Boranes in Synthesis. 5. The Hydroboration of Enamines with Mono- and Dialkylboranes. Asymmetric Synthesis of β-Amino Alcohols of Moderate Enantiomeric Purity from Aldehyde Enamines

Article abstract of DOI:10.1021/jo00112a026

The hydroboration of both acyclic and cyclic aldehyde and ketone enamines with such representative mono- and dialkylboranes as thexylborane and dicyclohexylborane, followed by an oxidative workup, yields the corresponding β-amino alcohols in good to excellent isolated yields.The hydroboration of ketone and aldehyde enamines with the asymmetric hydroboration reagents monoisopinocampheylborane (^dIpeBH2) and diisopinocampheylborane (^dIpc2BH) was also investigated. ^dIpc2BH is highly effective for the asymmetric hydroboration of acyclic aldehyde enamines, such as 1-(4-morpholino)-3-phenyl-1-propene and 1-(1-pyrrolidino)-1-octene.Oxidation of the intermediate trialkylborane furnishes the corresponding β-amino alcohols in 50-86percent ee.The stereogenic center of the carbinol carbon is consistently enriched in the R-enantiomer when ^dIpc2BH prepared from (+)-α-pinene is used as the hydroboration reagent.The enantiomeric excesses of the β-amino alcohols synthesized in this study were determined by HPLC using a chiral stationary phase.The absolute configuration of some of the β-amino alcohols synthesized in this study were determined by chiral HPLC comparison with authentic β-amino alcohols prepared from chiral epoxides of known absolute configuration.