Metathesis spiroketals

Stereodefined spiroketals are a common structural motif in physiologically-active natural products. Richard P. Hsung of the University of Minnesota recently reported (Organic Lett. 2005, 7, 2273) that Tf,NH is a particulary effective Brpnsted acid mediator for the stereoselective coupling of vinyl lactols such as 11 with homoallylic acids such as 12. The axial ethers so produced undergo smooth ringclosing metathesis to the spiroketals. 

Keywords Annulation Conjugate addition Cyclization Cycloaddition Macrocycles Metathesis Natural products Oxygen-containing heterocycles Prins reaction Spiroketalization... 

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