text: Chiral oxazolidinones have been employed most widely in stereoselective aldol reactions.
  Soft enolization with the Lewis acid dibutylboron triflate and the base diisopropylethylamine gives the (Z)-enolate, which undergoes a diastereoselective aldol reaction with an aldehyde substrate.  The transformation is particularly powerful because it establishes two contiguous stereocenters simultaneously.
  A model for the observed stereoselectivity can be found below. The syn-stereo relationship between the methyl group and the new secondary alcohol results from a six-membered ring Zimmerman-Traxler transition state, wherein the enolate oxygen and the aldehyde oxygen both coordinate boron. The aldehyde is oriented such that the hydrogen is placed in a pseudo-axial orientation to minimize 1,3-diaxial interactions. The absolute stereochemistry of the two stereocenters is controlled by the chirality in the auxiliary. In the transition structure, the auxiliary carbonyl is oriented away from the enolate oxygen so as to minimize the net dipole of the molecule; one face of the enolate is blocked by the substituent on the chiral auxiliary.
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