Probing the alkylidene carbene-strained alkyne equilibrium in polycyclic systems via the Fritsch-Buttenberg-Wiechell rearrangement.

Strained cycloalkynes are valuable building blocks in synthetic chemistry due to their high degree of reactivity and ability to form structurally complex scaffolds, common features of many pharmaceuticals and natural products. Alkylidene carbenes provide a pathway to the formation of strained cycloalkynes through Fritsch-Buttenberg-Wiechell rearrangements, but this strategy, like other methods of alkyne generation, is believed to depend upon a thermodynamic equilibrium that favors the alkyne over the carbene. Herein three highly strained, polycyclic alkynes, previously thought to be thermodynamically inaccessible, are generated under mild conditions and intercepted through Diels-Alder cycloaddition with a diene trapping agent. The use of a different trapping agent also allows for the interception of the alkylidene carbene, providing the first instance in which both an exocyclic alkylidene carbene and its cycloalkyne Fritsch-Buttenberg-Wiechell rearrangement product have been trapped.