Naphthalene converts magnesiated ω-alkenylnitriles into bi- and tricyclic ketones via a polar-radical addition-cyclization cascade. One-electron oxidation of magnesiated nitriles generates nitrile-stabilized radicals that cyclize onto a pendant olefin and then rebound onto the nitrile through a reduction-cyclization sequence; subsequent hydrolysis affords a diverse array of bicyclo[3.2.0]heptan-6-ones. Combining the polar-radical cascade with a 1,2:1,4-carbonyl-conjugate addition generates complex cyclobutanones containing four new carbon-carbon bonds and four chiral centers in one synthetic operation.