The preparation of pimecrolimus, a synthetic derivative of ascomycin endowed with immunomodulatory activity, requires the selective protection of 24-hydroxy group of the ascomycin, before elaboration of the 32-hydroxy group. The aim was achieved by means of two regioselective Candida antarctica lipase-catalyzed steps. The structure of the new key intermediates, 24-, 32-monoacetates, and 24, 32-diacetate, was established by means of ...