An oxidative [3 + 2] C–H spiroannulation reaction of 2-alkenylphenols with ynamides has been developed toward the synthesis of spiro­[4,5]­decane derivatives. This dearomative reaction employs earth-abundant cobalt as the metal catalyst and occurs under rather mild reaction conditions (room temperature). The use of ynamides confers unique reactivity and exclusive regioselectivity. The products bearing an all-carbon quaternary stereogenic center were constructed in generally good yields with good functional group tolerance being observed. Experimental mechanistic studies were conducted, and a possible reaction mechanism is proposed.

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