The WestphalenâLettrĂ© rearrangement is a classic organic reaction in organic chemistry describing a rearrangement reaction of cholestane-3ÎČ,5α,6ÎČ-triol diacetate with acetic anhydride and sulfuric acid. In this reaction one equivalent of water is lost, a double bond is formed at C10âC11 and importantly the methyl group at the C10 position migrates to the C5 position. [1] [2] [3]
The reaction is first-order in steroid in the presence of an excess of sulfuric acid [4] and the first reaction step in the reaction mechanism is likely the formation of a sulfate ester followed by that of a carbocation at C5 after which the actual re-arrangement takes place.[ citation needed]