Figure 9.

E- and Z-isomers of AviCys derivatives and enamides prepared by Taylor et al.¹¹ Left: Due to an electrostatic interaction between the sulfur and both the nitrogen and oxygen atoms, the (Z)-AviCys is thermodynamically more stable than the corresponding (E)-AviCys.¹¹ Right: Synthesis of enamides (i.e., substrates lacking a sulfur atom) using the same methods as used for AviCys formation favor the E-isomer.¹¹ Without an electrostatic interaction stabilizing the Z-configuration, the (E)-enamide predominates as it is less sterically hindered than the (Z)-enamide.