[Synthesis of cyclic analogues of the nerve growth factor loop 4]

Synthesis of Aib-containing cyclopeptides via the ‘azirine/oxazolone method’

The cyclic pentapeptide cyclo[Gly-Phe(2Me)-Aib-Aib-Gly], containing three α,α-disubstituted α-amino acids, was prepared by cyclization of H-Gly-Phe(2Me)-Aib-Aib-Gly-OH with diphenyl phosporazidate (DPPA) in 60% yield. The synthesis of the linear precursor was performed by using the ‘azirine/oxazolone method’, in which the α,α-disubstituted α-amino acids were introduced by coupling of the corresponding amino or peptide acid with a 2,2-disubstituted 3-amino-2H-azirine. Whereas the cyclization of an analogous hexapeptide afforded the cyclopeptide in 42% yield, the corresponding tetrapeptide yielded a 1:2 mixture of the monomeric and dimeric cyclopeptide. In the case of the tripeptide H-Gly-Phe(2Me)-Aib-OH, the cyclization with DPPA led to the dimeric cyclohexapeptide exclusively.