Abstract: The invention relates to a process for the preparation of the compound of the formula I, ##STR1## which comprises (a.sub.1) splitting off the ethylene ketal protecting group from the azetidinone derivative of the formula IVa, ##STR2## reducing the compound of the formula III thus obtained, ##STR3## treating the resulting compound of the formula II ##STR4## with concentrated aqueous hydrogen chloride solution; or (a.sub.2) reducing the compound of the formula III, treating the compound of the formula II thus obtained with concentrated aqueous hydrogen chloride solution; or (a.sub.3) treating the compound of the formula II with concentrated aqueous hydrogen chloride solution; and isolating the compound of the formula I thus obtained from the reaction mixture. The compound of the formula I is useful as an intermediate in the production of thienamycin, a highly potent antibiotic.