Synthesis of Enantiopure ε-Oxapipecolic Acid.

A six-step synthesis of orthogonally protected ()-ε-oxapipecolic acid is described, starting from a commercially available glutamate diester. The approach features CPBA-mediated amine oxidation and an intramolecular Mitsunobu reaction to form the tetrahydrooxazine ring. The enantiopure building block was employed in the synthesis of a short model peptide to determine the amide rotamer preference -terminal to the cyclic residue. In contrast to pipecolic acid, which exhibits a high amide population, the ε heteroatom in oxapipecolic acid exerts a strong substantiating effect through lone pair repulsion.