The 2-phenyl-2H-chromene structural core is a widespread element in natural flavonoids and has attracted intense attention since a wide range of biological activities associated with the scaffold have been identified. Accordingly, the development of facile synthetic strategies to access such heterocycles is of considerable interest and a few asymmetric synthetic methods have been reported recently. Although the asymmetric methods could provide enantioenriched flavene derivatives, their development for more functionalized flavene skeleton has proven to be a challenging synthetic task. In this communication, we wish to report diastereoselective Reformatsky reaction of flavene-3-carbaldehydes to provide β-(2H-3-flavenyl)-β-hydroxy esters and disclose a new enantioselective synthetic method for 3substituted flavene derivatives by asymmetric dehydration via kinetic resolution. We have initially investigated the substrate controlled diastereoselective addition of Reformatsky reagent to flavene-3-carbaldehydes 1, in which the chiral center at the C(2) position of 2H-chromene exerts control over the formation of new stereogenic center. In an initial experiment, to a solution of BrZnCH2CO2t-Bu (3 equiv) in THF was added a solution of flavene-3-carbaldehyde 1a in THF, and the resulting mixture was heated to reflux to afford the products 2a and 3a in 91% yield with a diastereomeric ratio (dr) of 71:29. (entry 1, Table 1) Different experimental conditions have been tested on flavene-3-carbaldehyde 1a to improve the diastereoselectivity of the addition. Since the diastereoselectivity could be enhanced by effecting the reaction at low temperature, different reaction temperatures have been tested on flavene-3-carbaldehyde 1a as summarized in Table 1. As the reaction temperature decreases, the dr is improved as shown in entries 1-5. The ratio of major 2a to minor product 3a was 79:21 at rt, whereas it was 90:10 at −15 °C. At −30 °C, much slower reaction gave products 2a and 3a with only 15% conversion after 5 h and a slightly improved dr of 92:8. (entry 5) Polar solvents were then screened in an attempt to increase the solubility of the reagent and to enhance the rate and the selectivity of the reaction. The use of THF-DMSO (1:1), THF-p-dioxane (1:1) and p-dioxane respectively as solvent (entries 6-9) did not increase the stereoselection and decreased the yield of the reaction. Under the optimized condition at −15 °C in THF, the reaction of o-methoxy substituted flavene aldehyde 1b gave slightly lower dr (entry 10) and p-methoxy substituted flavene aldehydes 1c-e reacted with Reformatsky reagent leading to 2c-e and 3c-e with drs ranging from 89:11 to 91:9, which are practically independent on the presence of the substituents on the aromatic rings and substitution pattern. While 4chloro substituent increase the selectivity up to 94:6 dr (entry 16), 2-methyl substituted chromenes 1f and 1g showed much lower selectivity (entries 14-15). Non-enzymatic kinetic resolution of racemic compounds using chiral catalyst is an area of great importance in contemporary organic synthesis. We recently reported the first example of the kinetic resolution in dehydration of alcohols using D-Phg-L-Pro-derived chiral ligand 5. Given
결정핵-매개 소프트-용액 방법을 이용하여 판상형 ZnO 나노입자를 합성하였다. 합성된 판상형 ZnO 나노입자는 많은 결함이 존재하는 (0001)면이 발달하여 상업적인 ZnO 나노입자 보다 광촉매 특성이 우수하였다. 또한, 판상형 ZnO 나노입자의 세포독성이 실리카 코팅을 통해 감소함을 확인하였다. 마지막으로 판상형 ZnO를 합성하는데 필요한 성장용액의 다양한 시약 조건들을 변화시키면서 상업적인 적용이 가능한 수백 그램 스케일로 대량합성 하는데 처음으로 성공하였다. ZnO nanoplates were prepared by seed-mediated soft-solution process. Photocatalytic property of ZnO nanoplates was superior to that of conventional ZnO nanoparticles owing to the enhanced (0001) plane with large defect sites. In addition, we found that silica coating method could provide to reduce cytotoxicity of ZnO nanoplates. Finally, we have successfully synthesized for the first time large-scale synthesis of plate-type ZnO as few hundreds gram scale for industrial applications through controlling various reagents of growth solution.
In recent, next-generation power semiconductor materials, such as SiC and GaN, are renowned for their high withstand voltage and low on-resistance characteristics, as opposed to traditional Si-based semiconductors. While extensive research is underway on these advanced power semiconductors, there is also a notable emphasis on investigating bonding materials crucial for connecting semiconductor devices to metal or ceramic substrates. Our attention is particularly drawn to the development of a joining material capable of replacing conventional high-temperature solders, exhibiting superior electrical and thermal conductivity. Sintering bonding, a technology enabling bonding at temperatures comparable to existing high-temperature solder while ensuring high-temperature reliability, has gained prominence. Although sintering bonding with Ag powder is gaining traction, the associated material cost remains a concern. Cu paste, offering a more economical alternative to Ag, faces challenges related to relatively high processing temperature, low bonding strength, and copper oxidation. In our study, we explore pastes for sintering of chip attach and achieve high-strength bonding using Ag coated Cu (Cu@Ag) powder, leveraging the advantages of both materials.
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