A key step in the synthesis of the C1-C9 fragment of the ionophore antibiotic ionomycin involves the addition of an alkylcopper(I) reagent to an ester-functionalised cationic η3-allylicmolybdenum and an alkylzinc cuprate to the corresponding η3-allyliciron complex. The reaction is regioselective and the metal directs enantiofacial attack (