排序方式: 共有33条查询结果,搜索用时 15 毫秒
1.
2.
3.
对手性噁唑硼烷催化芳醛不对称炔基化还原反应的机理进行了量子化学半经验方法研究.研究表明,该反应是放热的,炔基转移是反应的控制步骤,炔基转移的过渡态具有扭曲的椅式结构,还原的主要产物是R-手性醇. 相似文献
4.
In recent years, many optically active β-amino alcohols, mostly derived from naturally occurring α-amino acids, have been incorporated into the asymmetric synthesis as chiral auxiliaries or ligands.1 An effective asymmetric catalysts, the oxazaborolidine-borane reagents, which were originally pioneered by Itsuno and Corey,2 were generally prepared from chiral β-amino alcohols by the reaction with boric acid or formed in situ in the presence of borane. These reagents provide excellent enanti… 相似文献
5.
The enantioselective reduction of prochiral ketones with NaBH4/Me2SO4/(S)-Me-CBS is described. Borane is generated in situ via the reaction of NaBH4 with Me2SO4 in tetrahydrofuran, which is as efficient as the commercial one. Such in situ–generated borane reagent was applied to reduce prochiral ketones in the presence of chiral oxazaborolidine catalyst directly. The corresponding chiral secondary alcohols were obtained with excellent enantiomeric excesses (93–99% ee) and good to excellent yield (80–99%). 相似文献
6.
7.
8.
In the current article, the structures and properties of intermediates during the hydride transfer for the prior coordination of the carbonyl oxygen of keto oxime ether at B(2) of oxazaborolidine are discussed. All the structures are optimized completely by means of the Hartree–Fock (HF) and the density functional methods at the HF/6‐31G(d) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP)/6‐31G(d) levels. The hydride transfer from BH3 to the carbonyl carbon in oxazaborolidine‐borane‐keto oxime ether adduct results in the formation of the adduct 4a* with a seven‐membered ring. This adduct has four stable structures. Another hydride of BH2 transfers to the oxime carbon in 4a* , leading to the adduct 5a* , which has also four stable structures. Among all the structures of 5a* , the most stable structure can generate (1S, 2R)‐cis amino alcohol, which is in agreement with that obtained in the experiment. This enantioselective reduction may go through the process in which oxazaborolidine‐borane‐keto oxime ether adduct is directly transformed into the adduct 4a* with a seven‐membered ring. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 307–316, 2003 相似文献
9.
Ming Li Rugang Xie Changwei Hu Xin Wang Anmin Tian 《International journal of quantum chemistry》2000,78(4):245-251
The ab initio molecular orbital method is employed to study the structures and properties of chiral cyclic sulfur‐containing oxazaborolidine, as a catalyst, and its borane adducts. All the structures are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The catalyst is a twisted chair structure and reacts with borane to form four plausible catalyst–borane adducts. Borane–sulfur adducts may be formed, but they barely react with aromatic ketone to form catalyst–borane–ketone adducts, because they are repulsed greatly by the atoms arising from the chair rear of the catalyst with a twisted chair structure. Borane–N adduct has the largest formation energy and is predicted to react easily with aromatic ketone to form catalyst–borane–ketone adducts. The formation of the catalyst–borane adducts causes the BBH3 HBH3 bond lengths of the BH3 moiety to be increased and thus enhances the activity of the enantioselective catalytic reduction. The borane–N adduct is of great advantage to hydride transfer. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 245–251, 2000 相似文献
10.
报道一种新的测定寡糖链糖体亚谱的NMR方法, 称为MMDY混合脉冲梯度选择一维HOHAHA法. 本方法采用MMDY混合脉冲与双梯度自旋回波脉冲相结合, 为解决高度重叠的1H NMR谱图分析困难提供了有力手段. 本方法的主要特点是: 采用双梯度自旋回波脉冲进行选择激发, 与差谱为基础的选择激发方法相比不但可节省测定时间, 而且谱图干净, 没有差谱伪峰; MMDY混合脉冲的磁化接力相干转移非常有效, 远程磁化转移信号的灵敏度高, 对于偶合常数大约为1 Hz的微弱偶合自旋体系, 如阿拉伯糖和鼠李糖, 也能够得到完整的亚谱; 梯度脉冲只是用于相干路径选择, 灵敏度比较高, 并且纯吸收形的高质量亚谱不经过复杂的Z-滤波步骤就可以快速得到, 因此比较适用于常规样品分析. 以苷类化合物为例讨论本方法的特点以及在谱峰高度的寡糖结构测定中的应用. 相似文献