Indigotin, structure

Baeyer assigned a cis-form to indigotin, but x-ray crystallographic studies indicated that the dye molecule has a center of symmetry that is only possible if the molecule has a trans-configutation (119). Many derivatives of indigotin have been prepared that would not have been possible if indigotin had a cis-structure, eg, (36) (120). 

Figure 13.8 Structures and ESI mass spectra of (a) indirubin and (b) indigotin. Reproduced from /VI. Puchalska, K. Potec Pawlak, I. Zadrozna, H. Hryszko and M. Jarosz. J. Mass Spectrom., 39, 1441 1449 (2004). By permission of John Wiley Sons, Ltd...

One more complexity needs to be considered. Some of the precursors are adducts (Y = -SCH3, -S02CH3 in the structures) and cannot be oxidized directly to the indigotins. Use of sunlight in a photochemical process was required in these cases.3... 

Fig. 5 Chemical structures of indigoid (indigotine) and azo (amaranth, allura red, black PN, and tartrazine) dyes.

Consideration of the circumstances surrounding the introduction of mauve and indigotin shows how rapidly organic chemistry had changed in 30 years. Perkin had been trying to prepare quinine by a process which, with our modem knowledge of molecular structure, we can see was hopeless. The laboratory synthesis of indigotin was first achieved in 1880, and its structure was established in 1883 by von Baeyer, and this enabled rational attempts to be made to find a commercially viable synthesis. 

Indigoid compounds are all based on the following structure, indigotin, or its isomer, indirubin ... 

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