Synthesis of Polysubstituted Furan Frameworks via [3 + 2] Annulation of -Enoxyimides with Chelated Alkynes Initiated by Rh(III)-Catalyzed C-H Activation.

A variety of 2,3,5-trisubstituted furans were synthesized by virtue of a Rh(III)-catalyzed vinylic C-H coupling of -enoxyimides with propargyl alcohols or amines. In this protocol, -enoxyimides served as a one-oxygen and two-carbon synthon and the -OH/-NHR moiety in the alkynes was found to be crucial for realizing the desired chemo-/regioselectivity. Detailed experimental mechanistic studies together with DFT calculations clarified the sequential C-H activation/regioselective alkyne insertion/simultaneous O-N bond cleavage and hydrogen transfer/intramolecular nucleophilic addition/aromatization reaction path, which involved a distinctive Rh(V) carbene species as the active intermediate.