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 five-or six-membered ring products. We show using quantum chemical calculations that formation of five-or six-membered rings in the overall process is a function of the substituents on the alkynes used to trap the ketenes.

Ring Forming Approaches to para-Quinones: Toward a General Diels–Alder Disconnection

“…32 With respect to the ketene–enol motif specifically, rapid ring closure to the corresponding lactone has been implicated in a number of studies, 33 whereas stabilized enol derivatives (e.g., silyl enol ethers) appear virtually unknown. 34 Synthetically, this approach has been achieved via DA reactions utilizing furans of general structure 25 (i.e., bearing appropriate oxygenation at C2), followed by a ring-opening/proton-transfer event. 35…”