Monoprotected phthalaldehydes 3 and 4, bearing a chiral imidazolidine auxiliary were reacted with various organometallic reagents. Lithium organocuprates gave almost exclusively one diastereomer whereas an organomanganese reagent gave the opposite one. Hydrolysis of the aminal moiety affords the enantiomerically pure lactols 7a–d. In both 3 and 4 the chiral imidazolidine auxiliary is very effective, but affords opposite results which are rationalized by chelation and/or steric factors.