The course of the Tl(III)-mediated intramolecular oxidative macrocyclization of phenolic residues to provide the M(2-4) diarylether ring of the vancomycin antibiotics is remarkably sensitive to transannular effects across the M(4-6) ring, brought about by variations in the degree of ring-6 chlorination as well as the conformational bias imparted by the distal M(5-7) ring. The effect of structure on this reaction is documented.