The synthesis of (2R,6R)-lanthionine, sn-(2S,6R)-lanthionine or (2S,3S,6R)-3-methyl-lanthionine, forming an intra-polypeptide cross-link between peptidyl-cysteine, and peptidyl-serine or peptidyl-threonine; dehydration of the serine or threonine residue to the alpha,beta-unsaturated amino acid is the first step; a bond then forms between the ethylene (ethene) group thus formed and the sulfur atom of a cysteine, with the inversion of the configuration of the alpha carbon of the serine or threonine occurring during the process.