Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 25, Issue 34, Pages 8135-8148Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201901304
Keywords
cycloaddition; enantioselectivity; Lewis acids; photochemistry; total synthesis
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Funding
- European Research Council under the European Union [665951-ELICOS]
- Fonds der Chemischen Industrie (Liebig fellowship)
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The intramolecular [2+2] photocycloaddition of 3-alkenyl-2-cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54-86 % yield, 76-96 % ee) upon irradiation at lambda=366 nm in the presence of an AlBr3-activated oxazaborolidine as the Lewis acid. An extensive screening of proline-derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3-position of the heterocycle. DFT calculations of the Lewis acid-substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (+/-)-italicene.
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