Tritium labelling

The introduction of tritium into molecules is most commonly achieved by reductive methods, including catalytic reduction by tritium gas, PH2], of olefins, catalytic reductive replacement of halogen (Cl, Br, or I) by H2, and metal pH] hydride reduction of carbonyl compounds, eg, ketones (qv) and some esters, to tritium-labeled alcohols (5). The use of tritium-labeled building blocks, eg, pH] methyl iodide and pH]-acetic anhydride, is an alternative route to the preparation of high specific activity, tritium-labeled compounds. The use of these techniques for the synthesis of radiolabeled receptor ligands, ie, dmgs and dmg analogues, has been described ia detail ia the Hterature (6,7). 

Boron Bromide. Approximately 30% of BBr produced in the United States is consumed in the manufacture of proprietory pharmaceuticals (qv) (7). BBr is used in the manufacture of isotopicaHy enriched crystalline boron, as a Etiedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Examples of use of BBr as a catalyst include copolymerization of butadiene with olefins (112) polymerization of ethylene and propylene (113), and A/-vinylcarbazole (114) in hydroboration reactions and in tritium labeling of steroids and aryl rings (5). 

The incorporation of a chapter on deuteration in a steroid monograph is quite reasonable since development of a number of the most important deuteration reactions have actually had their impetus through steroid research. The field of steroid chemistry offers possibly the largest variety of deuteration reactions of any area of organic chemistry. Many of these deuteration techniques have also been used for tritium labeling, which is especially pertinent in view of the large demand for tritiated steroids as tracers in biological experiments. 

The production of tritium-labelled organic compounds was enormously facilitated by K. E. Wilz-bach s discovery in 1956 that tritium could be introduced merely by storing a compound under tritium gas for a few days or weeks the radiation induces exchange reactions between the hydrogen atoms in the compound and the tritium gas. The excess of gas is recovered for further use and the tritiated compound is purified chro-matographically. Another widely used method of... 

Synthesis of tritium-labeled biologically important diazines 99UK254. 

Another example of this preference is found in the enantiospecific syntheses of tritium-labeled leukotrienes(/i). Commercially available 3-nonyn-l-ol was converted to its phosphorane (16) and Wittig-coupled with the unsaturated aldehyde (17) to afford 18, which was reduced over Lindlar catalyst to give 19. 

Epoxyfarnesol was first prepared by van Tamelen, Stomi, Hessler, and Schwartz 4 using essentially this procedure. It is based on the findings of van Tamelen and Curphey5 that N-bromosuccinimide in a polar solvent was a considerably more selective oxidant than others they tried. This method has been applied to produce terminally epoxidized mono-, sesqui-, di-, and triterpene systems for biosynthetic studies and bioorganic synthesis.6 It has also been applied successfully in a simple synthesis of tritium-labeled squalene [2,6,10,14,18,22-Tetracosahexaene, 2,6,10,15,19,23-hexamethyl-, (all-E)-] and squalene-2,3-oxide [Oxirane, 2,2-dimethyl-3-(3,7,12,16,20-pentamethyl-3,7,ll,-15,19-heneicosapentaenyl)-, (all-E)-],7 and in the synthesis of Cecropia juvenile hormone.8... 

However, 1,2 migration of alkyl groups has been shown to occur in certain diradicalsFor example, the following rearrangement has been established by tritium labeling. 

Preparation of Tritium-Labeled Dibenzo- -dioxin and 2,3,7,8-Tetrachlorodibenzo-//-dioxin... 

Muelder and Shadoff (3) prepared C-2,3,7,8-Cl4-DBpD (0.9 mCi/ mmole) by chlorination of C-2,7-dichlorodibenzo-p-dioxin made from potassium C-2,4-dichlorophenate. The preparation of tritium-labeled 2,3,7,8-Cl4-DBpD is justified because the radiolabeled intermediates are less expensive and more accessible and because a higher specific activity is potentially attainable. Here, we consider the optimal conditions for the reaction sequence designed to obtain products of high chemical and radiochemical purity shown at the top of p. 8. 

The uptake of TRA into cervical tissue was determined by measuring tissue radioactivity following insertion of the collagen sponge cervical cap containing tritium-labeled TRA. The TRA concentrations peaked at 4 hr and then diminished rapidly by 24 hr. Since measurements of blood samples revealed that no systemic absorption had occurred, high local concentrations over an extended period of time may be possible without systemic side effects. 

Figure 1. Semilogarithmic plot of brevetoxin ( iCi) in plasma over time after an intravenous injection of tritium-labeled PbTx-3. T 1/2 alpha = 30 sec T 1/2 beta = 112 min.

CP-55,940 (8.5) was developed during the search for novel analgesics [146]. Although it is more potent than morphine it was never approved. Nevertheless, in its tritium-labeled form it became a very important tool for research and helped in the first identification of the cannabinoid receptor. 

A thiazole core has also been utilised by Solvay Pharmaceuticals [309] in the search for a novel bioisosterie replacement of the rimonabant (382) pyrazole core. The affinity of several compounds for the human CBi receptor was determined in transfected CHO cells using tritium-labelled CP 55940. Antagonism was determined in the same cell line by WIN 55212-2-induced release of arachidonic acid. The pK[ of (456) was found to be 6.9, while the p 2 value was measured as 8.7. A series of six 1,2,4-triazole analogues has been prepared by Jagerovic et al. [310], via their corresponding A-acylbenzamides (Table 6.39). 

An azetidine motif was also present in two series of CBi antagonist compounds disclosed by Vernalis Research [335, 336]. In the former, compound (560) was claimed to have an affinity of 285 nM in transfected HEK293 cells using tritium-labelled (382). Among the preferred indications were psychosis, schizophrenia, smoking cessation and eating disorders associated with excessive food intake. Compound (561) was claimed to have an affinity of 0.8 nM in the same binding assay [336]. 

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