Balasubrahmanyam, SN and Bharati, Narasimha S (1981) 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. In: Organic Magnetic Resonance, 16 (3). pp. 224-229.Full text not available from this repository. (Request a copy)
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 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 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.
|Item Type:||Journal Article|
|Additional Information:||The copyright belongs to Heyden & Son Ltd.|
|Department/Centre:||Division of Chemical Sciences > Organic Chemistry|
|Date Deposited:||25 Oct 2005|
|Last Modified:||27 Aug 2008 11:31|
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