The aliphatic ketone Claisen rearrangement is believed to involve three distinct steps: 1) transacetalization to give the mixed methoxypropyl ether (MOP ether) 12 , 2) elimination of methanol to give the enol ether 3) and the [3,3]-sigmatropic rearrangement (Scheme 2). The first transacetalization reaction quickly establishes equilibrium following the elimination reaction. The first two steps are known to be acid catalyzed, but it is uncertain whether the acid catalyst participates in the last rearrangement step. Hydrogen bonding urea and thiourea-based catalysts are known to accelerate the sigmatropic rearrangement step, but the range of substrates amenable to such catalysts is generally limited. Scheme 2. Steps of the Aliphatic Ketone Claisen Rearrangement.