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Concerning the application of the 1H NMR ABX analysis for assignment of stereochemistry to aldols deriving from aldehydes lacking beta-branches.

Dias LC, Aguilar AM, Salles AG, Steil LJ, Roush WR

Instituto de Química, Universidade Estadual de Campinas, UNICAMP, C. P. 6154, CEP 13084-971, Campinas, SP, Brazil. ldias@iqm.unicamp.br

[reactions: see text] 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.

Published 5 December 2005 in J Org Chem, 70(25): 10461-5.
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