Peptide Coupling in the Presence of Highly Hindered Tertiary Amines

Previously, 2,4,6-trimethylpyridine (collidine), due to steric shielding around the N-atom, was found to be an efficient base for effecting peptide segment coupling via azabenzotriazole-based onium-style coupling reagents. A number of even more highly hindered bases, including 2,3,5,6-tetramethylpyridine, 2,6-di-tert-butyl-4-(dimethylamino)pyridine, triisopropylamine, and N-tert-butylmorpholine, have been compared with collidine in such reactions. Some of the newer bases showed advantages in terms of convenience in handling and maintenance of configuration during segment coupling processes, although dramatic differences based on steric effects were not observed. On the basis of results with a number of test peptides and many base-coupling reagent combinations, it was noted that most efficient results are obtained if 1 equiv of HOAt is present as an additive during the coupling process. For rapid activation of onium-style coupling reagents during stepwise solid-phase coupling reactions, the stronger base 2,6-di-tert-butyl-4-(dimethylamino)pyridine was more effective than collidine.