Stereoselective synthesis of cis- and trans-2,3-disubstituted tetrahydrofurans via oxonium-prins cyclization: access to the cordigol ring system.

SnBr(4)-promoted oxonium-Prins cyclizations to form 2,3-disubstituted tetrahydrofurans (THFs) are reported. In the absence of an internal nucleophile, the carbocation intermediates are trapped by bromide to give 2,3-cis- and 2,3-trans-configured products; two variations with intramolecular trapping are also reported. One of these allows a single-step stereocontrolled synthesis of the core 2,3-cis-THF ring system of cordigol, a fungicidal polyphenol from the stem bark of Cordia goetzei. For this latter transformation, a stepwise oxonium-Prins/cation trapping pathway rather than orthoquinonemethide formation/hetero-Diels-Alder cycloaddition is supported computationally.