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When talking about free rotation it is often helpful to determine about which geometric arrangement one is referring to. For example in the case of butane we talk about several eclipsed or staggered conformations when discussing the rotational barriers. This also often helps to explain the all trans (staggered) arrangement of fatty acids and surfactants.

In a wider sense even the pseudo rotation of cyclohexane has its own terminology, with boat, chair and twisted conformations.

I am interested in a way to name the two rotamers that can be thought of in α-keto esters (1, 2). My initial thought was calling these cis- and trans- configuration, in analogy of the partial double bond (blue, 3, 4), but for my taste this has a too rigid connotation. Has there been anything proposed in this area? Amides would also be comparable (5, 6).

rotameric structures of alpha keto esters

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  • $\begingroup$ With respect to the corresponding amides, my coauthors and I used the anti/syn designations in a paper, although we made sure to include a figure to make clear which conformations were anti and syn. pubs.acs.org/doi/abs/10.1021/jo025554u $\endgroup$ – jerepierre Jun 30 '15 at 17:22
  • $\begingroup$ @jerepierre Thanks for the practical example, nice little paper. $\endgroup$ – Martin - マーチン Jul 1 '15 at 2:40
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The IUPAC recommends s-cis and s-trans for the rotamers of conjugated dienes.

enter image description here

The nomenclature derives from having a "cis-like" or "trans-like" geometry about a sigma bond.

I feel like something similar should be appropriate, however that same IUPAC link recommends E/Z or sp/ap (syn-periplanar/anti-periplanar) for the N-alkyl amides (5,6).

enter image description here

Amide 5 would be Z (but ap w/r/t alkyl groups) and amide 6 would be E (and sp). E/Z is less ambiguous in my opinion.

Ketoester 1 is E, and ketoester 2 is Z. I suppose that ketoester 1 is also ap and ketoester 2 is also sp.

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  • $\begingroup$ +1 for E/Z. All of the other variants really require defining "what's trans to what" or "what's anti-periplanar to what". It's a little weird to think of the "double bond character" to a dicarbonyl, but if we're really just thinking about the planar conformations, then E/Z are perfect designations. $\endgroup$ – jerepierre Jun 30 '15 at 17:20
  • $\begingroup$ Thank you very much, this is exactly what I was looking for and what a fast reply. I will use that and make sure to link it back once we publish something with it. $\endgroup$ – Martin - マーチン Jul 1 '15 at 2:41

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