AbstractCyclic peptide analogs containing a β‐lactam moiety were prepared. Reacting Fmoc‐protected amino acid‐derived diazo ketones 1, 2 with benzylidene‐protected amino esters 3, 4 in a photochemically induced Staudinger‐type reaction, trans‐substituted β‐lactams 5a/b and 6a/b were formed in 35−70% yield (dr 60:40−70:30). N‐Terminal chain elongation to the respective acyclic precursors 14, 17a/b and 19a/b was achieved using conventional peptide synthesis (i.e. the pentafluorophenyl ester protocol). After saponification, activation as pentafluorophenyl esters and subsequent cleavage of the N‐terminal Boc‐protecting group, the pre‐strained (3R,4S)‐configured isomers could be cyclized without the need for high dilution or prolonged reaction times. Contrary to this, the (3S,4R)‐configured isomers did not cyclize but gave polymeric material. The conformational stability of the cyclic peptidomimetics 16, 20, and 21 which were obtained in high yield, was elucidated by means of NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)