Stereoselective synthesis of highly functionalized 1,5-oxa-bridged cyclooctenes via a 3-oxidopyrylium-cyclopropenone acetal cycloaddition [corrected].

[reaction: see text] A [5C+2C] oxidopyrylium-cyclopropenone acetal cycloaddition followed by ring opening of the cyclopropane unit of the resulting adduct leads to highly substituted 1,5-oxa-bridged cyclooctenes containing up to four stereocenters. The protocol formally constitutes a [5C+3C] annulation process.