title: New strategies for natural products containing chroman spiroketals.
abstract: Two cycloaddition strategies are described that lead to various chroman spiroketals from assorted exocyclic enol ethers. Unlike conventional thermodynamic ketalization strategies, the stereochemical outcome for this approach is determined by a kinetic cycloaddition reaction. Thus, the stereochemical outcome reflects the olefin geometry of the starting materials along with the orientation of the associated transition state. However, the initial kinetic product can also be equilibrated by acid catalysis and reconstituted into a thermodynamic stereochemical arrangement. Thus, these strategies uniquely enable synthetic access to either the thermodynamic or kinetic conformation of the spiroketal stereocenter itself. Applications of these strategies in the syntheses of berkelic acid, β-rubromycin, and paecilospirone are presented along with the use of a chroman spiroketal for the construction of heliespirones A and C.
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106-9510, USA.
authors.forename: Jason C
authors.initials: JC
authors.lastname: Green
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106-9510, USA.
authors.forename: G Leslie
authors.initials: GL
authors.lastname: Burnett
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106-9510, USA.
authors.forename: Thomas R R
authors.initials: TR
authors.lastname: Pettus
keywords: Diels–Alder; [4 + 2] cycloaddition; aromatic; aryl spiroketals; chroman; natural products; ortho-quinone methide; oxidative [3 + 2] cycloaddition; spiroketals; total synthesis