6, 6-Diiodopenicillanic acid was prepared from 6-aminopenicillanic acid using iodine-sodium iodide systems in good yield (60%). Enolates derived from 6-chloro-6-iodo-, 6,6-diiodo-penicillanate have been generated in situ by a metal-halogen exchange process at -$78^{circ}C$ using methylmangnesium iodide and reacted with acetaldehyde to yield aldols. As the size of remaining halogen atom increases, the proportion of ${eta}$-face attack was increased. In the case of 6,6-diiodopenicillanate, only ${eta}$-face attack was observed yielding a single isomer (6S, 8R).