Synthetic studies on oryzalexin S and structurally related diterpenes

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Oryzalexin S,a potent rice plant phytoalexin, has a stemarane-type diterpenoid structure which is very rare in nature. Triptoquinone G is a diterpenoid quinone with inhibitory activity against interleukin-1 releases, and has a similar AB-ring structure to oryzalexin S.In this research, we attempted the total syntheses of oryzalexin S and triptoquinone G.We chose the Wieland-Mischer ketone as the common starting material for both of the target compounds, considering its availability in optically active forms. This ketone was successively treated as follows : selective protection the C-9 carbonyl, reductive ethoxycarbonylation, enol triflation, reductive removal of the TfO group, allylic oxidation with the Collins reagent, catalytic hydrogenation, protection of the C-2 carbonyl, and enolate methylation at the C-4 position, to give a common intermediate. Selective deprotection of the C-2 carbonyl of this intermediate was followed by reduction with lithium in liquid ammonia to afford the AB-ring moiety of oryzalexin S,while convesion of the ester function to a carboxylic acid followed by selective deprotection of the C-2 acetal and the dissolving metal reduction gave the AB-ring portion of triptoquinone G.In a model system, the synthesis of an intermediate incorporating the C-ring structure of oryzalexin S was accomplished via the Robinson annelation reaction. A model compound of triptoquinone G possessing the complete C-ring structure was also synthesized via the double-Michael reaction.