Hypervalent-Iodine-Mediated Synthesis of 1,2-Dispirodienones: Experimental and Theoretical Investigations

The oxidation of phenol acetanilides mediated by hypervalent iodine is described. The transformation proceeds through the formation of a carbocationic species that evolves either into a [6,5,6]-tricyclic compound (1,2-dispirodienone) or a [6,6,6]-tricyclic compound (aryl spirodienone). In most examples, the 1,2-dispirodienones are obtained in good yields. In some cases, depending on the substitution pattern of the phenol acetanilides and the reaction conditions, aryl spirodienones are formed. We performed electrochemical experiments on a 1,2-dispirodienone to study its redox proper-ties. These data provide evidence that the 1,2-dispirodienone can ring-open and ring-close under redox conditions. The hypervalent-iodine-mediated oxidation of phenol acetanilides to 1,2-dispirodienones was rationalized on the basis of density functional theory (DFT) calculations. In addition, these DFT computations suggest that the formation of the aryl spirodienones occurs by a stepwise reaction pathway, which first involves an ipso attack to yield a [6,5,6]-tricyclic carbocationic intermediate and then a 1,2-sigmatropic shift of the bond that connects the two six-membered rings.