Stereospecific pseudoproline ligation of N-terminal serine, threonine, or cysteine-containing unprotected peptides

This paper describes an orthogonal and stereospecific method for ligating f ree peptide segments to form a monosubstituted pseudoproline bond with a hy droxymethyl moiety at the C2 carbon. The pseudoproline ligation, comprising both the oxaproline and thiaproline ligations, initially involves an imine capture of a peptidyl glycoaldehyde ester with an N-terminal cysteine, ser ine, or threonine peptide segment and then two spontaneous cyclization reac tions. The thiazolidine or oxazolidine ester formed in the first cyclizatio n undergoes an O,N-acyl transfer to form an pseudoproline bond. The thiapro line ligation can be carried out exclusively with unprotected peptide segme nts in both aqueous and nonaqueous pyridine-acetic acid conditions. However , the oxaproline ligation is best performed in a nonaqueous pyridine-acetic acid mixture with unprotected peptide segments except for those containing N-terminal nucleophilic amino acids such as Cys, His, and Trp. Pseudoproli ne ligation is not only regioselective but also stereospecific. 2D H-1 NMR studies of dipeptide models, Z-Xaa-psi Pro-OMe, indicate that the newly cre ated C2 stereocenter of the pseudoproline ring affords only an li-epimer an d the C2-hydroxymethyl-substituted pseudoproline exhibits high preference f or cis conformation. Three of the model peptides have more than 50% cis iso mers. Finally, this novel method has been used successfully in ligating two segments of 24 and 35 amino acids under mild conditions to synthesize thre e analogues of bactenecin 7, an antimicrobial peptide containing 59 amino a cid residues.