The first synthesis of 1,3-thiazine fused peptide mimics is described from N-(3-hydroxypropyl)thioamides under MsCl/NEt 3 conditions. The method is amenable to oligopeptidomimics with polar and apolar side chains. Substantial epimerization occurs at chiral C(2) exo methines in non-Pro fused mimics even under neutral conditions. 1 H NMR and crystal structure analyses indicate that the Thi analogues primarily associate with each other through intermolecular hydrogen bonds, involving the nitrogen of 1,3-thiazine and the N–H of the fused residue, which may be the basis for enamination–racemization process in these peptide mimics.