Inverse Electron Demand Diels-Alder Reactions of Heterocyclic Azadienes, 1-Aza-1,3-Butadienes, Cyclopropenone Ketals, and Related Systems. A Retrospective.

A summary of the investigation and applications of the inverse electron demand Diels-Alder reaction is provided that have been conducted in our laboratory over a period that now spans more than 35 years. The work, which continues to provide solutions to complex synthetic challenges, is presented in the context of more than 70 natural product total syntheses in which the reactions served as a key strategic step in the approach. The studies include the development and use of the cycloaddition reactions of heterocyclic azadienes (1,2,4,5-tetrazines; 1,2,4-, 1,3,5-, and 1,2,3-triazines; 1,2-diazines; and 1,3,4-oxadiazoles), 1-aza-1,3-butadienes, α-pyrones, and cyclopropenone ketals. Their applications illustrate the power of the methodology, often provided concise and nonobvious total syntheses of the targeted natural products, typically were extended to the synthesis of analogues that contain deep-seated structural changes in more comprehensive studies to explore or optimize their biological properties, and highlight a wealth of opportunities not yet tapped.