Catalytic reduction, preparation tritiated

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). 

Tritiated GAs of very high specific radioactivity have been prepared by catalytic reduction. The 1,2 double bond of GAq can be selectively reduced, using a partially poisoned palladium catalyst, to give [1,2-3h2]GA] (7 -,(75,76), although some reduction of the 16,17 double bond and the lactone also occurs (76). Introduction of 3h at sites other than carbon atoms 1 and 2 has also been found (76). [3h]GAI has been prepared from GAy by a similar method JT7). [3h]GA was converted... 

Tracers used in this study were 24,28-tritiated castasterone and brassinolide, which were prepared by tritium catalytic reduction of dohchosterone and dolicholide, respectively (7). Figure 4 shows a reverse phase HPLC profile of tritiated products of dolicholide, in which brassinolide was a major component but, in addition, its 24-epimer was also present in the product. Tritiated brassinolide was separated from its 24 epimer by this HPLC technique. In the case of castasterone, die product was likewise purified by HPLC to separate tritiated castasterone from its 24-epimer. 

Tritiated oleate was prepared by catalytic reduction of one of the two double bonds of linoleic acid. To 5 mCi of NaBT4 (13.9 pmol) in 0.5 mL diethyl ether and 5 mg of 10% PdA3 was added 156 pL of 100 mg linoleic acid (55.7 pmol) (in 1 ml diethyl ether [7]). The free linoleic acid was added slowly (15 pL per min) formed sufficient tritium gas to stoichiometrically hydrogenate one of the two double bonds. The reaction mixture was taken to dryness under nitrogen, treated with 0.250 mL 10% methanolic HCl, heated to 70° for 1 hour to prepare the methyl ester. The product was separated by preparative TLC on silver impregnated silica gel as above. Three major radioactive peaks were observed, in addition to some material near the origin. The major radioactive peak corresponded with methyl oleate, the second peak with methyl vaccinate and the third peak was unidentified. 

Tritiated ethers can be prepared by controlled catalytic reduction by Wilzbach exchange using the unsaturated aUcoxyglycerols. Haigh and Hanahan (1965) reported that the main radiation decomposition products of extremely high specific activity H -selachyl alcohol were hydrocarbons. 

Properties of T2O. Some important physical properties of T2O are Hsted in Table 2. Tritium oxide [14940-65-9] can be prepared by catalytic oxidation of T2 or by reduction of copper oxide using tritium gas. T2O, even of low (2—19% T) isotopic abundance, undergoes radiation decomposition to form HT and O2. Decomposition continues, even at 77 K, when the water is fro2en. Pure tritiated water irradiates itself at the rate of 10 MGy/d (10 rad/d). A stationary concentration of tritium peroxide, T2O2, is always present (9). AH of these factors must be taken into account in evaluating the physical constants of a particular sample of T2O. 

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 ... 

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