The solid-phase synthesis of a cyclic peptide containing the 21-residue epitope found in the A−Bloop of the Cε3 domain of human immunoglobulin E has been carried out. The key macrocyclizationstep to form the 65-membered ring is achieved in ∼15% yield via an “on-resin” Sonogashira couplingreaction which concomitantly installs a diphenylacetylene amino acid conformational constraintwithin the loop.