Synthetic Studies on the Ingenane Diterpenes. Interand Intramolecular [6 + 4] Tropone-Diene Cycloaddition Reactions1Summary: Thermally allowed interand intramolecular [6 + 4] tropone-diene cycloadditions have been employedfor the construction of intermediates in the synthesis of the cocarcinogenic diterpene ingenol.(9) While this manuscript was in preparation, another example of an intramolecular [6 + 4] tropone cycloaddition surfaced: Funk, R. L., personal communication.(10) 2-Substituted tropones are relatively difficult to prepare. Compound 9 is available in 61% from the reaction of 2-chlorotropone11 and the Grignard reagent derived from l-bromohept-4,6-diene. Details of this procedure will be reported in a separate paper: Rigby,
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