Stereoselective synthesis of trans-fused tetrahydrofuran derivatives of 5H-dibenzo[a,d]cycloheptene

The epoxides derived from 5H-dibenzo[a,d]cycloheptene and its 2-fluoro derivative were converted to trans-fused hydrofurans 4a,b (55 and 44% overall yields) via a five-step sequence, i.e. (i) epoxide ring opening using propargylmagnesium bromide, (ii) mercury(II)-induced cyclisation and in situ bromination to give the bromomethylene substituted hydrofurans, (iii, iv) acid catalysed hydration and stereoselective reduction of the hemiacetal intermediates, and (v) base promoted cyclisation.