Advances in Chemistry of Isothiocyanate-derived Colourants

Abstract

This study is focused on the reactions of isothiocyanates (ITCs) in the presence of amino compounds leading to coloured structures via substituted 2-thiohydantoins. A series of complementary experiments has been done and appropriate reaction conditions and structural prerequisites have been defined. Low-molecular colourants isolated and characterised from the model systems can be sorted into three groups. Yellow to red diastereomeric dehydrodimers of 2-thiohydantoin derivatives that contain an acidic methylene group are formed in mixtures consisted of ITCs and amino acids with α-methylene group in mild acidic to mild alkaline systems. The condensation products of the 2-thiohydantoins with reactive aromatic or heterocyclic carbaldehydes from the Maillard reaction, essential oils etc. comprise a heterogeneous group of mostly yellow colourants. Blue compounds of two types are structurally more complicated structures that arise from N-substituted amino acids and ITCs in alkaline media.

Keywords

• isothiocyanates
• amino acids
• 2-thiohydantoins
• carbaldehydes
• colour
• pigments