Carbon-14 labelled acetylenes preparation

The yield of labelled acetylene is 90-100%. A variant, using labelled barium carbonate and excess of barium to make labelled BaC., then hydrolysis, has been used to prepare and . The isotopic yield is excellent if all traces of paraffin... 

Propane-l,2- C, specific activity of about 110 mCi mmol" has been synthesized from doubly labelled acetylene in reaction series shown in equation 12. This synthesis included preparation of doubly labelled acetylene from barium carbonate-according to Cox and Warne, nearly quantitative hydrogenation of acetylene to ethylene, addition of hydroiodic acid to the latter to form iodoethane, preparation of ethylmagnesium iodide followed by carbonation to yield propionic acid and reduction o n-propanol with 70-75% yield. The latter yielded a tosylate which was finally reduced to doubly labelled propane 2 with sodium borohydride, and purified by gas chromatography on alumina or silica. Its specific activity was close to the maximal possible specific activity of acetylene (i.e. 124.9 mCi mmol " ). 

Synthetic chemical methods for the preparation of carbon-14-labeled chemicals utilize a number of basic building components that are prepared from barium carbonate ( " C). These are versions of carbon dioxide, acetylene, benzene, sodium acetate (1- and 2- " C), methyl iodide, methanol, sodium cyanide, and urea. Many complicated labeled compounds may be synthesized from these materials. 

The possibilities of labeling P-ionone have been reviewed by Isler et al, (1960). Total synthesis of P-ionone via citral, developed by Kimel et al. (1957), could be used to introduce carbon-14 in all positions by such basic building blocks as labeled acetylene, acetone, or acetoacetic ester. Preparation of all-rran5-retinoic-6,7- C2 acid, although unpublished, has been referred to in this review (Isler et al., 1960). The methodology was also used to synthesize 13-cw-retinoic-6,7- C acid and is discussed in Section II,A,2,a,i. 

This method was adapted for a preparation of C-labeled retinoic acid as shown in Fig, 5 (Kaegi et al, 1982c). Acetylene- " 2, prepared from barium carbonate-was converted to the monobromomagnesium acetylide- C2 (XVI) with the aid of ethylmagnesium bromide. Treatment of p-ionone (II) with this acetylide... 

Various [l- C]carboxylic acids have been prepared by carboxylation of carbanions stabilized by -I or -M substituents. The examples in Figure 5.7 have been selected here as prototypes, since they are of strategic interest. Deprotonation of diethyl methylphos-phonate (18 with n-BuLi followed by [ CJcarboxylation and esterification with diazoethane provided triethyl phosphonoll- Clacetate (19) in an overall radiochemical yield of 62%. Compound 19 has been widely exploited for chain extensions by a labeled two-carbon unit via its alkenylation reaction with carbonyl compounds (Homer-Wadsworth-Emmons reaction). Similarly, reaction of alkyl halides, tosylates or carbonyl compounds with LiC = CH or LiC=CH H2NCH2CH2NH2 followed by deprotonation and [ C]carboxylation of the resulting terminal alkynes has been used as a strategic tool for the incorporation of a labeled three-carbon unit, as exemphfied in a steroid platform (20 to 21). This chemistry provides outcomes complementary to those using [ C2]acetylene (Chapter 8, Section 5.1). Finally, the [ CJcarboxylation of lithiated dimethylsulfide provided an alkylthio[l- C]acetic acid 22 and thence a functionalized 2-alkylthio[l- C]ethyl derivative 23 useful, in this case, for elaboration into e.p. 

Acetylene, which can be prepared in high yield from barium [ C]carbonate, is most often used for the production of [U- C]benzene, and thence avast array of labeled aromatic compounds. The preparation and handling of [ C2]acetylene is not difficult to achieve in the average radiochemistry laboratory, and it has been used as a two-carbon building block in a variety of syntheses. In addition, it is the source of a number of other two- to four-carbon building blocks, making its utility relatively broad, even aside from its uses as the source of [U- C]aromatic derivatives. 

Preparation of Carbon-14-Labeled Compounds via the [" Cz]Acetylene Tree 449... 

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