The Westphalen-Lettré rearrangement is a classic organic reaction in organic chemistry describing a rearrangement reaction of cholestane-3β,5α,6β-triol diacetate with acetic anhydride and sulfuric acid. In this reaction one equivalent of water is lost, a double bond is formed at C10-C11 and importantly the methyl group at the C10 position migrates to the C5 position. 
The reaction is first order in steroid with access of sulfuric acid and the first reaction step in the reaction mechanism is likely the formation of an sulfate ester followed by that of a carbocation at C5 after which the actual rearrangement takes place.
- T. Westphalen, Ber., 48, 1064 (1915)
- H. Lettré and I. Muller, Ber., 70, 1947 (1937)
- Rearranged Steroid Systems. I. Studies in the Pregnane Series O. R. RODIG, P. BROWN, and P. ZAFFARONI J. Org. Chem. 1961, 26(7), 2431-2435. (doi: 10.1021/jo01351a066)
- Acid catalysed reactions of 5α-hydroxy-steroids—III : The westphalen rearrangementTetrahedron, Volume 21, Issue 6, 1965, Pages 1567-1580 J. W. Blunt, A. Fischer, M. P. Hartshorn, F. W. Jones, Kirk D. N. and S. W. Yoong (doi:10.1016/S0040-4020(01)98321-8)