Total Synthesis and Stereochemical Assignment of Enteropeptin A.

The total synthesis and structural elucidation of the antimicrobial sactipeptide enteropeptin A is reported. Enteropeptin A contains a thioaminoketal group with an unassigned stereochemical configuration that is embedded in a highly unusual thiomorpholine ring. In this synthesis, a linear peptide containing a dehydroamino acid and a pendant cysteine residue is subjected to Markovnikov hydrothiolation by a dithiophosphoric acid catalyst. This cyclization reaction forms the central thiomorpholine ring found in the enteropeptins. Both diastereomers at the unassigned thioaminoketal stereocenter of enteropeptin A were prepared, and their comparison to an authentic standard allowed for the unambiguous stereochemical assignment of the natural product to be of the D configuration. This inaugural total synthesis of enteropeptin A represents the first total synthesis of a sactipeptide reported to date. Moreover, the strategy disclosed herein serves as a general platform for the synthesis of stereochemically defined thiomorpholine-containing peptides, which may enable the discovery of new cyclic peptide antibiotics.