N.C.A. 11C-labelling of benzenoid compounds in ring positions: synthesis of nitro-[1-11C]benzene and [1-11C]aniline

The paper describes the first method for n.c.a. 11C-ring labelling of benzenoid compounds having a reactive group for further derivatization by use of the known principle of synchronous six-electron cyclization of hexatriene systems into aromatics. Nitro-[11C]methane (1) prepared from cyclotron-produced [11C]carbon dioxide reacts in the presence of t-BuOK with 5-dimethylaminopenta-2,4-dienylidene-dimethylammonium perchlorate (2) to form 6-nitro-1-dimethylamino-[6-11C]hexatriene (3) followed by cyclization/aromatization into nitro-[1-11C]-benzene (4) at increased temperatures. Starting from 1, nitro-[1-11C]benzene of a radiochemical purity of about 92% and a mean specific radioactivity of 1 Ci/μmol was obtained within 7 min. Related to [11C]CO2, the reproducible radiochemical yield of 4 (decay-corrected) was 80±5%. Reduction of 4 by heating the above reaction mixture with aqueous Na2S gave [1-11C]aniline (5) of a radiochemical purity of about 81%. The reproducible radiochemical yield of 5 (decay-corrected) in relation to [11C]CO2 was 65±5%, the synthesis time from 1 was 18 min. (author)