HATU is a very promising coupling agent for chemical protein synthesis.
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This strategy was exploited to prepare macrocycles from the trimeric linear arylopeptoids (ortho-, meta-, and para-) containing isopropyl or ethyl side chains, synthesized as described by Hjelmgaard et al. The cyclization procedure reported for α,β-cyclopeptoids was applied. The linear arylopeptoids were cyclized in the presence of HATU and DIPEA in CH?Cl?/DMF (4:1) after the deprotection of the tert-butyl group in TFA/CH?Cl?.
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After the synthesis of the Fmoc-protected monomers, the oligomers were synthesized on the 2-chlorotrityl resin with excellent yield of coupling (>98%). The trimers and tetramers of the different isomers were synthesized in good overall yield (60-84%). Then, the crude oligomers were cyclized in DMF in the presence of HATU and DIPEA in high dilution (3 × 10?3 M) to furnish the cyclized trimers and tetramers in good yields ranging between 32% and 72%.