No‐carrier‐added 11 C‐labelling of benzenoid compounds in ring positions by condensation of nitro‐[ 11 C]methane with pyrylium salts

A new synthesis is described for three no‐carrier‐added nitro‐[1‐ 11 C]benzenes 3a–c by condensation of nitro[ 11 C]methane ( 1 ) with the appropriate pyrylium salts 2a–c in the presence of a base such as t‐BuOK in t‐BuOH. For synthesizing 4‐nitro‐[4‐ 11 C]anisole ( 3a ), tetrabutylammonium fluoride was successfully used as an auxiliary base. The best results were obtained in the synthesis of 3a . The conversion of 1 with 4‐methoxypyrylium perchlorate ( 2a ) yielded 3a of a radiochemical purity of up to 82 % and a mean specific radioactivity of 30 GBq/μmol (0.8 Ci/μmol) within 20 min. Related to 1 , the reproducible radiochemical yields of 3a are in the range of 77±5 % (decay‐corrected). 2,6‐Dimethyl‐4‐methoxy‐nitro‐[1‐ 11 C]benzene ( 3b ) was prepared by reaction of 1 with 2,6‐dimethyl‐4‐methoxy‐pyrylium perchlorate ( 2b ) in radiochemical yields of about 37 % (decay‐corrected) within 10 min. 2‐Nitro‐[2‐ 11 C]mesitylene ( 3c ) was obtained by condensation of 1 with 2,4,6‐trimethylpyrylium tetrafluoroborate ( 2c ) in radiochemical yields of about 29 % (decay‐corrected) within 20 min. 13 C/ 11 C Co‐labelling experiments were carried out in order to confirm the identity of 3a–c and the position of the label. Copyright © 2000 John Wiley & Sons, Ltd.