Studies on the dimerization reaction of 3-oxidopyrylium ion

Abstract

The [5+2] oxidopyrylium ion cyclization reactions with alkenes or allenes have been studies in the synthetic applications as well as in the theoretical studies and successfully applied to total synthesis of natural products. 3-Oxidopyrylium ion can be regarded as a 4π-electron component for addition across the C-2 and C-6 position, and as a 6π-electron component for reaction across the C-2 and C-4 position. Hendrickson reported that 3-oxidopyrylium ion readily dimerized to form a eight-membered carbocyclic ring compound as a single isomer. This dimerization incorporated one oxidopyrylium acting as a 4π-, and the other one as a 6π-component. This dimerization reaction attracted our attention since the reaction formed eight-membered carbocyclic ring system that was rich in functional groups for further manipulation and was rigid enough for stereoselective functionalization. Formally, this dimerization can be views as thermal [5C+3C] cycloaddition reaction for eight-membered ring construction, and could serve as a general synthetic methodology for total synthesis of cyclooctanoid natural products. However, since the Hendrickson’s report there has been only one other report of this dimerization reaction and no one reported a systematic study of the dimerization reaction of 3-oxidopyrylium ions. Therefore, we decided to investigate the scope of dimerization of 3-oxidopyrylium ion. Dimerization reaction of 3-oxidopyrylium ions with substituents at 2,4,5 or 6-position was tested to evaluate reactivity and stereoselectivity of the dimerization reaction.