Enantioselective deprotonation of the monoacetals of bicyclo[3.3.0]octan-3,7-dione. An approach to the asymmetric synthesis of chiral synthons for carbacyclins
作者:Hiroyuki Izawa、Ryuichi Shirai、Hisashi Kawasaki、Hee-doo Kim、Kenji Koga
DOI:10.1016/s0040-4039(01)93940-1
日期:1989.1
Kinetic deprotonation of the monoacetals (4) of bicyclo[3.3.0]octan-3,7-dione by chiral lithium amides (5) in the presence of excess trimethylsilyl chloride afforded the corresponding silyl enol ethers (6), useful synthons for the synthesis of optically active carbacyclins, in up to 94% ee.
Useful chiral lactones derived from cis-bicyclo[3.3.0]octan-3,7-dione via asymmetric deprotonation
作者:John Leonard、Jacqueline D. Hewitt、Dehimi Ouali、Shirley K. Rahman、Stephen J. Simpson、Roger F. Newton
DOI:10.1016/s0957-4166(00)82377-0
日期:1990.1
monoketal derived from cis-bicyclo[3.3.0]octane-3,7-dione was deprotonated using chiral lithium amide bases and the enolates were trapped as either enol acetates or trimethylsilyl enol ethers. Oxidative cleavage of the enol derivatives provided useful bicyclic lactones, the ee's of which could be determined using NMR chiral shift techniques. The lactones are also potentially useful as intermediates
Pseudo-C2 -symmetrical ligands have been prepared efficiently: The attachment of the chiral alkyl group to the heteroatom (P or N) through a nonstereogenic, chirotopic carbon center facilitates their synthesis as the configuration at this carbon atom no longer needs to be controlled. Two such ligands were combined, for example, in the base 1, which is especially useful for asymmetric deprotonation of prochiral ketones [Eq. (a)].
New Homochiral Ligands Bearing Nonstereogenic Chirotopic Centers. Lithiated <i>N</i>,<i>N</i><i>‘</i>-Dialkylureas as Chiral Bases and Sterically Crowded Phosphines for Asymmetric Catalysis
The synthesis of new homochiral pseudo-C-2-symmetrical ligands of type 1 bearing a nonstereogenic chirotopic center is reported. Two such structural units are combined in the ureas 2 and 10, which proved to be useful for enantioselective deprotonation and alkylation of ketones (up to 88% ee). The monophosphine 3 induced a high enantiomeric excess in the Pd-catalyzed allylation of dimethyl malonate (up to 73% ee). Furthermore, we have shown that the achiral sterically crowded alkyl diphosphine 23 efficiently stabilized Pd(0) at high temperatures.