Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications.

ABSTRACT: 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 ?=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.

SUBMITTER: Poplata S 

PROVIDER: S-EPMC6618137 | biostudies-literature | 2019 Jun

REPOSITORIES: biostudies-literature