Asymmetric <i>Syn</i>-Selective Aldol Reactions of γ-Oxygenated Vinylogous Urethane with a Second Generation Chiral Auxiliary: Application in Construction of (+)-3-Deoxy-<scp>d</scp>-<i>manno</i>-2-octulosonic Acid
作者:Richard H. Schlessinger、Liping H. Pettus
DOI:10.1021/jo981916f
日期:1998.11.1
Various examples of highly diastereoselective aldol reactions are presented where the nonracemic lithium enolate 6 derived from a C4-oxygenated vinylogous urethane reacts in syn fashion to provide upon intramolecular lactonization useful gamma-alkoxy-delta-lactone synthons 12a-f. In one particular example, the result of reaction with an acrolein surrogate, the lactone product 12e is applied in an efficient asymmetric synthesis of(+)-KDO (10 steps, 34% overall yield). Notable transformations include (1) hydrolysis of the vinylogous urethane functionality, (2) stereoselective reduction of the resulting beta-keto-lactone 2, (3) stereoselective dihydroxylation of the vinyl side chain of delta-lactone 17, and (4) addition of alpha-ethoxy-vinyllithium to the lactone carbonyl of 19 to procure the aldulosonic acid residue in 1 upon ozonolysis.