Synthesis and spectroscopic characterisation of ¹³C‐labelled anthranilic acid

The three isotopomers (1– 13 C)‐, (2‐ 13 C)‐, and (6‐ 13 C)‐ anthranilic acid (2‐aminobenzoic acid) have been synthesised from simple 99% 13 C‐labelled starting materials. The isotopomers have been characterised using mass spectrometry, 1 H‐NMR spectroscopy and 13 C‐NMR spectroscopy. The spectroscopic results confirm that, within experimental error, the syntheses were accomplished without scrambling or dilution of label. The labelled anthranilic acids are used as substrates for bioconversion into the correspondingly labelled L‐tryptophans. For this purpose, the (2‐ 13 C)‐ label has been synthesised on the gram scale.