Cyclopenta[b]annulation of Heteroarenes by Organocatalytic γ'[C(sp )-H] Functionalization of Ynones.

A new approach for the cyclopenta[b]annulation of heteroarenes through metal-free and directing-group-free γ'[C(sp )-H] functionalization and intramolecular hydroalkylation of ynones has been developed. In an unprecedented event, nucleophilic addition of an organophosphine to the designed ynones triggers γ'[C(sp )-H] functionalization, leading to the formation of heteroaryl-based ortho-quinodimethane (oQDM) intermediates that undergo carbocyclization to provide cyclopentannulated heteroarenes in good yields and excellent stereoselectivities. Deuterium-labeling experiments substantiated the proposed reaction mechanism as well as the speculated epimerization.