142 research outputs found
C2-Selective, Functional-Group-Divergent Amination of Pyrimidines by Enthalpy-Controlled Nucleophilic Functionalization
No full textSynthesis of heteroaryl amines has been an important topic in organic chemistry because of their importance in small-molecule discovery. In particular, 2-aminopyrimidines represent a highly privileged structural motif that is prevalent in bioactive molecules, but a general strategy to introduce the pyrimidine C2-N bonds via direct functionalization is elusive. Here we describe a synthetic platform for site-selective C-H functionalization that affords pyrimidinyl iminium salt intermediates, which then can be transformed into various amine products in situ. Mechanism-based reagent design allowed for the C2-selective amination of pyrimidines, opening the new scope of site-selective heteroaryl C-H functionalization. Our method is compatible with a broad range of pyrimidines with sensitive functional groups and can access complex aminopyrimidines with high selectivity.11Nsciescopu
Intra-operative MRI with MR detectable endoscope using tunable lens filled with MR contrast agent
No full textDuring brain surgery, location of lesions can change in real-time due to leakage of cerebrospinal fluid. Therefore, navigating an MR-Endoscope probe with real-time intraoperative MRI is important in clinical application. However, conventional tracking system attached to the endoscope probe induces severe artifacts and is expensive and bulky. In this study, we propose a technique for navigating the endoscope probe without additional tracking system through segmentation of signals from tunable lens filled with gadolinium contrast agents. We also demonstrated tunable liquid-filled lens endoscope for intraoperative MRI. The proposed system/approach would be a good alternative as a tracking system for intraoperative MRI
High-efficiency Bidirectional Buck-Boost Converter for Residential Energy Storage System
No full textThis paper proposes a bidirectional dc-dc converter for residential micro-grid applications. The proposed converter can operate over an input voltage range that overlaps the output voltage range. This converter uses two snubber capacitors to reduce the switch turn-off losses, a dc-blocking capacitor to reduce the input/output filter size, and a 1:1 transformer to reduce core loss. The windings of the transformer are connected in parallel and in reverse-coupled configuration to suppress magnetic flux swing in the core. Zero-voltage turn-on of the switch is achieved by operating the converter in discontinuous conduction mode. The experimental converter was designed to operate at a switching frequency of 40-210 kHz, an input voltage of 48 V, an output voltage of 36-60 V, and an output power of 50-500 W. The power conversion efficiency for boost conversion to 60 V was >= 98.3% in the entire power range. The efficiency for buck conversion to 36 V was >= 98.4% in the entire power range. The output voltage ripple at full load was <3.59 V-p.p for boost conversion (60 V) and 1.35 V-p.p for buck conversion (36 V) with the reduced input/output filter. The experimental results indicate that the proposed converter is well-suited to smart-grid energy storage systems that require high efficiency, small size, and overlapping input and output voltage ranges.11Ysciescopu
다중탄성 원판을 가진 강체회전축-탄성지지계의 진동모드 규명
No full text학위논문(박사) - 한국과학기술원 : 기계공학과, 1999.2, [ xiv, 142, [2] p. ]The recent trend toward high rotational speed and low vibration level for high performance and efficiency in the design of rotating machines demands the inclusion of both the disk flexibility and shaft dynamics in the analysis model for accurate vibration analysis. This is especially important in the areas such as gas turbines, industrial compressors, hard disks and CD-ROM drive systems. However, the analysis model consisting of both flexible shaft and disks is so complex that the coupling between the disk and shaft is not yet clearly understood, which necessitates a simpler model. In the case that disk flexibility influences significantly the vibrational characteristics of the whole system, it is very difficult to identify the modal parameters by the conventional modal testing method utilizing mode shapes, since the shaft and disk modes are coupled with each other and their modes tend to be closely packed in the frequency domain.
In this thesis, the coupled vibrations between the shaft and disk elastic motions and the corresponding mode splits, which can be used to identify effectively the vibrational modes of rotating machines, are investigated. The analysis model assumes that the shaft is rigid, the bearings are isotropic and the disks behave according to the classical plate theory. The equations of motion are derived by using the extended Hamiltion``s principle, and discretized by expressing the disk deformation as a series of the eigenfunctions of the disk. According to the coupling mechanism of shaft and disk modes, the equations are classified as: bending coupled motion associated with the disk modes with one nodal diameter, axially coupled motion associated with the disk modes with no nodal diameter and uncoupled motion associated with the disk modes with more than one nodal diameter.
The bending coupled modes are classified as the balanced and unbalanced modes. The differences in magnitude between forward and backward modal frequencies, referred to as ...한국과학기술원 : 기계공학과
Unified balancing approach using complex modal analysis
No full text학위논문(석사) - 한국과학기술원 : 기계공학과, 1986.2, [ [vi], 65 p. ]한국과학기술원 : 기계공학과
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Polar- and Enthalpic Effects on the Selectivity of Aromatic Substitution by Cationic Radicals
No full textThe field of radical aromatic substitution consistently receives attention from synthetic organic community, in parts due to the versatile reactivity of radicals towards formation of valuable carbon–heteroatom bonds. One of the biggest goals associated with the development of useful radical aromatic substitution reactions is to achieve sufficient degree of regiocontrol, so that the maximum yield of a specific target molecule can be obtained.
On the other hand, it may also be of synthetic value to develop an aromatic substitution reaction that is able to generate a wide spectrum of constitutional product isomers from a single starting material. It would be especially so if such a reaction also does not discriminate much between differentially activated aromatic substrates, thereby achieving a broad substrate scope.
The subject of this dissertation is on the efforts to understand fundamental chemical effects that govern radical reactivity, and to address the above-mentioned challenges based on the reactivity principles.
In Chapter 1 is described a non-chelation-assisted, highly para-selective aryl C–H substitution reaction with a doubly cationic nitrogen radical. High degree of charge-transfer in the transition state of radical addition is attributed to the predictable, exquisite positional selectivity. The high electron affinity of the bond-forming species engenders significant polar effects that are responsible for the sensitivity of the reaction towards substituent directing effects.
In Chapter 2, the synthetic utility of the para-selective C–H functionalization is extended by using the functionalized product as a branch point for installation of valuable functional groups. Efficient transition-metal-catalyzed cross-coupling reactions convert the aryl ammonium intermediate into methylarenes, constituting an overall para-selective aryl C–H methylation sequence.
In Chapter 3, aromatic substitution by another highly reactive nitrogen radical cation is introduced. While also possessing high electron affinity, the novel sp2-nitrogen radical cation exhibits unexpectedly low positional selectivity, and at the same time, is able to functionalize an unprecedentedly broad range of aromatic substrates in terms of electronic activation. Dominant reactivity control by enthalpic effects is elucidated by investigation of selectivity relationships. The C–N bond forming reaction is applied towards late-stage amination of (hetero)arenes with a substrate scope that addresses the limitations of current C–H amination methodology.Chemistry and Chemical Biolog
FOXC2 and CLIP4 : a potential biomarker for synchronous metastasis of ≤7-cm clear cell renal cell carcinomas
No full textRenal cell carcinomas (RCC) smaller than 7-cm are heterogeneous and exhibit metastatic potential in approximately 15% of cases. Although large-scale characterization of mutations in clear cell RCC (ccRCC), the most common RCC subtype, has been established, the genetic alterations related to ≤7-cm ccRCCs undergoing synchronous metastasis are poorly understood. To discover biomarkers that can be used to estimate the risk of synchronous metastasis in these ccRCC patients, we performed whole exome sequencing on the formalin-fixed paraffin-embedded (FFPE) samples of 10 ccRCC patients with ≤7-cm tumors and synchronous metastasis and expanded our study using The Cancer Genome Atlas (TCGA) ccRCC dataset (n = 201). Recurrent mutations were selected according to functional prediction and statistical significance. Mutations in three candidate genes, RELN (1 out of 10), FOXC2 (1 out of 10), and CLIP4 (2 out of 10) were found in expanded analysis using a TCGA cohort. Furthermore, siRNA-mediated target gene knockdown (FOXC2 and CLIP4) and overexpression (RELN) assays showed that FOXC2 and CLIP4 significantly increased cell migration and viability in ccRCCs. Our study demonstrated that FOXC2 and CLIP4 activity correlates to the presence of ≤7-cm ccRCCs with synchronous metastasis and may be potential molecular predictors of synchronous metastasis of ≤7-cm ccRCCs.ope
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