Sunlight is the ultimate driver of biosynthesis but photochemical steps late in biosynthetic pathways are very rare. They appear to play a role in the formation of certain furanocembranoids isolated from Caribbean corals. One of these compounds, intricarene, has been suspected to arise from an intramolecular 1,3-dipolar cycloaddition involving an oxidopyrylium. Here we show, by a combination of experiments and theory, that the oxidopyrylium forms under photochemical conditions and that its cycloaddition occurs via a triplet state. The formation of a complex by-product can be rationalized by another photochemical step that involves a conical intersection. Our work raises the question whether intricarene is biosynthesized in the natural habitat of the corals or is an artefact formed during workup. It also demonstrates that the determination of exact irradiation spectra, in combination with quantum chemical calculations, enables the rationalization of complex reaction pathways that involve multiple excited states.
Mapping the matrix: Several coralloidolides, members of a Mediterranean branch of the furanocembranoid family of diterpenes, have been synthesized. The total syntheses include biomimetic transformations that often occur with high chemoselectivity, thus obviating the need for protecting‐group manipulations. The fascinating reactivity of 2,5‐diene‐1,4‐dione moieties was explored in detail.
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