Photochemical synthesis of carbon-14 labelled dibenz[a,h]anthracene

Abstract

Carrier-free [5,12-¹⁴C]dibenz[a,h]anthracene (DBA, 12*) has been prepared for the first time using [¹⁴C]methyl iodide as the source of the label. The key step in this radiosynthesis consisted of the regiospecific photocyclization of 2′,5′-divinyl-1,1′:4′,1″-terphenyl 10* to 5,6,12,13-tetrahydro-DBA 11* followed by dehydrogenation to 12* with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone). Divinylterphenyl 10* was obtained from 2′,5′-bis(bromomethyl)-1,1′:4′,1″-terphenyl 3via the diformyl derivative 9 and its Wittig olefination with the triphenylphosphonium salt of ¹⁴CH₃I. Thus 12* was synthesized with a radiochemical purity exceeding 99%. The chemical yield amounted to 58% from 9 in the case of 12* with a specific activity of 391 MBq mmol^–1 and to 12% in the case of carrier-free 12* with a specific activity of 4.14 GBq mmol^–1. Wide applicability of this radiosynthetic approach to the preparation of many other carbon-14 labelled polycyclic aromatic hydrocarbons can be envisaged.