Synthesis and resolution of new paramagnetic α-amino acids

New, paramagnetic unnatural α-amino acids were synthesized by the OʹDonnell method. In the new amino acids nitroxide is condensed with thiophene, benzene, and tetrahydroisoquinoline ring, or linked through a methylene, benzyl or propargyl spacer. Some of the racemic paramagnetic α-amino acid esters described earlier or in this work were resolved by fractional crystallization of diastereomeric salts. Another approach for optically active paramagnetic amino acids is the modification of S-tyrosine derivatives with Pd-catalyzed cross-coupling reactions with paramagnetic acetylene and with a paramagnetic boronic acid.