| Abstract
| - Boron trifluoride-etherate promotes the endo-selective oxacyclization of polyepoxides derived fromvarious acyclic terpenoid polyalkenes, including geraniol, farnesol, and geranylgeraniol, providingan efficient and stereoselective synthesis of substituted oxepanes and fused polyoxepanes. Themechanism of the oxacyclization reaction probably involves intramolecular nucleophilic additionof epoxide oxygen to open another epoxide that is activated as an electrophile by the Lewis acid.These oxacyclizations proceed stereospecifically with inversion of configuration upon opening ofeach epoxide to provide trans-fused polycyclic products. The oxacyclization cascade is terminatedby a tethered nucleophile, which may be the carbonyl oxygen of a ketone, ester, or carbonate, or atrisubstituted alkene. The best oxacyclization yields are generally observed with tert-butyl carbonateas the terminating nucleophile, although in some cases the oxacyclization products include formationof tert-butyl ethers as a minor product. The oxacyclization transformations described herein maymimic ring-forming steps in the biosynthesis of trans-syn-trans-fused polycyclic ether marine naturalproducts.
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