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Interrupted carbonyl-olefin metathesis via oxygen atom transfer

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Science  28 Sep 2018:
Vol. 361, Issue 6409, pp. 1363-1369
DOI: 10.1126/science.aar8238

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Two ways out of an oxetane

Oxetanes are highly reactive four-membered rings that contain three carbon atoms and an oxygen atom. Recently, they were implicated as transient intermediates in Lewis acid–catalyzed intramolecular metathesis reactions of ketones with olefins. Ludwig et al. now report that by replacing the Lewis acid with a strong Brønsted acid, they can change the course of the oxetane ring-opening. In a so-called interrupted metathesis, the oxygen atom migrates and then departs through dehydration, while the remaining carbon framework cyclizes to form tetrahydrofluorene compounds.

Science, this issue p. 1363

Abstract

Some of the simplest and most powerful carbon-carbon bond forming strategies take advantage of readily accessible ubiquitous motifs: carbonyls and olefins. Here we report a fundamentally distinct mode of reactivity between carbonyls and olefins that differs from established acid-catalyzed carbonyl-ene, Prins, and carbonyl-olefin metathesis reaction paths. A range of epsilon, zeta-unsaturated ketones undergo Brønsted acid–catalyzed intramolecular cyclization to provide tetrahydrofluorene products via the formation of two new carbon-carbon bonds. Theoretical calculations and accompanying mechanistic studies suggest that this carbocyclization reaction proceeds through the intermediacy of a transient oxetane formed by oxygen atom transfer. The complex polycyclic frameworks in this product class appear as common substructures in organic materials, bioactive natural products, and recently developed pharmaceuticals.

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