Oxidative cyclisation of 3,4-dibenzyltetrahydrofurans using ruthenium tetra(trifluoroacetate)

A series of trans-3,4-dibenzyltetrahydrofurans has been synthesised and subjected to oxidative cyclisation using ruthenium tetra(trifluoroacetate), affording dibenzocyclooctadiene lignans belonging to the isostegane series, in high yields. Since no evidence was found for the formation of the corresponding stegane isomers it is assumed that the reactions proceed with complete diastereoselectivity.