In this review the recent progress in the development of suitable precursors for 11C‐labelling is discussed. Especially the last few years' advancement of the use of [11C] carbon monoxide as a versatile and useful precursor in labelling chemistry is presented. The development is set in perspective of its potential in applying molecular imaging tools in drug and tracer development.The possibility of exploring small tracer libraries utilizing the microdosing concept is explored. Copyright © 2007 John Wiley & Sons, Ltd.

Photoinitiated radical carbonylation with [(11)C]carbon monoxide at low concentration was employed in syntheses of carbonyl-(11)C-labeled amides using alkyl iodides and amines as precursors. Eleven (11)C-amides were synthesized in up to 74% decay-corrected radiochemical yields with reaction times of 400 s and with up to 95% conversion of carbon monoxide. Starting with 26.3 GBq of [(11)C]carbon monoxide, 10.6 GBq of 1-cyclohexane [(11)C]carbonyl-4-phenyl-piperazine (15) was obtained within 35 min from the end of bombardment (33 microA) and with a specific radioactivity of 192 GBq/micromol at the same time point. The influence of solvents was investigated. The described procedure extends the range of accessible labeling methods. The method may also be useful for preparation of (13)C- and (14)C-substituted compounds.

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Aliphatic esters were labelled with a short-lived radionuclide, 11C with t ½ = 20.3 min, at the carbonyl position using [ 11 C]carbon monoxide via rapid (6 min) photoinduced radical-mediated carbonylation reactions. The esters were prepared from primary, secondary, and tertiary alkyl iodides, and various alcohols, including tert-butyl alcohol and phenol. The use of strong bases was necessary to achieve good radiochemical yields in short reaction times. Isolated decay-corrected radiochemical yields were in the range of 40-68 %.

[(11)C]Carbon monoxide ((11)CO) is a versatile building block for the synthesis of Positron Emission Tomography (PET) radioligands. However, the difficulty of trapping (11)CO in a small solvent volume has limited its utility. We here report an evaluation of a simple, fully automated high-pressure synthesizer prototype for the use in (11)C-carbonylation reactions. [(11)C]Carbon monoxide was easily prepared by online reduction of [(11)C]carbon dioxide using either Mo(s) or Zn(s) as the reducing agent. The conversion yield of (11)CO was >99% when zinc was used as the reducing agent, and the corresponding value for Mo was approximately 71%. When the Zn or Mo column was constantly kept under inert atmosphere, no significant decrease in reducing properties was observed for more than 100 (11)CO productions. However, in our hands, Mo reductant was much easier to service. A total of nine functional groups were successfully radiolabeled using the (11)CO synthesizer prototype. All measured radiochemical yields exceeded 37%, and the (11)CO trapping efficiency was generally above 90%, except for the Suzuki coupling where the trapping efficiency was 80%. This high-pressure synthesizer using [(11)C]carbon monoxide as the labeling precursor is easy to operate allowing for (11)C-carbonylation reactions to be performed in a high yield and in a routinely fashion.

[reaction: see text] [carboxyl-(11)C]Carboxylic acids were prepared from alkyl iodides via photoinitiated radical reactions using 10(-)(8) mol of [(11)C]carbon monoxide in binary and ternary homogeneous solvent mixtures. Short- (isobutyric), medium-, and long-chain saturated fatty acids (heptadecanoic) were labeled with isolated decay-corrected radiochemical yields ranging from 55% to 70% in 5-7-min reactions. The conversion of [(11)C]carbon monoxide to products reached 80-90%. To obtain good yields in the reactions performed in water-acetonitrile and water-THF mixtures, the addition of tetrabutylammonium hydroxide or potassium hydroxide was essential. The carboxylation was efficient for primary and secondary alkyl iodides. The carboxylation of tertiary iodides was feasible for 1-iodoadamantane but not for tert-butyl iodide. The dependence of the radiochemical yields on reaction time, photoirradiation conditions, and organic and inorganic additives was studied. The method provides a one-step route to [carboxyl-(11)C]carboxylic acids; traditional methods, in contrast, would require several steps. For example, using the devised reaction conditions, 3.19 GBq of purified [1-(11)C]1,10-decanedicarboxylic acid (specific radioactivity 188 GBq/mumol) was obtained within 35 min of the end of 10 muAh bombardment. (1-(13)C)4-Phenylbutyric acid was synthesized using ((13)C)carbon monoxide for identifying the labeling position with (1)H and (13)C NMR.

Here we present a protocol for labeling aliphatic carboxylic acids with the positron-emitting radionuclide 11C (t(1/2) = 20.4 min) at the carboxyl position using [11C]carbon monoxide via photoinitiated free radical-mediated carbonylation. A solution of an alkyl iodide in a homogenous binary organic solvent-water mixture is introduced into a high-pressure photochemical reactor containing [11C]carbon monoxide. Then the reactor contents are pressurized to 40 MPa and irradiated with ultraviolet light for 6 min. The labeled product is purified using HPLC. All manipulations with radioactivity including the labeling synthesis are carried out on an automated Synthia system. In a typical case, 3.19 GBq of purified [1-(11)C]1,10-decanedicarboxylic acid (with a specific radioactivity of 188 GBq/micromol) can be obtained within 35 min after the end of a 10-microAh bombardment. Compared to previous labeling methods, this protocol is compatible with a wider range of functional groups, utilizes less-sensitive precursors, and is less subject to isotopic dilution.

Photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide (11C t1/2=20.4 min) is enhanced by ketones that have lowest-lying excited triplet state of npi* character. For example, adding 5 mol % of acetophenone increases radiochemical yields from 3 to 59% in brief 6-min long reactions. Similar or higher yields were achieved by adding di-tert-butyl peroxide. Since radicaloid npi* exited-state ketones and tert-butoxyl radicals have similar reactivity, the photosensitization proceeds most likely via a H-atom transfer mechanism rather than via energy transfer. We propose a mechanism that can account for the enhancement as well as for the formation of observed byproducts. The energy profile obtained by DFT calculations support the feasibility of the mechanism, and observed experimental differences in reactivity could be well rationalized by the calculated data. NBO calculations were performed to further analyze the obtained energetics. Various [carbonyl-11C]esters and some [carbonyl-11C]amides were synthesized in good radiochemical yields from primary and secondary alkyl iodides illustrating the utility of dialkyl peroxides to accelerate the carbonylations. These findings have potential in elaborating new synthetic protocols for the production of 11C-labeled tracers for positron emission tomography.