In view of the chemical and structural interest of cyclopeptides bearing an electron withdrawing substituent directly bonded at the amide nitrogen atom, the two N-p-toluensulfonyl (N-tosyl) derivatives cycl~-Phe(Tos)-~-Phe-] (I) and cycld-Phe(Tos)-D-Pro-] (11) have been synthesized and their stereochemistry defined. The molecular structure of I, as determined by x-ray difhaction analysis, is reported together with 'H-nmr parameters indicating the preferred rotameric conformation in chloroform solution. The N-tosyl group alters the geometry of the cycldpeptide ring by lengthening both the N-C bonds departing from the tosylated nitrogen and reducing the corresponding ring angle. The 6-membered pep9de ring adopts an unusual "sofa" confonnation with the Tos-Phe C a atom deviating 0.230(3) A out of the mean plane of the other five ring atoms. One of the two S-0 bonds forms a planar system that involves the tosylated nitrogen and the corresponding amide carbonyl. In the crystal, both the benzylic side chains are folded over the heterocyclic ring, whereas in chloroform solution, the benzylic side chain of the D-Phe prefers an extended conformation.