Indigotin derivatives

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). 

Oxidising agents convert indigotin to isatin [16]. Chlorine produces chlorine derivatives of isatin first, and then chlorinated phenols and chloranil [16]. Bromine acts in a similar manner. 

Halogen derivatives [33] of indigotin have been prepared from the corresponding derivatives of isatin and of orthonitrobenz-aldehyde [37]. r... 

A patent of Meister, Lucius, Briining [46] depends on a similar reaction. Claisen s benzylidene-acetone (cinnamylmethyl-ketone CeHj —CH = CHCO—CH3) [47], obtained by condensation of benzaldehyde and acetone, is nitrated. A mixture of ortho and paranitro derivatives is formed, and the former yields indigotin by action of alkalies. 

Excess of alkali is necessary to keep the compound in solution because there is a tendency for hydrolysis to occur, accompanied by the formation of the insoluble hydroxyl derivative. The sodium compound is substantive to cellulose and very readily oxidized in situ in the fibre to insoluble blue Indigotin by the action of atmospheric oxygen. 

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