Generation of alkoxyalkynylketenes from a bicyclic precursor. Cycloaddition chemistry with alkynes and theoretical studies regarding the formation of five- versus six-membered ring products

Abstract

A bicyclic [2.2.2]octadiene framework in which the ethano bridge contains a carbonyl group, an alkynyl and a trimethylsiloxy or hydroxy group has been utilized to generate alkoxyalkynylketenes via a retro-Diels-Alder reaction under relatively mild conditions (138 °C). This is an outstanding result since the retro-Diels-Alder reaction of [2.2.2]bicyclic compounds proceeds quantitatively but at unusually high temperatures (>500°C). The ketenes generated undergo [2+2] cycloadditions to alkynes in a sequence of events that lead to the formation of fiveor six-membered ring products. We show using quantum chemical calculations that formation of fiveor six-membered rings in the overall process is a function of the substituents on the alkynes used to trap the ketenes.