Labeled compound preparation

Zentralinstitut Kernforschung Rossendorf, Dresden, Annual Report ZfK 711 46-47 Kalincak M, Machan V, Vilcek S (1982) " Tc-labelled compounds prepared with Ti(III) as reducing agent. Isotopenpraxis 18 334-338... 

The stereochemistry of many of the biological reactions involving serine and its derivatives has been studied using the labeled compounds prepared above. The carbon atom, C-3, of serine acts as a source of the one carbon unit transferred by the coenzyme tetrahydrofolic acid 87 (95) (Scheme 24). It is initially transferred to the coenzyme 87 to give 5,10-methylenetetrahydro-folic acid 56a and glycine 23 in a reaction catalyzed by the enzyme serine... 

Labeled compound Preparation Isotope recovered in (or as) Remarks Ref. ... 

Before sample preparation, surrogate compounds must be added to the matrix. These are used to evaluate the efficiency of recovery of sample for any analytical method. Surrogate standards are often brominated, fluorinated, or isotopically labeled compounds that are not expected to be present in environmental media. If the surrogates are detected by GC/MS within the specified range, it is... 

Spectrometric Analysis. Remarkable developments ia mass spectrometry (ms) and nuclear magnetic resonance methods (nmr), eg, secondary ion mass spectrometry (sims), plasma desorption (pd), thermospray (tsp), two or three dimensional nmr, high resolution nmr of soHds, give useful stmcture analysis information (131). Because nmr analysis of or N-labeled amino acids enables determiaation of amino acids without isolation from organic samples, and without destroyiag the sample, amino acid metaboHsm can be dynamically analy2ed (132). Proteia metaboHsm and biosynthesis of many important metaboUtes have been studied by this method. Preparative methods for labeled compounds have been reviewed (133). 

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). 

Generally, labeled compounds are prepared by procedures which introduce the radionuchde at a late stage of the synthesis. This allows for maximum radiochemical yields, and reduces the handling time of radioactive material. When dealing with short half-life isotopes, a primary consideration is the time required to conduct synthetic procedures and purification methods. 

Alternatively, radiohalogen-labeled compounds may be prepared by way of isotopic labeling from the unlabeled bromo or iodo derivatives by various two-step reaction sequences. Examples include the use of trialkylsilyl synthons as described in References 10—13, and the use of boronic acid synthons as described in References 14 and 15. 

Radioactively labelled compounds have been employed in biology for the clarification of metabolic processes since the mid-1940s It has, thus, been necessary to prepare such substances and to check on their punty... 

An example is the preparation of 18-trideuterio 5a-steroids bearing a side chain at C-17. Labeling of this position with three deuteriums was accomplished by utilizing the Johnson procedure for steroid total synthesis. This synthesis involves, in part, introduction of the 18-angular methyl group by methylation of the D-homo-17a-keto-17-furfurylidene intermediate (243). By substituting d3-methyl iodide in this step, the C/D cis- and ra/J5-18,18,18-d3 labeled ketones [(244) and (245)] are obtained. Conversion of the C/D tra 5-methylation product (245) into 18,18,18-d3-d /-3)8-hydroxy-5a-androstan-17-one (246) provides an intermediate which can be converted into a wide variety of C-18 labeled compounds of high (98%) isotopic... 

When specifically labelled compounds are required, direct chemical synthesis may be necessary. The standard techniques of preparative chemistry are used, suitably modified for small-scale work with radioactive materials. The starting material is tritium gas which can be obtained at greater than 98% isotopic abundance. Tritiated water can be made either by catalytic oxidation over palladium or by reduction of a metal oxide ... 

All labeled compounds including n-Bu6Li (from 6Li ingot) were prepared by us. When n-Bu6Li was added to a solution of cyclopropylacetylene (37) and chiral modifier 46 (1 1 ratio) in THF-pentane, the 6Li NMR at -125 °C (A) is shown in Figure 1.7. A few sets of aggregates could be identified. (See Ref [35a] for full assignment). 

Preparative methods of 99mTc-labeled radiopharmaceuticals are discussed in detail by Volkert and Jurrison in this book [49]. Accordingly, limited examples of kinetics for the preparation of 99mTc-labeled compounds by ligand substitution are discussed in this section. 

The incorporation of 14C into compounds at a suitable site often requires extensive and complicated syntheses, and thus a relatively long time. This usually means that 14C-labeled compounds are unsuitable for studies to be carried out during discovery. There are however, very rapid methods for incorporating 3 H into compounds. The newer methods, generally involving metal-catalyzed exchange reactions [15-18], in our experience, mean that suitable labels can often be prepared in 2 or 3 weeks. These time scales make the approach viable for discovery support. Additionally, and importantly, these methods can lead to specific incorporation of tritium. 

This tritiated water can become incorporated into the normal bases of DNA and may contribute significantly to the overall apparent binding levels measured in DNA (4). Such problems are circumvented in the case of 14C labeled hydrocarbons. Tritium labeled compounds can be prepared at sufficiently high specific activities (often 20-80 Ci/mmole) that fentomole amounts of adduct may be detected. 

The incorporation of a fluorine-18 label can also be achieved by standard aliphatic nucleophilic substitution chemistry, as exemplified in Scheme 6.171. Here, the widely used reagent [18F]-/f-fluoroethyl tosylate was utilized to prepare several important 18F-labeled compounds [323],... 

It is interesting to remark that classical hydrogenation is a method used to prepare deuterium-labeled compounds by aromatic dehalogenation, but the usual reaction conditions suffer from several limitations. A microwave-enhanced catalytic dehalogenation procedure for rapid and specific deuterium labeling of N-4-picolyl-4-halogenobenzamide 68, by use of deuterated formate, [41] was recently reported (Scheme 8.25). 

The standard work of Evans [2] as well as a survey of the papers produced in the Journal of Labeled Compounds and Radiopharmaceuticals over the last 20 years shows that the main tritiation routes are as given in Tab. 13.1. One can immediately see that unlike most 14C-labeling routes they consist of one step and frequently involve a catalyst, which can be either homogeneous or heterogeneous. One should therefore be able to exploit the tremendous developments that have been made in catalysis in recent years to benefit tritiation procedures. Chirally catalyzed hydrogenation reactions (Knowles and Noyori were recently awarded the Nobel prize for chemistry for their work in this area, sharing it with Sharpless for his work on the equivalent oxidation reactions) immediately come to mind. Already optically active compounds such as tritiated 1-alanine, 1-tyrosine, 1-dopa, etc. have been prepared in this way. 

In the preparation of tritium-labeled compounds there are four stages ... 

The model compound anilinostearamide 78, labelled in the aliphatic chain only, prepared subsequently by the reduction of linoleic precursor 79 with hydrogen or deuterium (equation 30), exhibited a chromatographic isotope effect of similar magnitude. The labelled compound elutes on the re versed-phase HPLC prior to the unlabelled one. 

Electrochemical synthesis was utilized to prepare labeled compounds. Tetramethyllead labeled with 14C was prepared in a double compartment cell in DMF with NaClC>4, by electrolyzing 14CH3l on lead electrodes. The method is reported as superior to transmet-allation with methylmagnesium halide. It is also possible to incorporate lead isotopes. 2i°Pb2+ ions were deposited on a Cu foil and the latter was used as a sacrificial electrode in solutions of CH3I. The yield of labeled tetramethyllead was 85%65. Synthesis of 210Pb-labeled chlorotrimethylplumbane was also described66. 

---