Anisochrony of the O-Methylene Protons of the C-2-Ethyl Ester Function in Triethyl meso, cis-1,3-Dimethylcyclohexane-1,2,3-tricarboxylate-Atropisomerism of an Ester Group

Article abstract of DOI:10.1002/mrc.1270160312

The methylene protons of the ethyl ester function at C-2 in triethyl meso,cis-1,3-dimethylcyclohexane-1,2,3-tricarboxylate (A) are anisochronous, despite the apparent symmetry of structure A.The necessary diastereotopy is thoght to be engendered by the central ester being stabilized in a rotamerization in which the O:C:O plane is parallel to the general plane of the 6-membered ring while fast exchanges take place among rotamers about the alkyl-oxygen bond; other rotamers about the ester axis are thought to encounter high order repulsive steric/polar interactions with the flanking ester functions and C-methyl groups.Evidence for the axial orientation of the C-1 and C-2 ester functions in A, based on comparisons of its 1H NMR spectral characteristics with those of its RS-cis,trans and meso,trans analogues, is presented.The role of ion-pairing in the stereoselectivity of methylation of precursor enolates, leading to the formation these three systems, is briefly discussed.A comment on the appropriateness, or otherwise, of drawing conformational conclusions from the magnitudes of the anisochrony in comparable systems is included.Triester A is thought to be the first instance where atropisomerism about an sp³-sp² bond involving an ordinary ester function has been detected employing a prochiral sensor group.