How can you determine E and Z isomers in a molecule?

1 Answer
Mar 23, 2018

Priority is assigned to substituents about the double bond (remember absolute configuration?). When both of the highest priority groups are on the same side, it is #Z#, when they are on opposite sides, it is #E#.

For instance, consider pladienolide B, #""^1#

I highlighted the bond in consideration so you wouldn't be playing Where's Waldo?!

The ring system to the right would be considered highest priority per the normal rules, and the #"sp"^2# carbon to the left it is connected to would be considered highest priority.

They are on different sides. Hence, this is an #E# geometric isomer of pladienolide B.

For fun, it has approximately 256 stereoisomers it could be, and here is its enantiomer,

where all of its chirality centers (not the one on the #"sp"^2# carbon because it has three connections, not four!) are inverted.

#""^1# Org. Lett. 2012, 14, 4730-4733