Application of Parahydrogen-Induced Polarization to Unprotected Dehydroamino Carboxylic Acids

One focus of current nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) investigation is the hyperpolarization of biologically relevant substrates. In this study, the application of parahydrogen-induced polarization (PHIP) to amino carboxylic acids was enabled by protonation of the amino group as well as of the carboxylic acid. Due to the donor character of these functional groups, they usually act as ligands at the active catalytic sites. To enable parahydrogenation, blocking of the catalytic sites by the functional groups has to be avoided. In a new approach, this was realized via protonation of the starting material. For the first time PHIP spectra of allylglycine, vigabatrin and gamma-amino-butyric acid (GABA) were generated. The feasibility of the hydrogenation of amino carboxylic acids without using a protection group supersedes the deprotection reaction usually required. Hence, hydrogenation after protonation of the substrate opens the class of free dehydroamino carboxylic acids to PHIP.