ACYCLIC STEREOSELECTION .48. REVERSAL OF STEREOCHEMISTRY IN THE ALDOL REACTIONS OF A CHIRAL BORON ENOLATE

A novel route to optically active anti aldols of certain aldehydes is presented. The boron enolate derived from oxazolidinone 3 reacts with various aldehydes to give either syn or anti aldols, depending on the amount of dibutylboron trifiate used in the enolization of 3. The amine used in the enolization is also found to have a dramatic effect on the subsequent aldol reaction. High anti selectivity is observed when 3 and 3-(arylthio)propenals are used, whereas moderate selectivity is observed when 3 and aromatic aldehydes are used. An open transition state is suggested to account for the formation of the novel anti aldols. NMR studies provide support for the suggested mechanism. Although findings are at present limited to a few specific aldehydes, the ability to obtain either syn or anti aldols from the same chiral enolate by a choice of reagent and stoichiometry could have significant synthetic applicability. © 1990, American Chemical Society. All rights reserved.