Control of Relative Stereochemistry in the Cycloadditive Route to β-Hydroxy Carbonyls. Stereoselective Exo Aldol Reactions of Δ2-Isoxazolines

Article abstract of DOI:10.1016/S0040-4020(01)89051-7

Aldol reactions with lithium exo-azaenolates derived from fused isoxazoline 10 and a variety of aldehydes have been investigated in the presence and absence of ZrCp2Cl2.The lithium azaenolate exhibits good (aldehydes) to excellent (ketones) face selectivity, but the syn/anti selectivity is poor.The zirconium enolate exhibits both high face selectivity, which is reserved from the lithium azaenolates, and high syn selectivity.Deoxygenation of the Zr-promoted aldol products complements direct azaenolate alkylation because it provides the opposite diastereomer.Methylation of dianions derived from 12 and 13 also provides access to the aldol products derived from 10, and the stereoselectivities of these alkylations are either reinforced or eroded by the hydroxy-bearing stereogenic center.