A method for incorporation of a 14C‐label in the ring geminal methyl moiety of Vitamin A is described. The procedure involves mono methylation of 2,6‐dimethylcyclohexanone with 14C‐methyl iodide as catalyzed by potassium hydride to afford 2‐14C, 2,6‐trimethylcyclohexanone. Subsequent condensation with an 11‐carbon acetylenic side chain, followed by reduction, acetylation and dehydration yielded 14C‐retinyl acetate. Final purification was achieved by preparative thin layer chromatography.
The synthesis of tetralabeled histidine 15 described. Glycine‐15N was converted essentially by literature procedures to 4‐chloromethylimidazole‐3‐15N‐2,5‐2H2. Alkylation of ethyl acetamidocyanoacetate‐2‐13C, followed by acid hydrolysis and decarboxylation yielded DL‐histidine‐α‐ 13C‐3‐15N‐2,5‐2H2. The compound is useful as a mass spectrometric standard for the measurement of histidine and metabolites in biological media.
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