排序方式: 共有39条查询结果,搜索用时 140 毫秒
21.
PURPOSE: This study aimed to investigate the use of anatomically tailored hexagonal sampling for scan-time and error reduction in MRI. MATERIALS AND METHODS: Anatomically tailored hexagonal MRI (ANTHEM), a method that combines hexagonal sampling with specific symmetry in anatomical geometry, is proposed. By using hexagonal sampling, aliasing artifacts are moved to regions where, due to the nature of the anatomy, aliasing is inconsequential. This can be used to either reduce scan time while maintaining spatial resolution or reduce residual errors in speedup techniques like UNFOLD and k-t BLAST/SENSE, which undersample k-space and unwrap fold-over artifacts during reconstruction. Computer simulations as well as phantom and volunteer studies were used to validate the theory. A simplified reconstruction algorithm for hexagonally sampled and subsampled k-space data was also used. RESULTS: A reduction in sampling density of 13.4% and 25% in each hexagonally sampled dimension was achieved for spherical and conical geometries without aliasing or reduction in spatial resolution. Optimal subsampling schemes that can be utilized by UNFOLD and k-t BLAST/SENSE were derived using hexagonal subsampling, which resulted in maximal, isotropic dispersal of the aliases. In combination with UNFOLD, ANTHEM was shown to move residual aliasing artifacts to the corners of the field of view, yielding reduced artifacts in CINE reconstructions. CONCLUSIONS: ANTHEM was successful in reducing acquisition time in conventional MRI and in reducing errors in UNFOLD imaging. 相似文献
22.
Lovely CJ Du H Sivappa R Bhandari MR He Y Dias HV 《The Journal of organic chemistry》2007,72(10):3741-3749
Various 4-vinylimidazole derivatives have been prepared from the corresponding 4-iodoimidazoles or from urocanic acid. Several methods for the elaboration of these vinylimidazoles and their Diels-Alder reactions are reported. All of the vinylimidazoles prepared in the course of this study react with N-phenylmaleimide quite readily with mild thermal activation providing a single cycloadduct, in most cases the initial, nonaromatic adduct. With more electron rich substrates, there is a tendency for these initial cycloadducts to undergo aromatization, ene reaction, and oxidation although this can be circumvented to a large extent by the choice of reaction conditions. Limited reactions were observed with other dienophiles, providing the expected cycloadducts in most cases, although an abnormal adduct was obtained in one case with dimethyl acetylene dicarboxylate. These substrates also participate in regioselective Diels-Alder reactions with monoactivated dienophiles, but require fairly forcing conditions, thus only providing the aromatized cycloadducts in modest yields. An investigation of substituent effects at the 2-position of the imidazole moiety was undertaken, in which electron-donating and weakly electron-withdrawing substituents are tolerated. In addition, several substrates with terminally substituted vinyl moieties have been investigated. 相似文献
23.
Dr. Venubabu Kotikam Prof. Dr. Scott D. Kennedy Prof. Dr. James A. MacKay Prof. Dr. Eriks Rozners 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(17):4367-4372
The development of new RNA-binding ligands is attracting increasing interest in fundamental science and the pharmaceutical industry. The goal of this study was to improve the RNA binding properties of triplex-forming peptide nucleic acids (PNAs) by further increasing the pKa of 2-aminopyridine ( M ). Protonation of M was the key for enabling triplex formation at physiological pH in earlier studies. Substitution on M by an electron-donating 4-methoxy substituent resulted in slight destabilization of the PNA–dsRNA triplex, contrary to the expected stabilization due to more favorable protonation. To explain this unexpected result, the first NMR structural studies were performed on an M -modified PNA–dsRNA triplex which, combined with computational modeling identified unfavorable steric and electrostatic repulsion between the 4-methoxy group of M and the oxygen of the carbonyl group connecting the adjacent nucleobase to PNA backbone. The structural studies also provided insights into hydrogen-bonding interactions that might be responsible for the high affinity and unusual RNA-binding preference of PNAs. 相似文献
24.
Dr. Venubabu Kotikam Julien A. Viel Prof. Dr. Eriks Rozners 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(3):685-690
The success of RNA interference (RNAi) as a research tool and potential therapeutic approach has reinvigorated interest in chemical modifications of RNA. Replacement of the negatively charged phosphates with neutral amides may be expected to improve bioavailability and cellular uptake of small interfering RNAs (siRNAs) critical for in vivo applications. In this study, we introduced up to seven consecutive amide linkages at the 3′-end of the guide strand of an siRNA duplex. Modified guide strands having four consecutive amide linkages retained high RNAi activity when paired with a passenger strand having one amide modification between its first and second nucleosides at the 5′-end. Further increase in the number of modifications decreased the RNAi activity; however, siRNAs with six and seven amide linkages still showed useful target silencing. While an siRNA duplex having nine amide linkages retained some silencing activity, the partial reduction of the negative charge did not enable passive uptake in HeLa cells. Our results suggest that further chemical modifications, in addition to amide linkages, are needed to enable cellular uptake of siRNAs in the absence of transfection agents. 相似文献
25.
Beldar Vishal Gokul Kale Balasaheb Shantilal Pagare Ravikiran Jadhao Manojkumar 《Chemistry of Natural Compounds》2022,58(4):754-756
Chemistry of Natural Compounds - 相似文献
26.
Nair V Deepthi A Poonoth M Santhamma B Vellalath S Babu BP Mohan R Suresh E 《The Journal of organic chemistry》2006,71(6):2313-2319
The zwitterion formed by the reaction of dimethoxycarbene and DMAD adds efficiently to one of the carbonyl groups of 1,2-dicarbonyl compounds and anhydrides to generate dihydrofurans and spirodihydrofurans in good yields. In many cases, the carbene inserts into the C-C bond of the dione to yield masked vicinal tricarbonyl systems. 相似文献
27.
Nair V Poonoth M Vellalath S Suresh E Thirumalai R 《The Journal of organic chemistry》2006,71(23):8964-8965
A novel protocol for the annulation of tropone to enals involving nucleophilic heterocyclic carbene (NHC) catalyzed homoenolate formation has been developed. Interestingly, the reaction led to bicyclic delta-lactones instead of the expected gamma-spirolactones, presumably by the uncommon [8 + 3] annulation pathway. The strategy works well with a variety of enals. 相似文献
28.
The most frequent method of removing malignant growth-causing tones from current effluents before releasing them into water sources such as rivers, lakes, and groundwater has become standard. Traditional waste-water treatment frameworks have trouble getting rid of these contaminants. This is a unique flavonoid that uses Fe3O4 nanorods as photocatalytic agents to corrupt material tone in the watery stage utilizing observable light enlightenment. Green technique was used to amalgamate [email protected]3O4 nanorod like gems. Disappearance of the bright (UV) maintenance top at 565 nm confirmed the elimination of Methyl orange tone. After 110 min, the sensational shading disposal of Fe3O4 nanorod was observed to be 100%. This is due to photochemical redox process and the use of Fe3O4 nanorods with a high energy gap of flavonoids. The findings show that Fe3O4 rod-like gems manufactured using green technology are extremely valuable in the photocatalytic annihilation of hazardous contaminants. 相似文献
29.
Sonia George Raja Chakraborty Manojkumar Parameswaran Anju Rajan Thengungal Kochupaappy Ravi 《Journal of heterocyclic chemistry》2015,52(1):211-214
A series of substituted phenyl methyl piperazine triazolyl benzotriazoles 4a , 4b , 4c , 4d , 4e , 4f , 4g have been synthesized through the Mannich reaction of substituted phenyl triazolyl benzotriazoles 3a , 3b , 3c , 3d , 3e , 3f , 3g . The substituted phenyl triazolyl benzotriazoles were prepared from benzotriazolyl acetohydrazide, where the cyclization was facilitated through ammonium acetate and aryl aldehydes. The IR, 1H NMR, mass spectral data and elemental analysis were performed to assign the structure. All the newly synthesized compounds were screened for their antimicrobial and antioxidant activity. 相似文献
30.
Manojkumar R. Shukla Prashant N. Patil Prakash P. Wadgaonkar Prafulla N. Josh Manikrao M. Salunkhe 《合成通讯》2013,43(1):39-42
Synthesis of substituted (±)-3,4 Dihydrocoumarins were prepared with the reaction between substituted cinnamic acid and phenol catalysed by ecofriendly solid-acid catalyst H-Y zeolite involving esterification followed by ring closure. 相似文献