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 lambda = 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 EtAICl(2), it was attempted to perform the reactions enantioselectively in the presence of a chiral Lewis acid. A complex of Cu(ClO4)(2)center dot 6H(2)O 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 = 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 lambda = 419 nm. (C) 2017 The Authors. Published by Elsevier Ltd.