With the exception of palladium-catalyzed cross-couplings, no other group of reactions has had such a profound impact on the formation of carbon-carbon bonds and the art of total synthesis in the last quarter of a century than the metathesis reactions of olefins, enynes, and alkynes. Herein, we highlight a number of selected examples of total syntheses in which such processes played a crucial role and which imparted to these endeavors certain elements of novelty, elegance, and efficiency. Judging from their short but impressive history, the influence of these reactions in chemical synthesis is destined to increase.

Metathesis Reactions in Total Synthesis

Catalytic olefin metathesis--through which pairs of C = C bonds are reorganized--transforms simple molecules to those that are complex and precious. This class of reactions has noticeably enriched chemical synthesis, which is the art of preparing scarce molecules with highly desirable properties (for example, medicinal agents or polymeric materials). Research in the past two decades has yielded structurally well-defined catalysts for olefin metathesis that are used to synthesize an array of molecules with unprecedented efficiency. Nonetheless, the full potential of olefin metathesis will be realized only when additional catalysts are discovered that are truly practical and afford exceptional selectivity for a significantly broader range of reactions.

The remarkable metal-catalysed olefin metathesis reaction

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

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Euphorbia Diterpenes: Isolation, Structure, Biological Activity, and Synthesis (2008–2012)

Mit Ausnahme der palladiumkatalysierten Kreuzkupplungsreaktionen hatte im vergangenen Vierteljahrhundert keine Reaktionsklasse einen derart weitreichenden Einfluss auf die Kohlenstoff-Kohlenstoff-Verknupfung und die Totalsynthese wie die Metathesereaktionen von Olefinen, Eninen, und Alkinen. In diesem Aufsatz prasentieren wir Glanzlichter aus einer Reihe ausgewahlter Totalsynthesen, in denen diese eleganten und effizienten Reaktionen entscheidend zum Erfolg beigetragen haben. Schon an ihrer kurzen, aber beeindruckenden Geschichte kann man ablesen, dass die Bedeutung dieser Reaktionen fur die chemische Synthese weiter zunehmen wird.

Metathesereaktionen in der Totalsynthese

A total synthesis of the biologically important diterpene ingenol has been completed. Ring-closing olefin metathesis was used to construct the strained "inside-outside" tetracyclic skeleton, and a series of diastereoselective reactions were employed to complete the synthesis. Another naturally occurring ingenane, 20-deoxyingenol, has also been prepared.

Toru Maruyama

Papers

Total Synthesis of Ingenol