Effect of R on the cis-trans ratio in the epoxidation of cis-cinnamates

Conclusion:

Electron-withdrawing substituents increase the formation of trans epoxides, because they can stabilize the radical intermediate.

Example: Synthesis of diltiazem (anti-hypertensive agent)




Reactivity of conjugated polyenes



Conclusion:

High regioselectivity due to

1. cis-double bonds are more reactive than trans- double bonds

2. electron-withdrawing groups slow down the epoxidation at the appropriate double bond.