Cyclobutyl-Containing Rigid Analogues of Threonine: Synthesis and Physical Chemical Properties

Hitherto unknown <i>cis</i>- and <i>trans</i>-1-amino-3-hydroxy-3-methylcyclobutanecarboxylic acids were synthesized in multigram scale. The obtained compounds can be considered as achiral conformationally restricted analogues of threonine with fixed spatial orientation of functional groups. p<i>K</i>_a values are noticeably different for both amino acids. According to the X-ray data the cyclobutane rings in both compounds are almost planar (the corresponding torsion angles are below 7°)