Iodomethane labelled

BPN, a potent opioid partial agonist265, 277, has been labelled with carbon-11 at the 6-methoxy position, [nC PPN], using a precursor 278 synthesized in high yield (89%) from BPN 277 in two steps (equation 111a) and employing [nC]iodomethane as the radiolabelling reagent266 (equation lib). 

An example of reaction type (d) in Table 5-4 is the Finkelstein halide exchange reaction between iodomethane and radioactive labeled iodide ion. The rate constant for this reaction decreases by about 10" on going from less polar acetone to water as solvent [66]. 

C-flumazenil is commonly labeled at the A7-me 1 11 yl position by N-methylation with 11 C-iodomethane, which is prepared from 11C-C02, and using the freshly prepared Grignard reagent, methylmagnesium bromide (Maziere et al, 1984). The specific activity is very important for this product and therefore is purified by HPLC to give an optimum value between 0.5 and 2Ci/Vmol (18.5-74GI k]/i imol). It remains stable for up to 3 h at room temperature at pH 7.0. The molecular structure of 11C-fumazenil is shown in Fig. 8.2b. 

Raclopride is labeled with nC either by iV-ethylation with [1-UC] iodoethane or by O-methylation with [UC] iodomethane, although the latter is more suitable for routine synthesis. Both 11C-labeled iodoethane and iodomethane are prepared from 11C-C02. The product is purified by HPLC giving a purity of greater than 98%. The specific activity should be in the range of 0.5-2 Ci/Vmol (18.5-74 GBq/Vmol). The product at pH between 4.5 and 8.5 remains stable for more than 1 h at room temperature. The molecular structure of 11C-raclopride is shown in Fig.8.2e. 

Figure 6.3 shows an Arrhenius plot for the reaction between iodomethane and ethoxide ion in a solution of ethanol (Equation 6.1) based on the data used to plot Figure 6.1. Note, as explained in the accompanying Maths Help Box, that the horizontal axis for the Arrhenius plot is labelled 10 JT. This means that any point on this axis represents the numerical value of the quantity (10 K/7).

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