Precursor Production

The development of new precursors [9] is important for the development of new labeled substances. A number of precursors more or less routinely available from target-produced [11C]carbon dioxide are shown in Fig. 10.2.

The most frequently employed precursor is [11C]methyl iodide [10]. There are two synthetic methods available: converting [11C]carbon dioxide to [11C]methoxide followed by reaction with hydroiodic acid, or by a gas phase reaction where [11C]methane is reacted with iodine. Methyl iodide is a useful alkylating

Table 10.1.

Nuclear reaction

Threshold energy (MeV)

Target produced precursor of practical interest

14N(p,a)11C

3.1

[11C]CO2, [11C]CH4

11B(p,n)11C

3.0

[11C]CO2

11CH3CuLi 11CH3NO2

1ch3l^ 11CH3I]

11CH2N2 11CCl 4

1CHCl3

1CH3OH

Ph3P11CH 2 H211CO

'COCl 2

H11CN

H11CN

11CNBr

R11COCl

RnCOPdL2-

RXCOSe-

R11COR

Figure 10.2. Examples of precursors available from [11C]carbon dioxide.

agent for nucleophiles such as carbanions and nucle-ophilic hetero atoms. It can also be used for preparation of several other valuable precursors such as [nC]methyl triflate [11], [nC]methyl lithium [12], [11C]methylcuprates [13], [uC]nitro methane [14] and [uC]methyltriphenylphosphorane [15].

Another precursor, routinely applied in labeling synthesis, is hydrogen [uC]cyanide obtained on-line from [uC]carbon dioxide or [uC]methane. The labeled ni-triles obtained from substitution reactions with [11C]cyanide can be converted to amines, amides and carboxylic acids.

Recently [nC] carbon monoxide has proved useful for synthesis of labeled carbonyl compounds. Some of these labeled carbonyl compounds (i.e., aldehydes and ketones) might themselves be valuable precursors. It is likely that [uC]carbon monoxide in the future will be as important as [nC]methyl iodide in the routine production of PET tracers.

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