Multi‐13 C‐labelled inhibitors of tubulin assembly: 5‐substituted methyl N‐(1H‐benzimidazol‐2‐YL)carbamates

Eight methyl N‐(1H‐benzimidazol‐2‐yl) carbamates with various 5‐substituents were synthesized, each 13C‐enriched at carbon 2, and the carbonyl and methoxy carbons. Five were prepared by cyclization involving the appropriate 4‐substituted 1,2‐diaminobenzene (C6H5CO‐, CH3CH2CH2O‐, CH3CH2S‐, C6H5S‐ and CH3CH2CH2‐), and methyl‐13C N‐[imino(methylthio) methyl‐13C]carbamate‐13C or methyl N, N'‐bis(methoxy‐13C‐carbonyl‐13C)carbamimidothionate‐ 13C. The latter were prepared from commercially available 13C‐enriched (91‐92 atom %) carbon tetrachloride, methanol, and thiourea. The remaining three (5‐substituents: C6H5CH(OH)‐, C6H5SO‐ and CH3CH2CH2SO‐) were prepared by side‐chain reduction or oxidation. The 1H‐ and 13C‐NMR, and the methane CI mass spectral data of the products and intermediates are presented.