A copper-promoted oxyalkylation of alkenes with alkylnitriles has been developed. The protocol provides rapid access to phthalides (g-lactones) or isochromanones (d-lactones) via the formation of a C(sp3)C(sp3) and a C(sp3)O bond with the generation of up to two quaternary carbon atoms. Mechanistic studies suggest that this reaction is initiated by the formation of the C(sp3)C(sp3) bond rather than the C(sp3)O bond. Catalytic conditions were subsequently developed using carboxylic acid as an internal nucleophile.