An efficient and
redox-neutral synthesis of 2H-chromene-3-carboxylic
acids from N-phenoxyacetamides
and methyleneoxetanones has been realized via a solvent-controlled
and rhodium(III)-catalyzed C–H activation/unusual [3 + 3] annulation
sequence. This transformation represents the first example of using
an α-methylene-β-lactone unit as the three-carbon source
in transition-metal-catalyzed C–H activations through selective
alkyl C–O bond cleavage. Synthetic applications and mechanistic
details, including further derivatization of 2H-chromene-3-carboxylic
acids, the isolation and identification of a five-membered rhodacycle,
as well as the theoretical studies for reasoning a plausible Rh(III)–Rh(V)–Rh(III)
process, have also been discussed.