Heterogeneous 1H and 13C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters

Magnetic resonance imaging of [1-C-13]hyperpolarized carboxylates (most notably, [1-C-13]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of H-1 and C-13 hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-C-13-enriched forms with parahydrogen over Rh/TiO2 catalysts in methanol-d(4) and in D2O. The maximum obtained H-1 polarization was 0.6 +/- 0.2 % (for propyl acetate in CD3OD), while the highest C-13 polarization was 0.10 +/- 0.03 % (for ethyl acetate in CD3OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum H-1 and C-13 NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.