951 research outputs found
Non-proper helicoid-like limits of closed minimal surfaces in 3-manifolds
We show that there exists a metric with positive scalar curvature on S2xS1
and a sequence of embedded minimal cylinders that converges to a minimal
lamination that, in a neighborhood of a strictly stable 2-sphere, is smooth
except at two helicoid-like singularities on the 2-sphere. The construction is
inspired by a recent example by D. Hoffman and B. White.Comment: 12 pages, 3 figures, replaced because of corrupted fil
Advances in palladium catalysed wacker-type oxidative transformations
The development and optimisation of conditions for oxidative Wacker-type cyclisations followed by establishing the reaction scope are reported. Building upon the achievements in the field of oxidative Wacker-type reactions that has recently gathered interest, hydroxylamines and hydrazines were converted to isoxazolidines and pyrazolidines respectively. Secondary hydroxylamines cyclised yielding syn-isoxazolidines with excellent diastereoselectivities, whereas secondary hydrazines cyclised yielding anti-pyrazolidines but still maintained a high level of diastereoselectivity. Additionally, an enantioselective variant was explored. Isoxazolidines were successfully transformed to the corresponding 1,3-amino alcohols, which were further converted to amino sugar derivatives
Vanadium(V) oxo and imido calix[8]arene complexes: synthesis, structural studies, and ethylene homo/copolymerisation capability
Interaction of p-tert-butylcalix[8]areneHâ (Lâ¸Hâ) with in-situ generated [NaVO(Ot-Bu)â] (from VOClâ and four equivalents of NaOtBu) afforded the dark brown complex [Na(NCMe)â
][(VO)âLâ¸H]¡4MeCN (1¡4MeCN), in which the calix[8]arene adopts a saddle-shaped conformation. Increasing (to four equivalents per Lâ¸) the amount of [NaVO(Ot-Bu)â] present in the reaction, led to the formation of the yellow octa-vanadyl complex {[(Na(VO)âLâ¸)(Na(NCMe))â] [Na(NCMe)â}â¡10MeCN (2¡10MeCN), in which the calix[8]arene adopts a pleated loop conformation. In the presence of adventitious oxygen, reaction of four equivalents of [VO(Ot-Bu)â] (generated from VOClâ and 3KOtBu) with Lâ¸Hâ afforded the alkali-metal free green complex [(VO)âLâ¸(Ο³-O)â] (3); the solvates 3¡3MeCN and 3¡3CHâClâ have been isolated. In both solvates, the L⸠ligand adopts a shallow saddle-shaped conformation, supporting a core comprising of a (VO)âOâ ladder. In the case of lithium, in order to obtain crystalline material, it was found necessary to reverse the order of addition such that lithium tert-butoxide was added to Lâ¸Hâ, and then subsequently treated (at â78 â°C) with two equivalents of VOClâ; crystallization from tetrahydrofuran (THF) afforded {(VOâ)âLiâ[Lâ¸](thf)â(OtBu)â(EtâO)â}¡EtâO (4¡EtâO). In the structure of 4¡EtâO, vanadium, lithium and oxygen form a central lattern-type cage, which is capped top and bottom by an LiâOâ2 diamond; the calix[8]arene is in a âdown, down, out, out, down, downâ conformation. When the âsame reactionâ was extracted into acetonitrile (MeCN), the salt complex [Li(NCMe)â][(VO)âLâ¸H]¡8MeCN (5.8MeCN) was formed. In 5¡8MeCN, the [Li(NCMe)â] cations reside between the anions in the clefts of Lâ¸H, the latter adopting a saddle-shaped conformation. Use of the imido precursors [V(Nt-Bu)(Ot-Bu)â] and [V(Np-tolyl)(Ot-Bu)â] and Lâ¸Hâ, afforded, via an imido exchange, the salt [t-BuNHâ]{[V(p-tolylN)]âLâ¸H}¡3½MeCN (6¡3½MeCN). The molecular structures of 1 to 6 are reported; data collections for complexes 2¡10MeCN, 3¡3MeCN and 3¡3CHâClâ required the use of synchrotron radiation. Complexes 1, 3 and 4 have been screened as pre-catalysts for the polymerization of ethylene in the presence of a variety of co-catalysts (with and without a re-activator) at various temperatures and for the co-polymerization of ethylene with propylene; results are compared versus the benchmark catalyst VO(OEt)Clâ. In some cases, activities as high as 136,000 g/mmol.v.h were achievable, whilst it also proved possible to obtain higher molecular weight polymers (in comparible yields) versus the use of VO(OEt)Clâ. In the case of the co-polymerization, the incorporation of propylene was 7.1 â 10.9 mol% (cf 10 mol% for VO(OEt)Clâ), though catalytic activities were lower versus VO(OEt)Clâ
Continuous N-alkylation reactions of amino alcohols using Îł-Al2O3 and supercritical CO2: unexpected formation of cyclic ureas and urethanes by reaction with CO2
The use of Îł-Al2O3 as a heterogeneous catalyst in scCO2, has be successfully applied to the amination of alcohols for the synthesis of N-alkylated heterocycles. The optimal reaction conditions (temperature and substrate flow rate) were determined using an automated self-optimising reactor, resulting in moderate to high yields of the target products. Carrying out the reaction in scCO2 was shown to be beneficial, as higher yields were obtained in the presence of CO2 than in its absence. A surprising discovery is that, in addition to cyclic amines, cyclic ureas and urethanes could be synthesised by incorporation of CO2 from the supercritical solvent into the product
Dynamic adjustment of receive window utilized by a transmitting device
A method of controlling size of a receive window includes transmitting packets over a communication channel from a transmitting device to a receiver, and receiving acknowledgment packets from the receiver, the received acknowledgement packets from the receiver including an advertised receive window size. The method further includes determining a backlog parameter for the receiver in accordance with the advertised receive window size, determining a queuing delay in accordance the received acknowledgment packets, resetting a size of a congestion window in accordance with a function of a current size of the congestion window and a factor proportional to the queuing delay, and resetting a size of a receive window in accordance with a function of a current size of the receive window and the backlog parameter. A network window is reset in accordance with the smaller of the size of the congestion window and the size of the receive window
Efficient preparation of TMSCCl2Br and its use in dichlorocyclopropanation of electron deficient alkenes
The reaction of excess TMSCl and LiCCl2Br at low temperature is a technically simple high yield route to TMSCCl2Br. The latter is a stable source of the dichlorobromomethide carbanion, which undergoes 1,4-addition with cyclic nitroalkenes and (E)-fumarates leading to dichlorocyclopropanes after bromide explusion. For nitrostyrenes the reaction arrests at the 1,4-addition product. Low temperature NMR studies and DFT calculations suggest the formation of an âateâ species [(nitronate)SiFMe3]- which, upon boil off of TMSF at 10 - 20 °C, yields the cyclopropane. DFT calculations also support the experimental differences between fluoride and acetate as promotors
Photooxidation of Fulvenes in a Continuous Flow Photoreactor using Carbon Dioxide as a Solvent
A range of 3âsubstituted oxepinones have been synthesised by the photooxidation of fulvenes with photochemically generated singlet oxygen (1O2) using carbon dioxide (CO2) as a solvent and a CO2âsoluble porphyrin photosensitizer. The reactive intermediates generated in this reaction decompose to yield a range of useful and underutilized building blocks that are not easily accessible using more traditional reaction setups. Fulvenes containing an exocyclic allylic hydrogen are shown to form three products through potentially competing intramolecular pathways, which can be tuned to some extent by adjusting the conditions in the flow reactor. A range of substrates containing a variety of functional groups was examined; this led to lower yields of the desired lactones being observed in the case of compounds containing polar electronâdonating groups. We also demonstrate that one of the lactones can be further hydrogenated over a Pd/C catalyst to the unsaturated 7âmembered ring in high yield
Spatiotemporal dynamics of the postnatal developing primate brain transcriptome.
Developmental changes in the temporal and spatial regulation of gene expression drive the emergence of normal mature brain function, while disruptions in these processes underlie many neurodevelopmental abnormalities. To solidify our foundational knowledge of such changes in a primate brain with an extended period of postnatal maturation like in human, we investigated the whole-genome transcriptional profiles of rhesus monkey brains from birth to adulthood. We found that gene expression dynamics are largest from birth through infancy, after which gene expression profiles transition to a relatively stable state by young adulthood. Biological pathway enrichment analysis revealed that genes more highly expressed at birth are associated with cell adhesion and neuron differentiation, while genes more highly expressed in juveniles and adults are associated with cell death. Neocortex showed significantly greater differential expression over time than subcortical structures, and this trend likely reflects the protracted postnatal development of the cortex. Using network analysis, we identified 27 co-expression modules containing genes with highly correlated expression patterns that are associated with specific brain regions, ages or both. In particular, one module with high expression in neonatal cortex and striatum that decreases during infancy and juvenile development was significantly enriched for autism spectrum disorder (ASD)-related genes. This network was enriched for genes associated with axon guidance and interneuron differentiation, consistent with a disruption in the formation of functional cortical circuitry in ASD
- âŚ