119788-68-0Relevant articles and documents
Trimethylsilyl Cyanide - A Reagent for Umpolung, XVI. - Effect of Umpolung Moieties on the Diastereoselectivity of the Nucleophilic Acylation of α-Chiral Carbonyl Compounds
Huenig, Siegfried,Marschner, Claus
, p. 1329 - 1340 (2007/10/02)
Umpolung of benzaldehyde leads to derivatives 1a-h, which add to the aldehydes 2a and 2c as well as to the ketones 2b and 2d in high yields, if side reactions are excluded by proper choice of conditions.Determined with the acyloins 4 or their silyl ethers 5, the diastereoselectivity of this reaction depends only slightly on the groups used for umpolung (1e > 1c > 1d > 1a ca. 1b ca. 1g > 1h > 1f).Strong effects are observed for the substituents at the center of chirality of the electrophiles 2.Whereas a syn/anti selectivity of ca. 95:5 in products 4 and 5 is obtained by an α-phenyl group in 2, only a 52:48-71:29 ratio is produced by an α-ethyl group.In case of 4d the syn/anti ratio is raised from 63:37 to 80:20 by optimizing the conditions.The observed Cram selectivity is explained by the model of Anh. - Key Words: Umpolung/ Diastereoselectivity/ Acyloins/ Substituent effects/ Nucleophilic acylation