Stereocontrolled Syntheses of Peptide Thioesters Containing Modified Seryl Residues as Probes of Antibiotic Biosynthesis

Methods have been developed to synthesize tri- and pentapeptide thioesters containing one or more <i>p</i>-(hydroxyphenyl)­glycine (pHPG) residues and l-serine, some where the latter is <i>O-</i>phosphorylated, <i>O</i>-acetylated, or exists as a β-lactam. Selection of orthogonal protection strategies and development of conditions to achieve seryl <i>O</i>-phosphorylation without β-elimination and to maintain stereochemical control, especially simultaneously at exceptionally base-labile pHPG α-carbons, are described. Intramolecular closure of a seryl peptide to a β-lactam-containing peptide and the syntheses of corresponding thioester analogues are also reported. Modification of classical Mitsunobu conditions is described in the synthesis of the β-lactam-containing products, and in a broadly useful observation, it was found that simple exclusion of light from the P­(OEt)₃-mediated Mitsunobu ring closure afforded yields of >95%, presumably owing to reduced photodegradation of the azodicarboxylate used. These sensitive potential substrates and products will be used in mechanistic studies of the two nonribosomal peptide synthetases NocA and NocB that lie at the heart of nocardicin biosynthesis, a family of monocyclic β-lactam antibiotics