Cycloaddition Reactions of Indanedioneketene with Electron-Rich Dienophiles: An Experimental and a Theoretical Study.

Thermal decomposition of the phenyliodonium ylide of lawsone gives rise to a highly reactive cyclic alpha,alpha'-dioxoketene, indanedioneketene, which reacts with electron rich dienophiles such as enol ethers to afford [4 + 2] cycloadducts. The initially formed 2,3-dihydro-2-alkoxyindeno[1,2-b]pyrano-4,5-diones are labile compounds since through an opening of the pyranone ring upon heating they easily tautomerize to alkoxyallylidene-indenedione derivatives and under acid-catalysis they are additionally transformed to 2-(1,3-dihydroxyallylidene)-1H-indene-1,3(2H)-dione or by loss of alcohol to indeno[1,2-b]pyran-4,5-diones. A DFT study explains the polar nature of the cycloaddition reaction, the observed reactivity and suggests a possible mechanism operating in these reactions.