Pyroglutamic acid as a pseudoproline moiety: a facile method for its introduction into polypeptide chains

The acylation of benzyl (S)-pyroglutamate is reported by reaction with the pentafluorophenyl ester of protected alanine and threonine. The 1H NMR analysis of the acylated products shows that the α hydrogens of the protected amino acids are very deshielded. This effect is due to the presence of the carbonyl of the lactam ring and shows that the peptide bond is in the trans conformation. The deprotection of the amino acids is also reported.