Gold(I)-catalysed [2+4] cycloaddition of 1,1-difluoroallenes with conjugated enones: synthesis of ring-difluorinated dihydro-2H-pyrans.

1,1-Difluoroallenes underwent regioselective [2+4] cycloaddition with α,β-unsaturated ketones (enones) in the presence of an AuCl(IPr)-AgSbF6 catalyst. β-Aurated, charge-localized difluoroallylic carbocations, formed from 1,1-difluoroallenes and the cationic Au catalyst, were stabilised by the two fluorine substituents. This facilitated α-selective nucleophilic attack by the enone oxygen, followed by six-membered ring closure, leading to the formation of ring-difluorinated dihydro-2H-pyrans with (E)-alkylidene groups in high yields.