The title compound 3 was prepared by pyrolysis of the corresponding bis-sulfone 2 and subjected to X-ray structural analysis; space group P 1̅, a = 477.0(1), b = 625.6(2), c = 1363.6(3) pm, α = 82.79(2), β = 83.43(2), γ = 78.18(3)°, Ζ = 1, R = 0.058 for 900 reflections. The molecule contains a crystallographic symmetry centre, thus confirming the anti configuration.