Abstract
Non-recognition for dienes in ketene reactions has long been an important problem in organic chemistry since the diphenylketene-cyclopentadiene reaction was found by Staudinger in 1920. Recently, we have discovered that the ketene recognizes conjugated dienes. The ketene is a dienophile not for well-known [2+2] cycloadditions but for [4+2] (Diels-Alder) reactions across its C=O bond. This paper outlines the long history of ketene chemistry and describes the way of how the problem has been tackled. The frontier-orbital theory and ab initio calculations have predicted that ketene should react with cyclopentadiene via the [4+2] cycloaddition and a subsequent Claisen rearrangement. Careful low-temperature experiments and NMR spectroscopy of the reaction have demonstrated the first detection of the [4+2] -type cycloadduct and the conversion to the final product, cyclobutanone, by the rearrangement.
