Current diversity of cyclic anhydrides for the Castagnoli–Cushman-type formal cycloaddition reactions: prospects and challenges

The diversity of dicarboxylic acid anhydrides employable in the Castagnoli–Cushman reaction (CCR) was traditionally thought to be limited to the classical succinic and glutaric anhydrides—as well as to its remarkably more reactive counterpart, homophthalic anhydride. The diversity of the lactam products resulting from this extremely powerful, three-component reaction could have been perceived as somewhat limited and the reaction itself may have often been associated with forcing temperatures (thereby limiting, in public mind, the functional group tolerance that this reaction is capable of displaying). In this Digest we aim to provide a glimpse of the remarkable advancements the diversity of the anhydride component for the CCR has taken, summarize the consequences these advancements may have for the scope and applicability of the reaction and pencil in some directions this exciting area of organic chemistry could be taking in the near future, particularly based on the current efforts in our group.