Studies on the acetylation and NMR reassignment of indirubin derivatives.

The analysis of 1D- and 2D-NMR spectroscopic data confirmed that the amino N-1' protons of indirubin and indirubin-3'-oxime resonate at a higher frequency than N-1 protons. The amino N-1' protons in both indirubin and indirubin-3'-oxime are not favourable for acetylated reaction due to their intramolecular hydrogen bonding with the amide carbonyl group. The new N-1-acetylindirubin-3'-acetoxime has been synthesised using acetic anhydride. The reassignment of the NMR data of indirubin, indirubin-3'-oxime and N-1-acetylindirubin was confirmed with the aid of DEPT, HSQC, HMBC and NOESY methods.