Synthesis of the (E)-dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D

A procedure is reported for the introduction of both the thiazoline and (E)-dehydrobutyrine components into a tetrapeptide-derived fragment of vioprolides B and D. The (E)-dehydrobutyrine is introduced first but, as the carbon–carbon double-bond of the dehydrobutyrine appeared incompatible with an adjacent thiol, the thiazoline was assembled by dehydration of a serine containing thioamide not by dehydration of a cysteinyl analogue.