Selenocysteine derivatives are useful precursors for the synthesis of peptide conjugates and selenopeptides. Several diastereomers of Fmoc-3-methyl-Se-phenylselenocysteine (FmocMeSec(Ph)) were prepared and used in solid phase peptide synthesis (SPPS). Once incorporated into peptides, the phenylselenide functionality provides a useful handle for the site and stereospecific introduction of E- or Z-dehydrobutyrine residues into peptide chains via oxidative elimination. The oxidation conditions are mild, can be performed on a solid support, and tolerate functionalities commonly found in peptides, including variously protected cysteine residues. Dehydropeptides containing unprotected cysteine residues undergo intramolecular stereoselective conjugate addition to afford cyclic lanthionines and methyllanthionines, which have the same stereochemistry as found in lantibiotics, a family of ribosomally synthesized and post-translationally modified peptide antibiotics. The observed stereoselectivity is shown to originate from a kinetic rather than a thermodynamic preference.