7 alpha-(Dimethylstannyl)-24-nor-5 beta -cholane (12) and 3 beta-(dimethylstannyl)-24-nor-5 beta -cholane (11) have been prepared from cholic and lithocholic acid, respectively, as free-radical reducing agents. Limited enantioselectivity-testing data indicate that these compounds show promise as enantioselective free-radical reducing agents. For example, reaction of ethyl (rac)-2-bromo-2-cyclopentyl-2-phenylacetate with one equivalent of either (11) or (12) in the presence of one equivalent of (S,S)-(-)-N,N'-bis(3,5-di-tert-butylsalycidene)-1,2-cyclohexanediaminomanganese(III) chloride (7) in toluene at -78 degreesC provides ethyl (S)-2-cyclopentyl-2-phenylacetate, obtained with 62 and 90% ee, respectively.
Substituent effects in selenoxide elimination chemistry
作者:Phoebe E. Macdougall、Naomi A. Smith、Carl H. Schiesser
DOI:10.1016/j.tet.2008.01.044
日期:2008.3
prepared and their stabilities toward oxidative elimination assessed. Simple competitive experiments demonstrate that electron-withdrawing substituents stabilize arylselenides toward oxidation, while electron-donating groups accelerate the oxidation process. In addition, ab initio and density functional calculations on model systems reveal that selenoxides are relatively insensitive to the nature of substituents