A new type of thienamycin derivative (2) having a monothioacetal side chain at the C-2 position was prepared by means of a replacement reaction of protected thienamycin sulfoxide (13) with the monothiohemiacetal (16) liberated in situ by the retro-Michael reaction of the masked thiol (12) with 1, 8-diazabicyclo[5.4.0]undec-7-ene.