How can rotamers of alpha-keto esters be named to distinguish between different geometries? 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 &alpha;-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).
The IUPAC recommends s-cis and s-trans for the rotamers of conjugated dienes. 



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). 



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.