STUDIES ON INTRAMOLECULAR HIGHER-ORDER CYCLOADDITION REACTIONS

Thermal, metal-free, and metal-promoted intramolecular higher-order cy cloadditions are described. Substrates in which the tropone (2,4,6-cyc loheptatrien-1-one) nucleus is tethered to various diene moieties unde rgo exo-selective [6 pi + 4 pi] cycloaddition under thermal conditions to afford stereochemically defined tricyclic products. The substrates employed in this study were prepared by addition of the Grignard deri vative of the given diene side-chain unit to 2-chlorotropone, In each instance, the requisite 2-substituted tropone was obtained directly fr om these reaction sequences. A similar study was conducted on the corr esponding chromium(0) complexes of related tropone and cycloheptatrien e-based substrates. In the cases involving metal mediation both therma l and photochemical activation were effective for delivering cycloaddu cts. The metal-promoted reactions were shown to proceed exclusively vi a an endo-selective pathway, which is stereocomplementary to the cours e of the thermal, metal-free reactions. A tandem Cr(0)-promoted 1,5-H shift-[6 pi+2 pi] cycloaddition protocol is also reported with several systems.