Simple pyrrolidine-azole conjugates have been synthesized and found to be efficient catalysts for asymmetric Michael addition to nitrostyrenes. The identified optimal catalysts, pyrrolidine-azoles 2, 8 and 13, could catalyze the asymmetric Michael addition of a range of Michael donors and nitrostyrenes in high yields (up to 99%) and excellent stereoselectivities (up to 99:1 dr and 97% ee).
Benzoin condensation is an important carbon-carbon bond-forming reaction. The benzoin reaction with good yield, high enantioselectivity and broad substrate scope is highly desired in organic synthesis. The intermolecular benzoin condensation catalyzed by bifunctional N-heterocyclic carbenes has been investigated, and the corresponding α-hydroxyketones were obtained with yields up to 76% and enantioselectivities up to 99% ee.
Efficient synthesis and resolution of a series of meta-substituted inherently chiral aminocalix[4]arene derivatives have been described. Consequently,the meta-nitro,bromo and chloro substituted inherently chiral calix[4]arenes could be directly synthesized by the nitration,bromination,and chlorination of the acylating product of aminocalix[4]arene. meta-Cyano and phenyl substituted inherently chiral aminocalix[4]arenes were readily obtained by the nucleophilic substitution reaction or Suzuki coupling reaction from the meta-bromo substituted one under the Pd-catalyzed conditions. For kinetic resolution of the racemic inherently chiral aminocalix[4]arenes,it was found that the electron-withdrawing ability of substituent was helpful to improving the resolution efficiency of the acylation process,and the kinetic resolution could be efficiently applied to the resolution of meta-nitro substituted inherently chiral aminocalix[4]arene,providing (cS)-or (cR)-isomer in up to 95% or 99.9% ee value,respectively,with the corresponding chiral acylating reagent. Moreover,by introduction of the chiral auxiliary,enantiopure antipodes of meta-cyano and phenyl substituted inherently chiral aminocalix[4]arenes could also be readily obtained. These enantiopure amino-calix[4]arenes are potential building blocks for constructing novel chiral receptors and asymmetric catalysts.
XU ZhenXiang ZHANG Chun HUANG ZhiTang CHEN ChuanFeng
