Trans‐[13,14‐14C2]‐Retinoic acid has been synthesized by condensation of β‐ionylideneacetaldehyde and 14C‐labeled methyl senecioate in the presence of potassium amide in ether. A convenient synthesis of the key intermediate, methyl (diethoxyphosphinyl)‐[2‐14C]acetate, is also described.
The title work proceeds in five steps and 16% overall yield from sodium [1‐14C]acetate, which was converted by bromination followed by ethoxide displacement and reaction with phthaloyl dichloride to ethoxy[1‐14C]acetyl chloride. The thermal[2+2]cycloaddition of tetraethoxyethene with ethoxy[1‐14C]ketene, derived from base induced elimination from ethoxy[1‐14]acetyl chloride following the method of Bellus, provided an 80% yield of 1‐(ethoxy[1‐14C]acetoxy)‐2,3,3,4,4‐pentaethoxy‐l‐[1‐14C]‐cyclobutene. Acid hydrolysis and liquid‐liquid extraction afforded [14C]squaric acid.
Acetylene[14C2] is shown to be a useful starting material for the synthesis of a number of 14C‐labeled intermediates and small molecules. Its facile preparation from inexpensive barium carbonate[14C] and the range of transformations it undergoes allow the efficient syntheses of 2‐propyn‐[2,3‐14C2]‐1‐ol, 2‐propen[2,3‐14C2]‐1‐ol, 2‐propen[G‐14C]yl‐isothiocyanate, 2‐butanone[1,2‐14C2], furan[3,4‐14C2], p‐dioxane[U‐14C], and various other intermediates.
S W R Y C a t e~h o l -U -~~C and hydr~quinone-U-~~C were prepared from phenol-U-I4C by mononitration followed by chromatographic separation of 0-and p-nitrophenol-U-14C. and the resulting aminophenols were converted by diazotization followed by hydrolysis to the title dials. ciently from benzene by nitration, reduction, diazotization and hydrolysis. phenol, catechol and hydroquinone containing more than 88% total carbon-13.
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