Chan-Umlagerung
The Chan rearrangement in organic chemistry is a rearrangement reaction converting an acyloxy acetate in the presence of a strong base to a 2-hydroxy-3-keto-ester Vorlage:Ref.
The methylene group in the reactant with adjacent carbonyl and acetyl substituents is acidic and can be deprotonated by strong non-nucleophilic bases such as lithium tetramethylpiperidide or lithium diisopropylamide as in an aldol reaction. The thus formed enolate then gives a nucleophilic acyl substitution with the adjacent carbonyl of the acetyl group through a short lived intermediate oxirane. Acidic workup liberates the free hydroxyl group.
This procedure was rediscovered and employed in the Holton Taxol total synthesis.
References
- Vorlage:Note Rearrangement of α-acyloxyacetates into 2-hydroxy-3-ketoesters S. D. Lee, T. H. Chan, and K. S. Kwon Tetrahedron Letters Volume 25, Issue 32 , 1984, Pages 3399-3402 DOI Abstract