anti dihydroxylation: practice and mechanism
mCPBA first gives an epoxide, then water opens it under acid from the back. The two OH groups therefore end up trans (anti), unlike with OsO₄ (cis).
Example
mCPBA first forms the epoxide. Water opens it under acidic conditions from the back, so the two OH groups end up anti: cycloalkenes give the trans diol (OsO₄ gives the cis diol instead).
Mechanism step by step
- 1
mCPBA transfers an O atom to the double bond (concerted): an epoxide forms.
- 2
Aqueous acid protonates the epoxide oxygen.
- 3
Water attacks from the back and opens the ring. After deprotonation the two OH groups are trans.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: syn dihydroxylation · Epoxidation
Frequently asked questions
What is the difference from OsO₄?
OsO₄ adds both O atoms at once from one side (cyclic osmate ester): cis diol. Via the epoxide the second O comes from the opposite side: trans diol.
How can you see the difference in the product?
Only in the stereochemistry, e.g. with cyclohexene: trans-1,2-cyclohexanediol (anti) versus cis-1,2-cyclohexanediol (syn).