Coupling difficulty following replacement of Tyr with HOTic during synthesis of an analog of an EGF B-loop fragment

During Fmoc synthesis of an analog, [Abu^20,31, HOTic²²]hEGF(20- 31), of a fragment, Cys-Met-Tyr-Ile-Glu-Ala-Leu-Asp-Lys-Tyr-Ala-Cys, of the B-loop of human EGF, conductivity measurements showed that increased time was necessary for coupling and complete deprotection of the residues Met²¹ and Abu²⁰ which followed the HOTic²². Use of different active ester-forming reagents, including HOBt and BOP, did not increase the yield. Use of symmetrical anhydride with extended coupling time increased the yield but did not complete the coupling. It appears that inclusion of HOTic in place of Tyr to introduce conformational constraint to peptide analogs can cause or augment a tendency towards conformations with increasing occlusion of N- terminal amino groups and result in the need for altered coupling strategies for completion of analog synthesis.