Diels-Alder has been a beautiful and powerful cycloadition reaction for many masters such as Woodward. It is an atom-efficient, 4+2, environment-insensitive ring-formation reaction and fits click chemistry concept in many aspects .
Danishefsky's work expanded its scope by using cyclobutenone as a highly reactive dienophile. The strained cyclobutanone in the Diels-Alder product undergoes regioselective ring-expansion to give cyclopentanone, lactam, lactone. As an example indicated in the graph, the approach provides a variety of structures not accesible via direct Diels-Alder reaction.
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