Dimethyl 7b-methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylate. A new 10π-electron aromatic system

Abstract

The title compound (8a) is formed by the acid-catalysed elimination of methanol from the [8 + 2]-cycloadduct (7) of 3-methoxy-3a-methyl-3aH-indene and dimethyl acetylenedicarboxylate. Its physical and chemical properties are consistent with a 10π-electron aromatic periphery which sustains a dia-magnetic ring current. It is photochemically and thermally stable, but rearranges slowly in boiling xylene to the isomer (11) by a [1,5]methyl shift. With copper(II) nitrate and acetic anhydride, it gives a mixture of mononitro-substitution products. Preliminary experiments show that decarbonylation of the dialdehyde (8e) with tris(triphenylphosphine)rhodium(I) chloride provides a route to the unsubstituted tricyclic [10]annulene (8f).