An efficient and practical synthesis of [2-(11)C]indole via superfast nucleophilic [(11)C]cyanation and RANEY® Nickel catalyzed reductive cyclization.

ABSTRACT

A rapid method for the synthesis of carbon-11 radiolabeled indole was developed using a sub-nanomolar quantity of no-carrier-added [(11)C]cyanide as radio-precursor. Based upon a reported synthesis of 2-(2-nitrophenyl)acetonitrile (), a highly reactive substrate 2-nitrobenzyl bromide () was evaluated for nucleophilic [(11)C]cyanation. Additionally, related reaction conditions were explored with the goal of obtaining of highly reactive 2-(2-nitrophenyl)-[1-(11)C]acetonitrile () while inhibiting its rapid conversion to 2,3-bis(2-nitrophenyl)-[1-(11)C]propanenitrile (). Next, a RANEY® Nickel catalyzed reductive cyclization method was utilized for synthesizing the desired [2-(11)C]indole with hydrazinium monoformate as the active reducing agent. Extensive and iterative screening of basicity, temperature and stoichiometry was required to overcome the large stoichiometry bias that favored 2-nitrobenzylbromide () over [(11)C]cyanide, which both caused further alkylation of the desired nitrile and poisoned the RANEY® Nickel catalyst. The result is an efficient two-step, streamlined method to reliably synthesize [2-(11)C]indole with an entire radiochemical yield of 21 ± 2.2% (n = 5, ranging from 18-24%). The radiochemical purity of the final product was >98% and specific activity was 176 ± 24.8 GBq µmol(-1) (n = 5, ranging from 141-204 GBq µmol(-1)). The total radiosynthesis time including product purification by semi-preparative HPLC was 50-55 min from end of cyclotron bombardment.