Cyclization of azetidinone (I) by means of catalyst Rh2(OAc)4 in toluol provides carbapenam (II), which is treated with SiO2 in EtOAc to produce epimerization to the alpha-methyl compound (III). Methylation of (III) with diazomethane in dioxane affords carbapenem (IV), which is hydrogenolyzed over Pd/C in EtOAc and finally saponified with KHCO3 in H2O to furnish the desired compound.