Stereocontrolled Synthesis of 3-Substituted Azetidinic Amino Acids

A set of enantiomerically pure N-disubstituted (beta)-amino alcohols was chlorinated by treatment with thionyl chloride. This reaction gave a mixture of regioisomeric chlorides that could be equilibrated to the more stable regioisomer by heating in DMF. The chlorides thus obtained were engaged in an intramolecular anionic ringclosure and gave access to fully protected enantio- and dia­stereomerically pure 2,3-cis-disubstituted azetidinic amino acids. One of the latter was deprotected and included in a short peptide ­sequence.