Synthetic Studies on Pseudoguaianolides. I. Preparation of a Key Intermediate, 1β-tert-Butoxy-2,3,3aα, 4,5,8a-hexahydro-4α, 8aβ-dimethyl-6 (1H)-azulenone, for Helenanolides

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The preparation of the title compound (1), a key intermediate for the synthesis of helenanolides (pseudoguaianolides with a C_<10> α-methyl group), is described starting from the readily available enone (3). The correct stereochemistry of the C_<10>-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 Δ^6-double bond into 8 for the conversion to 1 was achieved regiospecifically via kinetic enolization.
公益社団法人日本薬学会の論文
1983-01-25