Acyclic diastereoselection of 1,3-relative centers is a topic of great interest1 and has important implication in the synthesis of polyether and macrolide antibiotics.2 Impressive progress has been made in special cases such as in the conversion of metal-chelated j3-hydroxyl ketones to 1,3-diol derivative^.^ However, high levels of asymmetric induction are less documented for substrates which do not involve metal-chelated c~mplexes.~ Geometrical constraints imposed by intramolecular cyclization reactions provide attractive means to control the newly-formed asymmetric centers in the process of ring as~embly,~ and acyclic stereocontrol would then result if the temporary ring can be subsequently elaborated.We describe here a new approach employing rigid fivemembered ring systems for diastereoselective constitution of stereocenters via intramolecular SN' reactions. It was presumed that enolates 1 would undergo O-displace-ment6 with the formation of enol ethers 2 which, after simple hydrolysis, would afford hemiacetals 3 or y-hydroxy ketones 4 with resulting stereocontrol of remote stereogenic centers (Scheme 1).We were initially intrigued with the possibility of stereoselective formation of bicyclic enol ether 7a via intramolecular SN' displacements (Scheme 2). In fact, treatment of cyclohexanone derivative 5l with potassium hydride gave a single isomer of vinyl ether product 7a which was similarly obtained from trans olefin 6.7 Thus, (1) For reviews, see: (a) Bartlett, P. A. Molander, G. A.; Etter, J . B.; Harring, L. S.; Thorel, P.-J. J. Am. Chem. SOC. 1991, 113, 8036. (d) Murakami, M.; Suginome, M.; Fujimoto, K.; Nakamura, H.; Andersson, P.; Ito, Y. J. Am. Chem. SOC. 1993, 115, 6487. (6) For palladium-catalyzed 0-displacment of acetoacetate derivatives, see: Trost, B. M.; Runge, T. A. J . Am. Chem. SOC. 1981, 103, 7550 and 7559.(7) By using 1 equiv of sodium hydride at 0 "C, monoalkylation of 2-phenylcyclohexanone with cisor trans-1,4-dibromide-2-butene gave the corresponding intermediates 5 and 6 in 45% and 65% yields, respectively. 0022-3263/95/1960-2668$09.00/0 Scheme 1 1 2 3 4 Scheme 2 5 l a 6 Scheme 3the olefin geometry of the starting materials has little influence on the resulting stereochemistry of the final product. The preference for 0rather than C-alkylation, which derives from stereoelectronic considerations, has been well documented for the process of five-membered ring formations.s Molecular modeling studiesg were performed to elucidate the nature of these results. In assessing the contribution of various transition states, we have chosen early transition state structures in which the new bond formation is stereodetennining (Scheme 3). In structures 5b and 6b, there are strong 1,3-interactions between phenyl and vinyl groups regardless of the geometry of the double bond. Therefore, the preferred transition states are Sa and 6a in which the vinyl group is oriented away from the phenyl during the five-membered ring formation, thus leading to an efficient 1,3-diastereoselection.An interesting feature of...
In this work, a double-layer honeycomb microperforated structure with adjustable back-cavity’s height is designed based on cylinder honeycomb structure and microperforated panel (MPP). The sound absorption performance can be changed by adjusting the height of back-cavity. Thus, a better absorption performance is achieved by changing the position of the inner MPP. Acoustic impedance of the structure was calculated based on transfer matrix method. The sound absorption coefficient of the structure was obtained by finite element method (FEM). Meanwhile, the 3D printing technology was used to produce the experimental samples. The experimental results demonstrate that the sound absorption coefficient of the structure is greater than 0.8 in the range of 750–1250 Hz, greater than 0.9 in the range of 2297–3592 Hz, and above 0.5 in the range of 500–4000 Hz. In addition, the feasibility of achieving adjustable sound absorption by means of changing the height of the back-cavity is verified by theoretical, simulation, and experimental results. The structure proposed in this work can realize the function of wide-band and better sound absorption performance by changing the position of the inner MPP, which can be applied to effectively reduce different frequencies noise.
A large part of research on membrane mufflers focuses on the sound insulation performance of the membrane, and there is less research on the sound absorption performance of the membrane. In most cases, membrane mufflers only have some narrow absorption peaks in the low frequency band. In this paper, a micro-perforated dielectric elastomer membrane sound absorber is introduced to broaden the sound absorption frequency band of the membrane in the low frequency band. Different initial thickness, perforation aperture, and perforation spacing of the dielectric elastomer membranes were designed and fabricated, and the effect of the variation of the parameters was investigated by testing the sound absorption performance of the structural specimens. It was found that increasing the initial thickness of the membrane, appropriately decreasing the perforation aperture, and appropriately controlling the perforation spacing could improve the sound absorption performance of the structure. And based on the perforated membrane, a ring electrode structure was further designed to realize the electrically adjustable sound absorption frequency of the membrane. After the membrane with the ring electrode structure was perforated in the central region, the sound absorption band of the membrane was broadened significantly, and the applied voltage could realize the electrically adjustable sound absorption performance of the structure in the low frequency band.
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