Alkene Isomerization Revitalizes the Coates–Claisen Rearrangement

The [3,3]‐sigmatropic rearrangement of allylic vinyl acetals, first investigated by Coates nearly four decades ago, is set apart from other variants of the Claisen rearrangement owing to the versatile monoprotected 1,5‐dicarbonyl motif featured in the products. Unfortunately, the synthetically elusive nature of the substrates has thus far precluded the widespread application of this attractive transformation. Herein, we show that the key allylic vinyl acetals can be efficiently generated through alkene isomerization of their readily available regioisomeric counterparts (derived from allylic alcohols and α,β‐unsaturated aldehydes), thus enabling the first systematic study of the substrate scope of this rearrangement, as well as the discovery of exceptionally mild conditions for its mediation by Lewis and Brønsted acids.