This invention relates generally to cyclic polypeptides comprising a 
thioether linkage and methods for their preparation. More particularly, this invention relates to halogenated polypeptides having at least one haloalanine-like 
amino acid, and methods for their preparation which involve converting the hydroxyl group (i.e., -OH) of a 
serine-like 
amino acid to a halo group (i.e., -X where X is Cl, Br, or I) with the aid of a 
phosphorus-based 
halogenation reagent such as a 
triphenylphosphine dihalide (i.e., (C6H5)3PX2, wherein X is Cl, Br, or I), a triphenylphosphite dihalide (i.e., (C6H5O)3PX2, wherein X is Cl, Br, or I), or a mixture of 
triphenylphosphine or triphenylphosphite with a 
halohydrocarbon (i.e., "halo-conversion"). This invention also relates to cyclic polypeptides having at least one polypeptide loop comprising a 
thioether linkage, and methods for their preparation which employ halogenated polypeptides and which involve intramolecular 
alkylation of the 
thiol group of a 
cysteine-like 
amino acid by the halo group of a haloalanine-like amino acid under suitable basic conditions to form a 
thioether linkage (i.e., "cyclization").