Conformationally constrained dipeptides. Obtention of enantiomerically pure 6‐acetamido‐5‐oxo‐1,2,3,5,6,7‐hexahydro‐3‐indolizine carboxylic acid

The separation of a stereoisomeric mixture of esters 6, and the synthesis of the new enantiomerically pure unsaturated 6,5‐fused bicyclic lactam 2 are described. The key‐step involves trimethylsilyl iodide cleavage, without racemization, of a vinylogous carbamate. The cis N‐ acetylamino acid 2 is a new scaffold for the synthesis of rigid β‐turn mimetics.     

Analysis and design of backbone architecture alternatives for IP optical networking

The unprecedented growth of IP traffic is leading Internet service providers and network operators worldwide to investigate architectural alternatives for cost effective, reliable, scalable, and flexible multiterabit IP backbones. In this paper, several overlay, service, and transport layer networking architectures, which employ IP, MPLS, SONET/SDH, and DWDM technologies, are proposed and analyzed. Multiple parameters, such as network capacity, cost, restoration strategy, reconfigurability, and accommodation of preemptable traffic, are considered for the architectural comparison. Detailed network design and economic analysis are provided for the different alternatives considering a typical nationwide U.S. backbone with projected IP traffic in approximately three years. Several sensitivity analysis results are also shown, to evaluate the effect of cost changes in some of the critical technological factors in these architectures, such as 10 Gb/s optics cost or IP router cost. The results show the value of transport layer networking architectures for multiterabit IP backbones, and how, when compared to service layer architectures, they provide additional desirable features such as wavelength reconfigurability and restoration scalability.     

Variational problems with topological constraints

We introduce a decomposition that captures much of the topology of level sets for functions in certain Sobolev spaces, and allows the definition of an analog of the decreasing rearrangement of a function which respects the topology of level sets. In a variety of settings this decomposition is preserved under weak limits, and so is useful in establishing existence of minimizers of various variational problems with prescribed topological properties. These include variational problems in which ‘topological rearrangements’ are fixed, and ones in which the functional depends on derivatives of rearrangements. Received: December 16, 1998 / Accepted: July 16, 1999     

Evaluation of steady and pulsatile flow with dynamic MRI using limited flip angles and gradient refocused echoes

Magnetic resonance imaging sequences utilizing limited flip angles and gradient echoes yield rapid (less than 2 min) dynamic images of the cardiovascular system. These images contain both accurate anatomical and functional information. Using a gradient refocused acquisition in the steady state (GRASS) in the CINE mode, we studied the relationship between gradient echo signal intensity and velocity of steady and pulsatile flow in a phantom simulating medium to large vessels. Images were acquired on a 1.5 Tesla system (repetition TIME = 21 ms, echo TIME = 12 ms, flip ANGLE = 30 degrees). Data from each pulse interval were sorted in 16 images. Signal intensities from flow tube lumina and surrounding stationary water jacket were used to calculate contrast ratios which were compared to velocity measurements made with electromagnetic (EM) flow probes outside the magnet room. During steady flow, signal intensity contrast ratios increased with increasing flow and in a 10 mm thick slice, reached a peak at 48 cm/s, and declined for velocities up to 90 cm/s. Changes in instantaneous velocity during pulsatile flow correlated well (r > .88) with signal intensity changes up to a maximum mean velocity of 17 cm/s. Total signal intensity from the lumen for an “R to R” interval correlated extremely well (r > .97) with mean pulsatile flow velocities up to 30 cm/s. The excellent correlation between gradient echo signal intensity and actual flow velocities suggests that this imaging sequence might be useful for evaluating normal and pathologic flow phenomena.     

责任制与业绩测量法并非万能管理手段

很难想象在商业活动中,会有过多的责任一说. 20年前,马里兰大学的爱德·洛克教授研究了能够带来更好成果的4种最流行的管理手段.其中,确立与责任挂钩的目标这一手段高居榜首,它是经理人可以实施的最有效的战略,可以把业绩平均提高16%.     

分享目标,推动员工全身心达成任务的伟大力量

我们并不在一个完美的世界里工作.问题是有什么东西挡住了我们的去路,一个我们没有看到的绊脚石. 这意味着经理人需要做的不仅仅是执行和清除障碍.领导们必须帮助员工找到一个专注于工作的理由,使他们战胜困难并发挥必要的主动性,确保他们达到组织的期望.     

Design Parameters for Ionic Liquid–Molecular Solvent Blend Electrolytes to Enable Stable Li Metal Cycling Within Li–O2 Batteries

Effective utilization of Li-metal electrodes is vital for maximizing the specific energy of lithium–oxygen (Li–O₂) batteries. Many conventional electrolytes that support Li–O₂ cathode processes (e.g._, dimethyl sulfoxide, DMSO) are incompatible with Li-metal. Here, a wide range of ternary solutions based on solvent, salt, and ionic liquid (IL) are explored to understand how formulations may be tailored to enhance stability and performance of DMSO at Li-metal electrodes. The optimized formulations therein facilitate stable Li plating/stripping performances, Columbic efficiencies >94%, and improved performance in Li–O₂ full cells. Characterization of Li surfaces reveals the suppression of dendritic deposition and corrosion and the modulation of decomposition reactions at the interface within optimized formulations. These observations are correlated with spectroscopic characterization and simulation of local solvation environments, indicating the persistent importance of DMSO–Li⁺-cation interactions. Therein, stabilization remains dependent on important molar ratios in solution and the 4:1 solvent-salt ratio, corresponding to ideal coordination spheres in these systems, is revealed as critical for these ternary formulations. Importantly, introducing this stable, non-volatile IL has negligible disrupting effects on the critical stabilizing interactions between Li⁺ and DMSO and, thus, may be carefully introduced to tailor other key electrolyte properties for Li–O₂ cells.     

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II(OAc)₂(XPhos). This Pd^II center is then reduced to the Pd⁰(XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I–Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰(XPhos) as the reactive species in equilibrium with the unreactive Pd⁰(XPhos)₂. Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.