Enantioselective photocyclisation reactions of 2-aryloxycyclohex-2-enones mediated by a chiral copper-bisoxazoline complex

The photocyclisation of the title compounds leads upon direct irradiation at λ = 366 nm in dichloromethane solution to racemic cis-2,3,4a,9b-tetrahydro-1H-dibenzofuran-4-ones (nine examples, 37-74% yield). Since it was found that the substrates show a significant bathochromic absorption shift upon treatment with EtAlCl2, it was attempted to perform the reactions enantioselectively in the presence of a chiral Lewis acid. A complex of Cu(ClO4)2·6H2O and a bisoxazoline ligand gave the best enantioselectivities (up to 60% ee). Two procedures are reported for the enantioselective photocyclisation. The first protocol is based on a direct irradiation at λ = 368 nm (LED) with a catalyst loading of 50 mol% and it delivered the products in 26-76% yield with 22-40% ee. The second protocol is applicable to electron rich 2-aryloxycyclohex-2-enones (31-62% yield, 29-46% ee) and relies on sensitization by thioxanthone (50 mol%) at λ = 419 nm.

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