抄録
The preparation of the title compound (1), a key intermediate for the synthesis of helenanolides (pseudoguaianolides with a C10 α-methyl group), is described starting from the readily available enone (3). The correct stereochemistry of the C10-metyl group was obtained because of the severe 1, 3-diaxial interaction in the perhydroindanone (5). The ring expansion of 5 by one carbon unit, the key step in our approach, was examined in three ways. Reaction of 5 with ethyl diazoacetate catalyzed by a Lewis acid was not highly regioselective, giving the perhydroazulenone (8) and (9) in a ratio of 1 to 4. Metal salt-catalyzed decomposition of the diazoester (10) produced only the β-ketoester (11). However, the desired ketone (8) was obtained by the rearrangement of the β-oxido carbenoid derivative from the dibromohydrin (12). Introduction of the Δ6-double bond into 8 for the conversion to 1 was achieved regiospecifically via kinetic enolization.
