Inhibition of radical reactions for an improved potassium<i>tert</i>-butoxide-promoted<sup>11</sup>C-methylation strategy for the synthesis of<i>α</i>-<sup>11</sup>C-methyl amino acids
作者:Chie Suzuki、Koichi Kato、Atsushi B. Tsuji、Ming-Rong Zhang、Yasushi Arano、Tsuneo Saga
DOI:10.1002/jlcr.3259
日期:2015.3
α-11C-Methyl amino acids are useful tools for biological imaging studies. However, a robust procedure for the labeling of amino acids has not yet been established. In this study, the 11C-methylation of Schiff-base-activated α-amino acid derivatives has been optimized for the radiosynthesis of various α-11C-methyl amino acids. The benzophenone imine analog of methyl 2-amino butyrate was 11C-methylated with [11C]methyl iodide following its initial deprotonation with potassium tert-butoxide (KOtBu). The use of an alternative base such as tetrabutylammonium fluoride, triethylamine, and 1,8-diazabicyclo[5.4.0]undec-7-ene did not result in the 11C-methylated product. Furthermore, the KOtBu-promoted 11C-methylation of the Schiff-base-activated amino acid analog was enhanced by the addition of 1,2,4,5-tetramethoxybenzene or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and inhibited by the addition of 1,10-phenanthroline. These results suggest that inhibition of radical generation induced by KOtBu improves the α-11C-methylation of the Schiff-base-activated amino acids. The addition of a mixture of KOtBu and TEMPO to a solution of Schiff-base-activated amino acid ester and [11C]methyl iodide provided optimal results, and the tert-butyl ester and benzophenone imine groups could be readily hydrolyzed to give the desired α-11C-methyl amino acids with a high radiochemical conversion. This strategy could be readily applied to the synthesis of other α-11C-methyl amino acids.
α-11C-甲基氨基酸是生物成像研究中的有用工具。然而,目前尚未建立起一种稳健的氨基酸标记程序。在本研究中,已对Schiff碱激活的α-氨基酸衍生物的11C-甲基化进行了优化,以实现各种α-11C-甲基氨基酸的放射合成。通过用氢氧化钾叔丁基(KOtBu)对其进行初步去质子化后,使用[11C]甲基碘化物对孟糖基2-氨基丁酸的苯基砜亚胺类似物进行了11C-甲基化。使用四丁基氟铵、三乙胺和1,8-二氮-双环[5.4.0]十一烯等替代性碱未能得到11C-甲基化产物。此外,KOtBu促进的Schiff碱激活氨基酸类似物的11C-甲基化在添加1,2,4,5-四甲氧基苯或2,2,6,6-四甲基哌啶-1-氧基(TEMPO)后得到了增强,而在添加1,10-菲啰啉后则被抑制。这些结果表明,抑制KOtBu诱导的自由基生成有助于提高Schiff碱激活氨基酸的α-11C-甲基化。将KOtBu和TEMPO的混合物加入Schiff碱活化的氨基酸酯与[11C]甲基碘化物的溶液中可获得最佳结果,叔丁基酯和苯基砜亚胺基团可被容易水解以生成所需的α-11C-甲基氨基酸,且具备高的放射化学转化率。这一策略可以方便地应用于其他α-11C-甲基氨基酸的合成。