8.7a Anti Dihydroxylation

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ANTI DIHYDROXYLATION
Less Substituted More Substituted
Two Groups Added OH OH
Reagents Added 1. RCO3H (usually MCPBA --a peroxyacid)
2. H3O+
Regioselectivity N/A
Stereospecificity Anti
Other Characteristics Intermediate is a 3-membered ring (an epoxide).

Anti Dihydroxylation

Anti dihydroxylation results in the anti-addition of two hydroxyl groups across an alkene.  If this occurs on a cyclic diene as in cyclohexene below then anti-addition will result in a trans relationship between the two hydroxyl groups.

anti dihyroxylation

Anti Dihydroxylation Mechanism

The first step in anti dihydroxylation is epoxidation of the alkene using a peroxyacid (a.k.a. peracid--MCPBA is the most common peroxyacid and is often specifically used).  The second step is acid-catalyzed ring opening of the epoxide intermediate with water as the nucleophile.  The epoxide is first protonated followed by back-side attack by water.  It is this back-side attack that is the ultimate reason that this reaction results in anti addition.

Step 1 - Epoxidation

anti dihyroxylation mechanism step 1

Step 2 - Acid-Catalyzed Ring Opening of an Epoxide

anti dihydroxylation step 2