and functional findings makes it possible to obtain thorough knowledge on basic physiological and pathological processes. Advanced applications, for example, optical biopsy in visceral cavities, have been enabled by combining OCT with established imaging modalities. This report gives an outline of the state of the art and novel trends of innovative OCT approaches in biomedical research in which the main focus is on applications in fundamental research and pre-clinical utilization.     

Hydrophilic interaction chromatography of nucleoside triphosphates with temperature as a separation parameter

Eight deoxynucleoside triphosphates (dNTPs) and nucleoside triphosphates (NTPs): ATP, CTP, GTP, UTP, dATP, dCTP, dGTP and dTTP, were separated with two 15 cm ZIC-pHILIC columns coupled in series, using LC-UV instrumentation. The polymer-based ZIC-pHILIC column gave significantly better separations and peak shape than a silica-based ZIC-HILIC column. Better separations were obtained with isocratic elution as compared to gradient elution. The temperature markedly affected the selectivity and could be used to fine tune separation. The analysis time was also affected by temperature, as lower temperatures surprisingly reduced the retention of the nucleotides. dNTP/NTP standards could be separated in 35 min with a flow rate of 200 μL/min. In Escherichia coli cell culture samples dNTP/NTPs could be selectively separated in 7 0min using a flow rate of 100 μL/min.     

NMR investigations on the proline-catalyzed aldehyde self-condensation: Mannich mechanism, dienamine detection, and erosion of the aldol addition selectivity

The proline-catalyzed self-condensation of aliphatic aldehydes in DMSO with varying amounts of catalyst was studied by in situ NMR spectroscopy. The reaction profiles and intermediates observed as well as deuteration studies reveal that the proline-catalyzed aldol addition and condensation are competing, but not consecutive, reaction pathways. In addition, the rate-determining step of the condensation is suggested to be the C-C bond formation. Our findings indicate the involvement of two catalyst molecules in the C-C bond formation of the aldol condensation, presumably by the activation of both the aldol acceptor and donor in a Mannich-type pathway. This mechanism is shown to be operative also in the oligomerization of acetaldehyde with high proline amounts, for which the first in situ detection of a proline-derived dienamine was accomplished. In addition, the diastereoselectivity of the aldol addition is evidenced to be time-dependent since it is undermined by the retro-aldolization and the competing irreversible aldol condensation; here NMR reaction profiles can be used as a tool for reaction optimization.     

trans‐Dichloridobis(4,8‐dimethyl‐2‐phenyl‐2‐phosphabicyclo[3.3.1]nonane‐κP)platinum(II)

The crystal structure of the title compound, trans‐[PtCl₂(C₁₆H₂₃P)₂], has been determined at 100 K. The Pt atom is located on a twofold axis and adopts a distorted square‐planar coordination geometry. The structure is only the second example of a coordination complex containing a derivative of the 4,8‐dimethyl‐2‐phosphabicyclo[3.3.1]nonane (Lim) phosphine ligand family. The ligand contains four chiral C atoms, with the stereochemistry at three of these fixed during synthesis, therefore resulting in two possible ligand stereoisomers. The compound crystallizes in the chiral space group P4₃2₁2 but is racemic, comprising an equimolar mixture of both stereoisomers disordered on a single ligand site. The effective cone angles for both isomers are the same at 146°.     

Heat Content Asymptotics for Oblique Boundary Conditions

We consider the short-time heat content asymptotics for oblique boundary conditions. The first few coefficients in the asymptotic expansion are calculated.     

吉林陨石钻孔样品中宇宙线成因核素的深度分布与宇宙线照射历史

成功地对吉林Ⅰ号陨石取得了平行与垂直第一阶段母体表面并相交于第二阶段母体中心的钻孔岩蕊.对两个钻孔的样品进行了系统的宇宙线成因核素(³He,^20,21,22Ne,²²Na,²⁶Al,⁵³Mn,⁶⁰Co)、放射成因气体和俘捕稀有气体的研究,证实吉林陨石具有典型的两阶段宇宙线照射历史,确证作者曾提出的两阶段母体年龄、形状、大小及各样品埋藏位置的模式是合理的.各样品散裂、放射和俘捕气体的研究表明:气体的丢失机制是第二阶段母体在行星际空间运行时受到太阳的周期性加热.根据Ⅱ号陨石各定位样品的^3,4He,^20,21,22Ne和⁴⁰Ar的研究,恢复了Ⅱ号陨石在两阶段母体中的相对位置.     

Root Systems and Length Functions

We introduce root systems for those imprimitive complex reflection groups which are generated by involutory reflections, and study the associated length functions. These have many properties in common with the usual length functions for finite Weyl groups.     

Notable features in the molecular structure of W2(OCH2-t-Bu)6(NCNMe2)3: Three different modes of bonding for NCNMe2(2-) ligands

Hydrocarbon solutions of W₂(OCH₂-t-Bu)₆(HNMe₂)₂(MM) and Me₂NCN (?3 equiv) react at 0°C to give a compound of formula W₂(OCH₂-Bu-t)₆(NCNMe₂)₃. The crystal and molecular struture of the latter compound, deduced from an X-ray study, reveals the loss of the WW triple bond (WW = 3.85 Å) and the formal reduction of each Me₂NCN molecule to a 2- ligand for which three different modes of bonding are seen.