BORANES IN SYNTHESIS .5. THE HYDROBORATION OF ENAMINES WITH MONOALKYLBORANES AND DIALKYLBORANES - ASYMMETRIC-SYNTHESIS OF BETA-AMINO ALCOHOLS OF MODERATE ENANTIOMERIC PURITY FROM ALDEHYDE ENAMINES

The hydroboration of both acyclic and cyclic aldehyde and ketone enami nes with such representative mono- and dialkylboranes as thexylborane and dicyclohexylborane, followed by an oxidative workup, yields the co rresponding beta-amino alcohols in good to excellent isolated yields. The hydroboration of ketone and aldehyde enamines with the asymmetric hydroboration reagents monoisopinocampheylborane ((d)IpcBH(2)) and dii sopinocampheylborane ((d)Ipc(2)BH) was also investigated, (d)Ipc(2)BH is highly effective for the asymmetric hydroboration of acyclic aldehy de enamines, such as 1-(4-morpholino)-3-phenyl-1-propene and 1-(1-pyrr olidino)-1-octene. Oxidation of the intermediate trialkylborane furnis hes the corresponding beta-amino alcohols in 50-86% ee. The stereogeni c center of the carbinol carbon is consistently enriched in the R-enan tiomer when (d)Ipc(2)BH prepared from (+)-alpha-pinene is used as the hydroboration reagent. The enantiomeric excesses of the beta-amino alc ohols synthesized in this study were determined by HPLC using a chiral stationary phase. The absolute configurations of some of the beta-ami no alcohols synthesized in this study were determined by chiral HPLC c omparison with authentic beta-amino alcohols prepared from chiral epox ides of known absolute configuration.