Download full text in PDF Opens in a new window. Article suggestions will be shown in a dialog on return to ScienceDirect. ... Please enable JavaScript to use all the features on this page. ... Photochemical ring contraction of a cis disubstituted pyrazolidin-3-one, prepared by hydrazinolysis of an α, β-unsaturated δ-lactone, gave the corresponding azetidinone which has been converted previously to the carbapenem antibiotic PS-5. ... Photochemical ring contraction of a 4, ...