3aH-indenes. Part 3. Base induced dimerisation of 3,7a-dihydro-4,7a-dimethyl-5H-inden-5-one. X-Ray structure determination of the hexacyclic dimer system

Abstract

When the enolate anion (3) of the bicyclic ketone (8) is treated with methyl fluorosulphonate and dienophiles, the normal sequence of O-methylation and cycloaddition is not observed; instead, addition of the anion (3) to the trienone (8) occurs to give, in the presence of 4-phenyltriazole-3,5-dione, the hexacyclic adducts (10) and (11). Structure (10) was determined by X-ray diffraction (Figure 1). When water is used to quench the enolate anion solution, the dimer (16) is formed. Thus three new carbon–carbon bonds are formed regio- and stereo-selectively, for which a sequence of three stereoelectronically favoured Michael additions is proposed (Scheme 2). The new hexacyclic system is highly concave with a self-filling cavity (Figure 2) having near-parallel sides with a separation of less than 3.4 Å. The initial red solution of the anion (3) rapidly changes to a persistent blue colour; it is suggested that this could be caused by intramolecular charge transfer across the molecular cavity, from the enolate anion donor to the dienone acceptor, in the hexacyclic anion (15).