Syntheses with isotopically labelled carbon. Methyl iodide, formaldehyde and cyanide

Gas Phase Transformations in Carbon-11 Chemistry

The short-lived positron-emitter carbon-11 (t1/2 = 20.4 min; β+, 99.8%) is prominent for labeling tracers for use in biomedical research with positron emission tomography (PET). Carbon-11 is produced for this purpose with a cyclotron, nowadays almost exclusively by the 14N(p,α)11C nuclear reaction, either on nitrogen containing a low concentration of oxygen (0.1-0.5%) or hydrogen (~5%) to produce [11C]carbon dioxide or [11C]methane, respectively. These primary radioactive products can be produced in high yields and with high molar activities. However, only [11C]carbon dioxide has some utility for directly labeling PET tracers. Primary products are required to be converted rapidly and efficiently into secondary labeling synthons to provide versatile radiochemistry for labeling diverse tracer chemotypes at molecular positions of choice. This review surveys known gas phase transformations of carbon-11 and summarizes the important roles that many of these transformations now play for producing a broad range of labeling synthons in carbon-11 chemistry.

High reactivity of [11C]CH3I with thiol group in the synthesis of C-11 labeled radiopharmaceuticals

High reactivity of [11C]-methyl iodide ([11C]CH3I) with the thiol group was demonstrated with cysteamine and other compounds containing a thiol and another functional groups in each structure. The methylation of the thiol group in cysteamine with [11C]CH3I was very rapid at 0 degree C with no catalyst, and gave a high radiochemical yield and purity without any detectable by-product. Moreover, this reaction was not disturbed by the other functional groups, such as -NH2, -OH and -COOH in the same structure. This S-methylation reaction is very useful for producing a new radiopharmaceutical labeled with the short lived positron emitting nuclide C-11.

An improved synthesis of carbon-11 labeled acetoacetic acid and an evaluation of its potential for the investigation of cerebral pathology by positron emission tomography

1-11C-acetoacetic acid was synthesized by carboxylation of the acetone carbanion. Purification was carried out using HPLC. The product was obtained with a radiochemical yield of up to 58%, corrected for decay, in a total preparation time of 30 min. The distribution of 1-11C-acetoacetic acid after injection into adult Wistar rats and cats was investigated by PET. When the tracer was injected into cats, 3 weeks after inflicting a unilateral freezing lesion upon the brain, accumulation of 1-11C-acetoacetic acid in the ipsilateral brain hemisphere was observed.

The radiosynthesis of [N-methyl-11C]-sertraline

[N-methyl-11C]Sertraline, a potential agent for the study of the serotonergic system in vivo with positron emission tomography, was prepared by N-methylation of the corresponding norcompound with [11C]iodomethane, which was itself prepared from cyclotron-produced [11C]carbon dioxide. Under the best conditions found [norsertraline free base (20 mM) in DMF (0.70 mL), 120 degrees C for 8 min] [N-methyl-11C]-sertraline can be prepared in 43% radiochemical yield from [11C]iodomethane (decay-corrected), corresponding to 20% overall radiochemical yield from [11C]carbon dioxide (decay-corrected), with high specific radioactivity. Preparations can be ready for i.v. injection 50 min from the end of radionuclide production.

Preparation of [11C]carbon dioxide from [11C]cyanide

[11C]Carbon dioxide was prepared from [11C]cyanide. First, [11C]CH4 was produced by the (p, alpha) reaction on a N2/H2 mixture and was converted to [11C]CN- by reaction with ammonia over platinum at 1273 K. The [11C]CN- was adsorbed on a cobalt(II, III) oxide/ceramic furnace material at room temperature and was subsequently converted to [11C]CO2 by heating the cobalt(II, III) oxide to 948 K over a 10 min period and collecting the [11C]CO2 in a trap at 77 K. Specific activities as high as 600 mCi/mumol at end of bombardment (EOB) were obtained in a 15 microA irradiation for 10 min.

Routine synthesis of N-[11C-methyl]scopolamine by phosphite mediated reductive methylation with [11C]formaldehyde

A synthesis of [11C]scopolamine capable of clinical delivery of this agent in high specific activity is described. The precursor [11C]formaldehyde was produced by catalytic oxidation of [11C]CH3OH over metallic silver and was used to N-11C-methylate norscopolamine using aqueous neutral potassium phosphite as the reducing agent. The labeling reaction was complete after 5 min at 75-80 degrees C and the [11C]scopolamine (99% radiochemical purity) was isolated by preparative HPLC. Total synthesis time is less than 45 min. Decay corrected radiochemical yields from [11C]CO2 are presently 20-43%.