Concerning the Application of the ~1H NMR ABX Analysis for Assignment of Stereochemistry to Aldols Deriving from Aldehydes Lacking beta-Branches

摘要

Attempts to apply the ~1H NMR ABX method for assignment of stereochemistry of beta-hydroxy ketones to aldols 4-10 deriving from alpha-methyl aldehydes lacking beta-branches reveals that the presence of a beta-branch in the aldehyde reaction partner is necessary so that the average chemical environment of Ha and Hb is different for the Felkin and anti-Felkin aldols(see conformational pairs A/B and C/D,respectively).When the chiral alpha-methyl aldehyde lacks a beta-branch,as in the case of the aldehyde precursors to 4-10,the conformational energies of E and F(for the Felkin beta-hydroxy ketone derivatives),and conformers G and H for the anti-Felkin aldols,are too close in energy(within each pair),such that the average chemical and magnetic environments of Ha and Hb in the two diastereomers cannot be easily distinguished.This analysis provides a rational basis for application of the ~1H NMR ABX pattern analysis to other beta-hydroxy ketone derivatives.