Conversion of Conjugated Enones into Enantiomerically Pure β-Hydroxy Ketones or 1,3-Diols - Samarium(II) Bromide Reductions of Protected α,β-Dihydroxy Ketones

Asymmetric dihydroxylations of alpha,beta-unsaturated ketones in the presence of Sharpless' AD mix-beta (TM) delivered alpha,beta-dihydroxy ketones or, if phenylboronic acid was present, the corresponding phenylboronates. The C-alpha-O bonds of these species were removed at -78 degrees C -in the former case after acetonide formation, in the latter case directly - in an unprecedented manner, namely by treatment with a suspension of Sm-II bromide in THF/MeOH. The resulting monohydroxy ketones could be reduced if so desired to give syn- or anti-configured 1,3-diols. The same diols were produced in one-pot reductions of the alpha,beta-dihydroxy ketone diacetonides with Sm-II bromide at 0 degrees C. When the alpha,beta-dihydroxy ketone phenylboronates were treated likewise, the phenylboronates of the 1,3-diols were obtained. Diastereocontrol in the one-pot reductions varied from perfect to nearly absent.