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

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The paper describes the first method for n.c.a 11C‐ring labelling of benzenioid 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) follow 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 aradiochemical 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.