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

Magnetic resonance imaging of [1- 13 C]hyperpolarized carboxylates (most notably, [1- 13 C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Here we investigate the efficiency of 1 H and 13 C 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- 13 C-enriched forms with parahydrogen over Rh/TiO 2 catalysts in methanol-d 4 and in D 2 O. The maximum obtained 1 H polarization was 0.6 ± 0.2% (for propyl acetate in CD 3 OD), while the highest 13 C polarization was 0.10 ± 0.03% (for ethyl acetate in CD 3 OD). 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 1 H and 13 C 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.