6β-Phthalimidohomopenicillanic acid and (αR,2R,5S)-α-(4,4-dimethyl-6-oxo-1-aza-3-thiabicyclo[3,2,0]hept–2-yl)-α-phthalimidoacetic acid are formed by u.v. irradiation of 6β-phthalimidopenicillanoyldiazomethane.