Stereoselective 1,4-hydroboration of acyclic (E)-α,β-unsaturated ketones with dialkylboranes. Synthesis of (Z)-vinyloxyboranes.
作者:Gian Paolo Boldrini、Michele Bortolotti、Emilio Tagliavini、Claudio Trombini、Achille Umani-Ronchi
DOI:10.1016/s0040-4039(00)92052-5
日期:1991.2
Dialkylboranes add to acyclic disubstituted (E)-α,β-unsaturatedketones in a 1,4-fashion affording (Z)-vinyloxyboranes as major or sole products. The stereochemical outcome of the hydroboration reaction is consistent with a concerted pericyclic mechanism. The intermediate vinyloxyboranes can be trapped with aldehydes to give virtually pure syn β-hydroxyketones.
A new protocol for regio- and stereocontrolled aldol reactions through the conjugate addition of dialkylboranes to .alpha.,.beta.-unsaturated ketones
作者:Gian Paolo Boldrini、Michele Bortolotti、Fabrizio Mancini、Emilio Tagliavini、Claudio Trombini、Achille Umani-Ronchi
DOI:10.1021/jo00020a025
日期:1991.9
A one-pot, two-step procedure, consisting of the 1,4-addition of dialkylboranes to beta-substituted (E)-alpha,beta-unsaturated ketones followed by the reaction of the resulting configurationally pure (Z)-(vinyloxy)boranes with aldehydes, is reported. The overall process corresponds to a regio- and stereocontrolled aldol addition of an unsymmetrical ketone to an aldehyde. A concerted 1,4-addition mechanism accounts for the stereochemical outcome of the hydroboraton reaction; cyclic enones do not undergo conjugate addition, while (Z)-beta-substituted or beta,beta-disubstituted alpha,beta-unsaturated ketones still react in a 1,4-fashion, but with a slower rate and a lower degree of chemoselectivity with respect to beta-substituted (E)-alpha,beta-unsaturated ketones. In the cases of alpha,beta-disubstituted alpha,beta-unsaturated ketones and (E)-(S-phenylthio)cinnammate, which react with dicyclohexylborane to give a mixture of E and Z enolates, an alternative mechanism is proposed.
A new approach to (Z)-vinyloxyboranes via 1,4 hydroboration of (E)-α,β-unsaturated ketones. Synthesis of syn aldols
作者:Gian Paolo Boldrini、Fabrizio Mancini、Emilio Tagliavini、Claudio Trombini、Achille Umani-Ronchi
DOI:10.1039/c39900001680
日期:——
(Z)-Vinyloxy boranes are obtained via 1,4-hydroboration of acyclic disubstituted (E)-α,β-unsaturatedketones with dicyclohexylborane or diisopinocampheylborane in tetrahydrofuran, CH2Cl2 or CHCl3 at 20 °C; treatment of the hydroboration mixture with an aldehyde allows pure synaldols to be synthesized in good yields, and in excellent enantiomeric excesses, using the latter borane.