Tritiation procedures

The most widely used methods for tritiating organic compounds include the following. Some methods are able to introduce the tritium at specific sites whilst others are used to prepare more generally labelled compounds. 

Whilst tritiation technology has progressed greatly in recent years there is still room for improvement. Many of the reactions are slow and require many hours to come to completion. Sometimes the regiospecificity is low 

Regiospeciflc tritiation of pentamidine using homogeneous RhCls as catalyst. Reproduced with permission. 

Many pharmaceutical agents are imable to withstand these high temperatures and are also required at higher specific activities. As mentioned above the need is for catalysts that can operate at ambient temperatures and where T2 gas rather than tritiated water is used as the donor parallel and combinatorial-type studies will accelerate this process. 

Heterogeneous catalysts are also capable of achieving high regiospeci-ficity. Thus Raney Nickel, a catalyst that has not been widely used in triti-ation studies, labels only the side chains of toluene and butylbenzene under mild conditions.On the other hand the Lewis acids, EtAlCl2 and BBra, tritiate the equivalent 2,4,6-positions of 1,3,5-trimethylbenzene despite the obvious steric hindrance. 

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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. 

The tritiation procedures given in Tab. 13.1 all have serious limitations/disadvan-tages. Thus for all three hydrogen isotope exchange reactions HTO is used as the do-... 

I 78 Microivave-enhanced Radiochemistry Tab. 18.1. Main tritiation procedures. 

The tritiation procedures given in Table 18.1 all have serious limitations or disadvantages. Thus, for all three hydrogen isotope exchange reactions, HTO is used as the donor. For health and safety reasons 50 Ci mL (1 Ci = 37 GBq), corresponding to dose on 2% isotopic abundance, is the highest specific activity that we have used and this inevitably limits the maximum specific activity of the products that can be obtained via these routes. 

A one-pot tritiation procedure for aminoglycoside antibiotics has been reported, involving hypochlorite oxidation of the amino... 

Recently microwaves have been used to greatly accelerate the rates of many of these reactions whilst the development of microwave-enhanced solid state tritiation procedures offers considerable potential. 

The small synthetic scale used for production of many labeled compounds creates special challenges for product purification. Eirst, because of the need for use of micro or semimicro synthetic procedures, the yield of many labeled products such as high specific activity tritiated compounds is often low. In addition, under such conditions, side reactions can generate the buildup of impurities, many of which have chemical and physical properties similar to the product of interest. Also, losses are often encountered in simply handling the small amounts of materials in a synthetic mixture. As a consequence of these considerations, along with the variety of tracer chemicals of interest, numerous separation techniques are used in purifying labeled compounds. 

Since ivermectin (= 22,23-dihydroavermectin B ) is obtained by catalytic reduction of avermectin B, the same procedure using tritium gas convenientiy affords tritiated ivermectin (22,23- [JT]-22,23-dihydroavermectin B ). The preparation of a tritiated derivative containing a 22,23-double bond starts with the readily available 5-ketone, which is reduced with [JT]-sodium borohydride stereospecificaHy to a 5- [JT]-derivative (40). Carbon-14 labeled avermectins can be obtained by a biosynthetic process using sodium (l- C)propionate as labeled precursor (48). 

Hot atom reactions have also been used to label organic compounds with T. Irradiation of helium-3 with neutrons according to the nuclear reaction produces very energetic tritium atoms that can displace ordinary hydrogen in organic compounds. This procedure is not very selective, and the labeling pattern must be determined to enable the tritiated product to be used effectively as a tracer (34). 

SO the Sgl mechanism and not the usual arenium ion mechanism is operating. Aromatic rings can also be deuterated by treatment with D2O and a rhodium(III) chloride or platinum catalyst or with CeDs and an alkylaluminum dichloride catalyst," though rearrangements may take place during the latter procedure. Tritium ( H, abbreviated T) can be introduced by treatment with T2O and an alkylaluminum dichloride catalyst. " Tritiation at specific sites (e.g., >90% para in... 

Ease of separation of tritiated products from a reaction medium is an important feature in the choice of labeling procedure. Sometime ago we used polymer-sup-ported acid and base catalysts [12, 13] to good effect and with the current interest in Green Chemistry one can expect to see more studies where the rate accelerations observed under microwave-enhanced conditions are combined with the use of solid catalysts such as Nafion, or zeolites. 

The tritiated version could be prepared from tritiated formic acid which we had prepared at high specific activity (2.5 Ci mmol-1) by a metal-catalyzed hydrogen-tritium exchange procedure using T2 gas. The material can be stored either as a solid or as a solution if the latter any release of tritium by back exchange can be easily monitored by 3H NMR spectroscopy. In our experience very little exchange occurs over several weeks of storage [51]. 

Tritiated digoxin has been used as a tracer in most of the published procedures, but recently techniques employing [ I]tyrosine-labeled digoxin have been described. Such techniques not only are more sensitive, but have the added advantage of dispensing with the necessity to use scintillation counting. 

Labeled compounds have also been produced by the hydrolysis of reduced pyrimidine derivatives, and thus the basic hydrolysis of 5,5,6,6-tetradeuterouracil 547 (R = D) gave 2,2,3,3-tetradeutero-/5-alanine 548 (R=D) in 50% yield <2001JLR7>. 5,6,6-Trideuterothymine 547 (R = Me) behaved similarly to give 2,3,3-trideutero-3-amino-2-methylpropanoic acid 548 (R = Me), also in 50% yield <2001JLR7>. Similar procedures have been performed with tritiated uracil derivatives <2002MI295>. 

The original work on wet chemical functionalization of germanium was reported as early as 1962 by Cullen et al. (researchers in RCA labs) [105]. They exposed ethyl Grignard to a chloride-terminated Ge(lll) surface and used radiotracer studies with tritiated ethyl groups to show that ethylation of the surface occurs via formation of Ge—C bonds in a one-to-one ratio with surface atoms. Because the chlorination procedure used by the authors can lead to formation of mono-, di-, and trichloride species, there may be some uncertainty as to the Ge/C ratio. However, these surfaces did demonstrate hydrophobic behavior and stability in both atmosphere and aqueous solution. 

Allylic terpenic thiols were prepared in 70-80% yield by this procedure. The reversibility under mild conditions of the ene reaction with N-sulfinylsulfonamides RS02N=S=0 had been exploited in allylic deuteration and tritiation of olefins [516]. 

A useful variation of this technique is to measure the rate of exchange of tritium from tritiated water with the C-2 proton of the imidazole ring of a histidine.31,32 The rate constant for the exchange depends on the state of ionization of the imidazole, and is faster for the unprotonated form. This procedure is a useful adjunct to NMR experiments, which also measure the pKa s of histidine residues. ... 

The radiochemical analysis of salicylamide in plasma using the ring-labelled tritiated compound was reported by Stella et al (59) The tritiated salicylamide was synthesized by reacting cold salicylamide with trituim oxide in the presence of heptafluorobutyric acid. This procedure is effective for the analysis of salicylamide and its metabolites (the sulphate and glucuronide) in the presence of similar phenolic compounds. 

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