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The four-membered ring compound cyclobutenone has been used for the first time as the electron-poor dienophile component of a Diels-Alder reaction (J. Am. Chem. Soc., DOI: 10.1021/ja1056888). This development paves the way for making bicyclic cyclopentanones, lactones, and lactams, which abound in natural products and drugs but have been tough to access through Diels-Alder chemistry. Xiaohua Li and Samuel J. Danishefsky of Columbia University and Sloan-Kettering Institute for Cancer Research surmised that cyclobutenone’s ring strain might make it a more reactive dienophile than cyclopentenone or cyclohexenone, which both need activating groups to participate in the Diels-Alder reaction. Although Diels-Alder reactions of substituted cyclobutenones have been reported, no one had reported the reaction with cyclobutenone itself. By developing a mild route to cyclobutenone and finding a way to store it stably, Li and Danishefsky were able to explore its reactivity. As they suspected, it’s a highly reactive dienophile. In addition, ring expansions of their Diels-Alder products led to diverse bicyclic derivatives. “This is a valuable new reagent and transformation that I am sure others will now certainly begin to use,” says Boston College organic chemist Marc L. Snapper.