Metal-Free Ringing

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Science  23 Feb 2007:
Vol. 315, Issue 5815, pp. 1055
DOI: 10.1126/science.315.5815.1055a

Chiral Brønsted acids have recently proven effective alternatives to metallic Lewis acids in a range of enantioselective catalysis applications. Rueping et al. extend this approach to electrocyclic ring closures. Specifically, they explored the capacity of binaphthyl phosphate derivatives to catalyze the Nazarov cyclization, a reaction in which two alkenyl groups flanking a carbonyl moiety connect at the β carbons to form a cyclopentenone ring. The resulting product bears two new chiral centers, which the optimized catalyst (at 2 mol % loading) produced with enantiomeric excesses up to 93% for the major diastereomer. The cis diastereomer was generally favored (with selectivities ranging from 1.5 to 9.3), though the products could be epimerized selectively at the α carbon to the corresponding transisomers by treatment with basic alumina. The selectivity is sensitive to solvent, because it relies on the nature of the ion pair formed after proton transfer from the chiral anion to the substrate, and was found to be highest in chloroform. — JSY

Angew. Chem. Int. Ed. 46, 10.1002/anie.200604809 (2007).

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