Rhodium(III)-Catalyzed C(sp2)-H Functionalization of Cyclobutenes. Access to Cyclobuta[c]pyridones and -pyridines.

O-(ω-Alkynyl) hydroxamates derived from cyclobutenyl carboxylic acids were identified as viable substrates in intramolecular rhodium(III)-catalyzed heteroannulations, which led to diversely substituted cyclobuta[c]pyridones. Further functionalization of the resulting cyclobutapyridones enabled the synthesis of cyclobuta[c]pyridines and other nitrogen heterocycles after electrocyclic ring opening of the four-membered ring.