Westphalen-Lettré rearrangement

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. [Theodor Westphalen, Ber., 48, 1064 (1915) DOI|10.1002/cber.191504801149] [H. Lettré and I. Muller, Ber., 70, 1947 (1937) DOI|10.1002/cber.19370700918 ] ["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)]

The reaction is first order in steroid with access of sulfuric acid ["Acid catalysed reactions of 5α-hydroxy-steroids—III : The westphalen rearrangement"Tetrahedron, 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)] 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.

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