[darkdevil]

Dear all,

I have a very fundamental question about geometrical isomers. There are Z and E or Cis and Trans isomers. In the following case, compound A,B,C and D gives the same name in ChemDraw and are considered as identical compound. They adopt a trans or E geometry. However, if we consider the the pi-conjugation throughout the compound as a "locked" resonance structure (compound being planar). The four compounds would not be identical and the trans isomer would now split into 4 different trans-isomers. Is this possible? and Does this kind of isomer exist? and what are they called?

[Dan]

Is this possible? and Does this kind of isomer exist? and what are they called?

This isomerism has a name - s-cis and s-trans, see:

http://goldbook.iupac.org/S05508.html

I guess the effect might be observable sometimes, e.g. as it is with amide/carbamate rotamers in NMR, but I'm not aware of any specific examples.