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

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 thought to be engendered by the central ester being stabilized in a rotamerization in which the O:C·O plane is held 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 1 H 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 of 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 sp 3 sp 2 bond involving an ordinary ester function has been detected employing a prochiral sensor group.